flask/app.py

# -*- coding: utf-8 -*-
"""
    flask.app
    ~~~~~~~~~

    This module implements the central WSGI application object.

    :copyright: (c) 2015 by Armin Ronacher.
    :license: BSD, see LICENSE for more details.
"""
import os
import sys
import warnings
from datetime import timedelta
from functools import update_wrapper
from itertools import chain
from threading import Lock

from werkzeug.datastructures import ImmutableDict, Headers
from werkzeug.exceptions import BadRequest, HTTPException, \
    InternalServerError, MethodNotAllowed, default_exceptions, \
    BadRequestKeyError
from werkzeug.routing import BuildError, Map, RequestRedirect, Rule

from . import cli, json
from ._compat import integer_types, reraise, string_types, text_type
from .config import Config, ConfigAttribute
from .ctx import AppContext, RequestContext, _AppCtxGlobals
from .globals import _request_ctx_stack, g, request, session
from .helpers import _PackageBoundObject, \
    _endpoint_from_view_func, find_package, get_debug_flag, \
    get_flashed_messages, locked_cached_property, url_for
from .sessions import SecureCookieSessionInterface
from .signals import appcontext_tearing_down, got_request_exception, \
    request_finished, request_started, request_tearing_down
from .templating import DispatchingJinjaLoader, Environment, \
    _default_template_ctx_processor
from .wrappers import Request, Response

# a lock used for logger initialization
_logger_lock = Lock()

# a singleton sentinel value for parameter defaults
_sentinel = object()


def _make_timedelta(value):
    if not isinstance(value, timedelta):
        return timedelta(seconds=value)
    return value


def setupmethod(f):
    """Wraps a method so that it performs a check in debug mode if the
    first request was already handled.
    """
    def wrapper_func(self, *args, **kwargs):
        if self.debug and self._got_first_request:
            raise AssertionError('A setup function was called after the '
                'first request was handled.  This usually indicates a bug '
                'in the application where a module was not imported '
                'and decorators or other functionality was called too late.\n'
                'To fix this make sure to import all your view modules, '
                'database models and everything related at a central place '
                'before the application starts serving requests.')
        return f(self, *args, **kwargs)
    return update_wrapper(wrapper_func, f)


class Flask(_PackageBoundObject):
    """The flask object implements a WSGI application and acts as the central
    object.  It is passed the name of the module or package of the
    application.  Once it is created it will act as a central registry for
    the view functions, the URL rules, template configuration and much more.

    The name of the package is used to resolve resources from inside the
    package or the folder the module is contained in depending on if the
    package parameter resolves to an actual python package (a folder with
    an :file:`__init__.py` file inside) or a standard module (just a ``.py`` file).

    For more information about resource loading, see :func:`open_resource`.

    Usually you create a :class:`Flask` instance in your main module or
    in the :file:`__init__.py` file of your package like this::

        from flask import Flask
        app = Flask(__name__)

    .. admonition:: About the First Parameter

        The idea of the first parameter is to give Flask an idea of what
        belongs to your application.  This name is used to find resources
        on the filesystem, can be used by extensions to improve debugging
        information and a lot more.

        So it's important what you provide there.  If you are using a single
        module, `__name__` is always the correct value.  If you however are
        using a package, it's usually recommended to hardcode the name of
        your package there.

        For example if your application is defined in :file:`yourapplication/app.py`
        you should create it with one of the two versions below::

            app = Flask('yourapplication')
            app = Flask(__name__.split('.')[0])

        Why is that?  The application will work even with `__name__`, thanks
        to how resources are looked up.  However it will make debugging more
        painful.  Certain extensions can make assumptions based on the
        import name of your application.  For example the Flask-SQLAlchemy
        extension will look for the code in your application that triggered
        an SQL query in debug mode.  If the import name is not properly set
        up, that debugging information is lost.  (For example it would only
        pick up SQL queries in `yourapplication.app` and not
        `yourapplication.views.frontend`)

    .. versionadded:: 0.7
       The `static_url_path`, `static_folder`, and `template_folder`
       parameters were added.

    .. versionadded:: 0.8
       The `instance_path` and `instance_relative_config` parameters were
       added.

    .. versionadded:: 0.11
       The `root_path` parameter was added.

    .. versionadded:: 0.13
       The `host_matching` and `static_host` parameters were added.

    :param import_name: the name of the application package
    :param static_url_path: can be used to specify a different path for the
                            static files on the web.  Defaults to the name
                            of the `static_folder` folder.
    :param static_folder: the folder with static files that should be served
                          at `static_url_path`.  Defaults to the ``'static'``
                          folder in the root path of the application.
    :param host_matching: sets the app's ``url_map.host_matching`` to the given
                          given value. Defaults to False.
    :param static_host: the host to use when adding the static route. Defaults
                        to None. Required when using ``host_matching=True``
                        with a ``static_folder`` configured.
    :param template_folder: the folder that contains the templates that should
                            be used by the application.  Defaults to
                            ``'templates'`` folder in the root path of the
                            application.
    :param instance_path: An alternative instance path for the application.
                          By default the folder ``'instance'`` next to the
                          package or module is assumed to be the instance
                          path.
    :param instance_relative_config: if set to ``True`` relative filenames
                                     for loading the config are assumed to
                                     be relative to the instance path instead
                                     of the application root.
    :param root_path: Flask by default will automatically calculate the path
                      to the root of the application.  In certain situations
                      this cannot be achieved (for instance if the package
                      is a Python 3 namespace package) and needs to be
                      manually defined.
    """

    #: The class that is used for request objects.  See :class:`~flask.Request`
    #: for more information.
    request_class = Request

    #: The class that is used for response objects.  See
    #: :class:`~flask.Response` for more information.
    response_class = Response

    #: The class that is used for the Jinja environment.
    #:
    #: .. versionadded:: 0.11
    jinja_environment = Environment

    #: The class that is used for the :data:`~flask.g` instance.
    #:
    #: Example use cases for a custom class:
    #:
    #: 1. Store arbitrary attributes on flask.g.
    #: 2. Add a property for lazy per-request database connectors.
    #: 3. Return None instead of AttributeError on unexpected attributes.
    #: 4. Raise exception if an unexpected attr is set, a "controlled" flask.g.
    #:
    #: In Flask 0.9 this property was called `request_globals_class` but it
    #: was changed in 0.10 to :attr:`app_ctx_globals_class` because the
    #: flask.g object is now application context scoped.
    #:
    #: .. versionadded:: 0.10
    app_ctx_globals_class = _AppCtxGlobals

    # Backwards compatibility support
    def _get_request_globals_class(self):
        return self.app_ctx_globals_class
    def _set_request_globals_class(self, value):
        from warnings import warn
        warn(DeprecationWarning('request_globals_class attribute is now '
                                'called app_ctx_globals_class'))
        self.app_ctx_globals_class = value
    request_globals_class = property(_get_request_globals_class,
                                     _set_request_globals_class)
    del _get_request_globals_class, _set_request_globals_class

    #: The class that is used for the ``config`` attribute of this app.
    #: Defaults to :class:`~flask.Config`.
    #:
    #: Example use cases for a custom class:
    #:
    #: 1. Default values for certain config options.
    #: 2. Access to config values through attributes in addition to keys.
    #:
    #: .. versionadded:: 0.11
    config_class = Config

    #: The debug flag.  Set this to ``True`` to enable debugging of the
    #: application.  In debug mode the debugger will kick in when an unhandled
    #: exception occurs and the integrated server will automatically reload
    #: the application if changes in the code are detected.
    #:
    #: This attribute can also be configured from the config with the ``DEBUG``
    #: configuration key.  Defaults to ``False``.
    debug = ConfigAttribute('DEBUG')

    #: The testing flag.  Set this to ``True`` to enable the test mode of
    #: Flask extensions (and in the future probably also Flask itself).
    #: For example this might activate test helpers that have an
    #: additional runtime cost which should not be enabled by default.
    #:
    #: If this is enabled and PROPAGATE_EXCEPTIONS is not changed from the
    #: default it's implicitly enabled.
    #:
    #: This attribute can also be configured from the config with the
    #: ``TESTING`` configuration key.  Defaults to ``False``.
    testing = ConfigAttribute('TESTING')

    #: If a secret key is set, cryptographic components can use this to
    #: sign cookies and other things.  Set this to a complex random value
    #: when you want to use the secure cookie for instance.
    #:
    #: This attribute can also be configured from the config with the
    #: ``SECRET_KEY`` configuration key.  Defaults to ``None``.
    secret_key = ConfigAttribute('SECRET_KEY')

    #: The secure cookie uses this for the name of the session cookie.
    #:
    #: This attribute can also be configured from the config with the
    #: ``SESSION_COOKIE_NAME`` configuration key.  Defaults to ``'session'``
    session_cookie_name = ConfigAttribute('SESSION_COOKIE_NAME')

    #: A :class:`~datetime.timedelta` which is used to set the expiration
    #: date of a permanent session.  The default is 31 days which makes a
    #: permanent session survive for roughly one month.
    #:
    #: This attribute can also be configured from the config with the
    #: ``PERMANENT_SESSION_LIFETIME`` configuration key.  Defaults to
    #: ``timedelta(days=31)``
    permanent_session_lifetime = ConfigAttribute('PERMANENT_SESSION_LIFETIME',
        get_converter=_make_timedelta)

    #: A :class:`~datetime.timedelta` which is used as default cache_timeout
    #: for the :func:`send_file` functions. The default is 12 hours.
    #:
    #: This attribute can also be configured from the config with the
    #: ``SEND_FILE_MAX_AGE_DEFAULT`` configuration key. This configuration
    #: variable can also be set with an integer value used as seconds.
    #: Defaults to ``timedelta(hours=12)``
    send_file_max_age_default = ConfigAttribute('SEND_FILE_MAX_AGE_DEFAULT',
        get_converter=_make_timedelta)

    #: Enable this if you want to use the X-Sendfile feature.  Keep in
    #: mind that the server has to support this.  This only affects files
    #: sent with the :func:`send_file` method.
    #:
    #: .. versionadded:: 0.2
    #:
    #: This attribute can also be configured from the config with the
    #: ``USE_X_SENDFILE`` configuration key.  Defaults to ``False``.
    use_x_sendfile = ConfigAttribute('USE_X_SENDFILE')

    #: The name of the logger to use.  By default the logger name is the
    #: package name passed to the constructor.
    #:
    #: .. versionadded:: 0.4
    logger_name = ConfigAttribute('LOGGER_NAME')

    #: The JSON encoder class to use.  Defaults to :class:`~flask.json.JSONEncoder`.
    #:
    #: .. versionadded:: 0.10
    json_encoder = json.JSONEncoder

    #: The JSON decoder class to use.  Defaults to :class:`~flask.json.JSONDecoder`.
    #:
    #: .. versionadded:: 0.10
    json_decoder = json.JSONDecoder

    #: Options that are passed directly to the Jinja2 environment.
    jinja_options = ImmutableDict(
        extensions=['jinja2.ext.autoescape', 'jinja2.ext.with_']
    )

    #: Default configuration parameters.
    default_config = ImmutableDict({
        'DEBUG':                                get_debug_flag(default=False),
        'TESTING':                              False,
        'PROPAGATE_EXCEPTIONS':                 None,
        'PRESERVE_CONTEXT_ON_EXCEPTION':        None,
        'SECRET_KEY':                           None,
        'PERMANENT_SESSION_LIFETIME':           timedelta(days=31),
        'USE_X_SENDFILE':                       False,
        'LOGGER_NAME':                          None,
        'LOGGER_HANDLER_POLICY':               'always',
        'SERVER_NAME':                          None,
        'APPLICATION_ROOT':                     '/',
        'SESSION_COOKIE_NAME':                  'session',
        'SESSION_COOKIE_DOMAIN':                None,
        'SESSION_COOKIE_PATH':                  None,
        'SESSION_COOKIE_HTTPONLY':              True,
        'SESSION_COOKIE_SECURE':                False,
        'SESSION_REFRESH_EACH_REQUEST':         True,
        'MAX_CONTENT_LENGTH':                   None,
        'SEND_FILE_MAX_AGE_DEFAULT':            timedelta(hours=12),
        'TRAP_BAD_REQUEST_ERRORS':              None,
        'TRAP_HTTP_EXCEPTIONS':                 False,
        'EXPLAIN_TEMPLATE_LOADING':             False,
        'PREFERRED_URL_SCHEME':                 'http',
        'JSON_AS_ASCII':                        True,
        'JSON_SORT_KEYS':                       True,
        'JSONIFY_PRETTYPRINT_REGULAR':          False,
        'JSONIFY_MIMETYPE':                     'application/json',
        'TEMPLATES_AUTO_RELOAD':                None,
    })

    #: The rule object to use for URL rules created.  This is used by
    #: :meth:`add_url_rule`.  Defaults to :class:`werkzeug.routing.Rule`.
    #:
    #: .. versionadded:: 0.7
    url_rule_class = Rule

    #: the test client that is used with when `test_client` is used.
    #:
    #: .. versionadded:: 0.7
    test_client_class = None

    #: the session interface to use.  By default an instance of
    #: :class:`~flask.sessions.SecureCookieSessionInterface` is used here.
    #:
    #: .. versionadded:: 0.8
    session_interface = SecureCookieSessionInterface()

    def __init__(self, import_name, static_path=None, static_url_path=None,
                 static_folder='static', static_host=None,
                 host_matching=False, template_folder='templates',
                 instance_path=None, instance_relative_config=False,
                 root_path=None):
        _PackageBoundObject.__init__(self, import_name,
                                     template_folder=template_folder,
                                     root_path=root_path)
        if static_path is not None:
            from warnings import warn
            warn(DeprecationWarning('static_path is now called '
                                    'static_url_path'), stacklevel=2)
            static_url_path = static_path

        if static_url_path is not None:
            self.static_url_path = static_url_path
        if static_folder is not None:
            self.static_folder = static_folder
        if instance_path is None:
            instance_path = self.auto_find_instance_path()
        elif not os.path.isabs(instance_path):
            raise ValueError('If an instance path is provided it must be '
                             'absolute.  A relative path was given instead.')

        #: Holds the path to the instance folder.
        #:
        #: .. versionadded:: 0.8
        self.instance_path = instance_path

        #: The configuration dictionary as :class:`Config`.  This behaves
        #: exactly like a regular dictionary but supports additional methods
        #: to load a config from files.
        self.config = self.make_config(instance_relative_config)

        # Prepare the deferred setup of the logger.
        self._logger = None
        self.logger_name = self.import_name

        #: A dictionary of all view functions registered.  The keys will
        #: be function names which are also used to generate URLs and
        #: the values are the function objects themselves.
        #: To register a view function, use the :meth:`route` decorator.
        self.view_functions = {}

        # support for the now deprecated `error_handlers` attribute.  The
        # :attr:`error_handler_spec` shall be used now.
        self._error_handlers = {}

        #: A dictionary of all registered error handlers.  The key is ``None``
        #: for error handlers active on the application, otherwise the key is
        #: the name of the blueprint.  Each key points to another dictionary
        #: where the key is the status code of the http exception.  The
        #: special key ``None`` points to a list of tuples where the first item
        #: is the class for the instance check and the second the error handler
        #: function.
        #:
        #: To register an error handler, use the :meth:`errorhandler`
        #: decorator.
        self.error_handler_spec = {None: self._error_handlers}

        #: A list of functions that are called when :meth:`url_for` raises a
        #: :exc:`~werkzeug.routing.BuildError`.  Each function registered here
        #: is called with `error`, `endpoint` and `values`.  If a function
        #: returns ``None`` or raises a :exc:`BuildError` the next function is
        #: tried.
        #:
        #: .. versionadded:: 0.9
        self.url_build_error_handlers = []

        #: A dictionary with lists of functions that will be called at the
        #: beginning of each request. The key of the dictionary is the name of
        #: the blueprint this function is active for, or ``None`` for all
        #: requests. To register a function, use the :meth:`before_request`
        #: decorator.
        self.before_request_funcs = {}

        #: A list of functions that will be called at the beginning of the
        #: first request to this instance. To register a function, use the
        #: :meth:`before_first_request` decorator.
        #:
        #: .. versionadded:: 0.8
        self.before_first_request_funcs = []

        #: A dictionary with lists of functions that should be called after
        #: each request.  The key of the dictionary is the name of the blueprint
        #: this function is active for, ``None`` for all requests.  This can for
        #: example be used to close database connections. To register a function
        #: here, use the :meth:`after_request` decorator.
        self.after_request_funcs = {}

        #: A dictionary with lists of functions that are called after
        #: each request, even if an exception has occurred. The key of the
        #: dictionary is the name of the blueprint this function is active for,
        #: ``None`` for all requests. These functions are not allowed to modify
        #: the request, and their return values are ignored. If an exception
        #: occurred while processing the request, it gets passed to each
        #: teardown_request function. To register a function here, use the
        #: :meth:`teardown_request` decorator.
        #:
        #: .. versionadded:: 0.7
        self.teardown_request_funcs = {}

        #: A list of functions that are called when the application context
        #: is destroyed.  Since the application context is also torn down
        #: if the request ends this is the place to store code that disconnects
        #: from databases.
        #:
        #: .. versionadded:: 0.9
        self.teardown_appcontext_funcs = []

        #: A dictionary with lists of functions that are called before the
        #: :attr:`before_request_funcs` functions. The key of the dictionary is
        #: the name of the blueprint this function is active for, or ``None``
        #: for all requests. To register a function, use
        #: :meth:`url_value_preprocessor`.
        #:
        #: .. versionadded:: 0.7
        self.url_value_preprocessors = {}

        #: A dictionary with lists of functions that can be used as URL value
        #: preprocessors.  The key ``None`` here is used for application wide
        #: callbacks, otherwise the key is the name of the blueprint.
        #: Each of these functions has the chance to modify the dictionary
        #: of URL values before they are used as the keyword arguments of the
        #: view function.  For each function registered this one should also
        #: provide a :meth:`url_defaults` function that adds the parameters
        #: automatically again that were removed that way.
        #:
        #: .. versionadded:: 0.7
        self.url_default_functions = {}

        #: A dictionary with list of functions that are called without argument
        #: to populate the template context.  The key of the dictionary is the
        #: name of the blueprint this function is active for, ``None`` for all
        #: requests.  Each returns a dictionary that the template context is
        #: updated with.  To register a function here, use the
        #: :meth:`context_processor` decorator.
        self.template_context_processors = {
            None: [_default_template_ctx_processor]
        }

        #: A list of shell context processor functions that should be run
        #: when a shell context is created.
        #:
        #: .. versionadded:: 0.11
        self.shell_context_processors = []

        #: all the attached blueprints in a dictionary by name.  Blueprints
        #: can be attached multiple times so this dictionary does not tell
        #: you how often they got attached.
        #:
        #: .. versionadded:: 0.7
        self.blueprints = {}
        self._blueprint_order = []

        #: a place where extensions can store application specific state.  For
        #: example this is where an extension could store database engines and
        #: similar things.  For backwards compatibility extensions should register
        #: themselves like this::
        #:
        #:      if not hasattr(app, 'extensions'):
        #:          app.extensions = {}
        #:      app.extensions['extensionname'] = SomeObject()
        #:
        #: The key must match the name of the extension module. For example in
        #: case of a "Flask-Foo" extension in `flask_foo`, the key would be
        #: ``'foo'``.
        #:
        #: .. versionadded:: 0.7
        self.extensions = {}

        #: The :class:`~werkzeug.routing.Map` for this instance.  You can use
        #: this to change the routing converters after the class was created
        #: but before any routes are connected.  Example::
        #:
        #:    from werkzeug.routing import BaseConverter
        #:
        #:    class ListConverter(BaseConverter):
        #:        def to_python(self, value):
        #:            return value.split(',')
        #:        def to_url(self, values):
        #:            return ','.join(super(ListConverter, self).to_url(value)
        #:                            for value in values)
        #:
        #:    app = Flask(__name__)
        #:    app.url_map.converters['list'] = ListConverter
        self.url_map = Map()

        self.url_map.host_matching = host_matching

        # tracks internally if the application already handled at least one
        # request.
        self._got_first_request = False
        self._before_request_lock = Lock()

        # Add a static route using the provided static_url_path, static_host,
        # and static_folder if there is a configured static_folder.
        # Note we do this without checking if static_folder exists.
        # For one, it might be created while the server is running (e.g. during
        # development). Also, Google App Engine stores static files somewhere
        if self.has_static_folder:
            assert bool(static_host) == host_matching, 'Invalid static_host/host_matching combination'
            self.add_url_rule(self.static_url_path + '/<path:filename>',
                              endpoint='static', host=static_host,
                              view_func=self.send_static_file)

        #: The click command line context for this application.  Commands
        #: registered here show up in the :command:`flask` command once the
        #: application has been discovered.  The default commands are
        #: provided by Flask itself and can be overridden.
        #:
        #: This is an instance of a :class:`click.Group` object.
        self.cli = cli.AppGroup(self.name)

    def _get_error_handlers(self):
        from warnings import warn
        warn(DeprecationWarning('error_handlers is deprecated, use the '
            'new error_handler_spec attribute instead.'), stacklevel=1)
        return self._error_handlers
    def _set_error_handlers(self, value):
        self._error_handlers = value
        self.error_handler_spec[None] = value
    error_handlers = property(_get_error_handlers, _set_error_handlers)
    del _get_error_handlers, _set_error_handlers

    @locked_cached_property
    def name(self):
        """The name of the application.  This is usually the import name
        with the difference that it's guessed from the run file if the
        import name is main.  This name is used as a display name when
        Flask needs the name of the application.  It can be set and overridden
        to change the value.

        .. versionadded:: 0.8
        """
        if self.import_name == '__main__':
            fn = getattr(sys.modules['__main__'], '__file__', None)
            if fn is None:
                return '__main__'
            return os.path.splitext(os.path.basename(fn))[0]
        return self.import_name

    @property
    def propagate_exceptions(self):
        """Returns the value of the ``PROPAGATE_EXCEPTIONS`` configuration
        value in case it's set, otherwise a sensible default is returned.

        .. versionadded:: 0.7
        """
        rv = self.config['PROPAGATE_EXCEPTIONS']
        if rv is not None:
            return rv
        return self.testing or self.debug

    @property
    def preserve_context_on_exception(self):
        """Returns the value of the ``PRESERVE_CONTEXT_ON_EXCEPTION``
        configuration value in case it's set, otherwise a sensible default
        is returned.

        .. versionadded:: 0.7
        """
        rv = self.config['PRESERVE_CONTEXT_ON_EXCEPTION']
        if rv is not None:
            return rv
        return self.debug

    @property
    def logger(self):
        """A :class:`logging.Logger` object for this application.  The
        default configuration is to log to stderr if the application is
        in debug mode.  This logger can be used to (surprise) log messages.
        Here some examples::

            app.logger.debug('A value for debugging')
            app.logger.warning('A warning occurred (%d apples)', 42)
            app.logger.error('An error occurred')

        .. versionadded:: 0.3
        """
        if self._logger and self._logger.name == self.logger_name:
            return self._logger
        with _logger_lock:
            if self._logger and self._logger.name == self.logger_name:
                return self._logger
            from flask.logging import create_logger
            self._logger = rv = create_logger(self)
            return rv

    @locked_cached_property
    def jinja_env(self):
        """The Jinja2 environment used to load templates."""
        return self.create_jinja_environment()

    @property
    def got_first_request(self):
        """This attribute is set to ``True`` if the application started
        handling the first request.

        .. versionadded:: 0.8
        """
        return self._got_first_request

    def make_config(self, instance_relative=False):
        """Used to create the config attribute by the Flask constructor.
        The `instance_relative` parameter is passed in from the constructor
        of Flask (there named `instance_relative_config`) and indicates if
        the config should be relative to the instance path or the root path
        of the application.

        .. versionadded:: 0.8
        """
        root_path = self.root_path
        if instance_relative:
            root_path = self.instance_path
        return self.config_class(root_path, self.default_config)

    def auto_find_instance_path(self):
        """Tries to locate the instance path if it was not provided to the
        constructor of the application class.  It will basically calculate
        the path to a folder named ``instance`` next to your main file or
        the package.

        .. versionadded:: 0.8
        """
        prefix, package_path = find_package(self.import_name)
        if prefix is None:
            return os.path.join(package_path, 'instance')
        return os.path.join(prefix, 'var', self.name + '-instance')

    def open_instance_resource(self, resource, mode='rb'):
        """Opens a resource from the application's instance folder
        (:attr:`instance_path`).  Otherwise works like
        :meth:`open_resource`.  Instance resources can also be opened for
        writing.

        :param resource: the name of the resource.  To access resources within
                         subfolders use forward slashes as separator.
        :param mode: resource file opening mode, default is 'rb'.
        """
        return open(os.path.join(self.instance_path, resource), mode)

    def create_jinja_environment(self):
        """Creates the Jinja2 environment based on :attr:`jinja_options`
        and :meth:`select_jinja_autoescape`.  Since 0.7 this also adds
        the Jinja2 globals and filters after initialization.  Override
        this function to customize the behavior.

        .. versionadded:: 0.5
        .. versionchanged:: 0.11
           ``Environment.auto_reload`` set in accordance with
           ``TEMPLATES_AUTO_RELOAD`` configuration option.
        """
        options = dict(self.jinja_options)
        if 'autoescape' not in options:
            options['autoescape'] = self.select_jinja_autoescape
        if 'auto_reload' not in options:
            if self.config['TEMPLATES_AUTO_RELOAD'] is not None:
                options['auto_reload'] = self.config['TEMPLATES_AUTO_RELOAD']
            else:
                options['auto_reload'] = self.debug
        rv = self.jinja_environment(self, **options)
        rv.globals.update(
            url_for=url_for,
            get_flashed_messages=get_flashed_messages,
            config=self.config,
            # request, session and g are normally added with the
            # context processor for efficiency reasons but for imported
            # templates we also want the proxies in there.
            request=request,
            session=session,
            g=g
        )
        rv.filters['tojson'] = json.tojson_filter
        return rv

    def create_global_jinja_loader(self):
        """Creates the loader for the Jinja2 environment.  Can be used to
        override just the loader and keeping the rest unchanged.  It's
        discouraged to override this function.  Instead one should override
        the :meth:`jinja_loader` function instead.

        The global loader dispatches between the loaders of the application
        and the individual blueprints.

        .. versionadded:: 0.7
        """
        return DispatchingJinjaLoader(self)

    def init_jinja_globals(self):
        """Deprecated.  Used to initialize the Jinja2 globals.

        .. versionadded:: 0.5
        .. versionchanged:: 0.7
           This method is deprecated with 0.7.  Override
           :meth:`create_jinja_environment` instead.
        """

    def select_jinja_autoescape(self, filename):
        """Returns ``True`` if autoescaping should be active for the given
        template name. If no template name is given, returns `True`.

        .. versionadded:: 0.5
        """
        if filename is None:
            return True
        return filename.endswith(('.html', '.htm', '.xml', '.xhtml'))

    def update_template_context(self, context):
        """Update the template context with some commonly used variables.
        This injects request, session, config and g into the template
        context as well as everything template context processors want
        to inject.  Note that the as of Flask 0.6, the original values
        in the context will not be overridden if a context processor
        decides to return a value with the same key.

        :param context: the context as a dictionary that is updated in place
                        to add extra variables.
        """
        funcs = self.template_context_processors[None]
        reqctx = _request_ctx_stack.top
        if reqctx is not None:
            bp = reqctx.request.blueprint
            if bp is not None and bp in self.template_context_processors:
                funcs = chain(funcs, self.template_context_processors[bp])
        orig_ctx = context.copy()
        for func in funcs:
            context.update(func())
        # make sure the original values win.  This makes it possible to
        # easier add new variables in context processors without breaking
        # existing views.
        context.update(orig_ctx)

    def make_shell_context(self):
        """Returns the shell context for an interactive shell for this
        application.  This runs all the registered shell context
        processors.

        .. versionadded:: 0.11
        """
        rv = {'app': self, 'g': g}
        for processor in self.shell_context_processors:
            rv.update(processor())
        return rv

    def run(self, host=None, port=None, debug=None, **options):
        """Runs the application on a local development server.

        Do not use ``run()`` in a production setting. It is not intended to
        meet security and performance requirements for a production server.
        Instead, see :ref:`deployment` for WSGI server recommendations.

        If the :attr:`debug` flag is set the server will automatically reload
        for code changes and show a debugger in case an exception happened.

        If you want to run the application in debug mode, but disable the
        code execution on the interactive debugger, you can pass
        ``use_evalex=False`` as parameter.  This will keep the debugger's
        traceback screen active, but disable code execution.

        It is not recommended to use this function for development with
        automatic reloading as this is badly supported.  Instead you should
        be using the :command:`flask` command line script's ``run`` support.

        .. admonition:: Keep in Mind

           Flask will suppress any server error with a generic error page
           unless it is in debug mode.  As such to enable just the
           interactive debugger without the code reloading, you have to
           invoke :meth:`run` with ``debug=True`` and ``use_reloader=False``.
           Setting ``use_debugger`` to ``True`` without being in debug mode
           won't catch any exceptions because there won't be any to
           catch.

        .. versionchanged:: 0.10
           The default port is now picked from the ``SERVER_NAME`` variable.

        :param host: the hostname to listen on. Set this to ``'0.0.0.0'`` to
                     have the server available externally as well. Defaults to
                     ``'127.0.0.1'`` or the host in the ``SERVER_NAME`` config
                     variable if present.
        :param port: the port of the webserver. Defaults to ``5000`` or the
                     port defined in the ``SERVER_NAME`` config variable if
                     present.
        :param debug: if given, enable or disable debug mode.
                      See :attr:`debug`.
        :param options: the options to be forwarded to the underlying
                        Werkzeug server.  See
                        :func:`werkzeug.serving.run_simple` for more
                        information.
        """
        # Change this into a no-op if the server is invoked from the
        # command line.  Have a look at cli.py for more information.
        if os.environ.get('FLASK_RUN_FROM_CLI_SERVER') == '1':
            from .debughelpers import explain_ignored_app_run
            explain_ignored_app_run()
            return

        from werkzeug.serving import run_simple
        _host = '127.0.0.1'
        _port = 5000
        server_name = self.config.get("SERVER_NAME")
        sn_host, sn_port = None, None
        if server_name:
            sn_host, _, sn_port = server_name.partition(':')
        host = host or sn_host or _host
        port = int(port or sn_port or _port)
        if debug is not None:
            self.debug = bool(debug)
        options.setdefault('use_reloader', self.debug)
        options.setdefault('use_debugger', self.debug)
        try:
            run_simple(host, port, self, **options)
        finally:
            # reset the first request information if the development server
            # reset normally.  This makes it possible to restart the server
            # without reloader and that stuff from an interactive shell.
            self._got_first_request = False

    def test_client(self, use_cookies=True, **kwargs):
        """Creates a test client for this application.  For information
        about unit testing head over to :ref:`testing`.

        Note that if you are testing for assertions or exceptions in your
        application code, you must set ``app.testing = True`` in order for the
        exceptions to propagate to the test client.  Otherwise, the exception
        will be handled by the application (not visible to the test client) and
        the only indication of an AssertionError or other exception will be a
        500 status code response to the test client.  See the :attr:`testing`
        attribute.  For example::

            app.testing = True
            client = app.test_client()

        The test client can be used in a ``with`` block to defer the closing down
        of the context until the end of the ``with`` block.  This is useful if
        you want to access the context locals for testing::

            with app.test_client() as c:
                rv = c.get('/?vodka=42')
                assert request.args['vodka'] == '42'

        Additionally, you may pass optional keyword arguments that will then
        be passed to the application's :attr:`test_client_class` constructor.
        For example::

            from flask.testing import FlaskClient

            class CustomClient(FlaskClient):
                def __init__(self, *args, **kwargs):
                    self._authentication = kwargs.pop("authentication")
                    super(CustomClient,self).__init__( *args, **kwargs)

            app.test_client_class = CustomClient
            client = app.test_client(authentication='Basic ....')

        See :class:`~flask.testing.FlaskClient` for more information.

        .. versionchanged:: 0.4
           added support for ``with`` block usage for the client.

        .. versionadded:: 0.7
           The `use_cookies` parameter was added as well as the ability
           to override the client to be used by setting the
           :attr:`test_client_class` attribute.

        .. versionchanged:: 0.11
           Added `**kwargs` to support passing additional keyword arguments to
           the constructor of :attr:`test_client_class`.
        """
        cls = self.test_client_class
        if cls is None:
            from flask.testing import FlaskClient as cls
        return cls(self, self.response_class, use_cookies=use_cookies, **kwargs)

    def open_session(self, request):
        """Creates or opens a new session.  Default implementation stores all
        session data in a signed cookie.  This requires that the
        :attr:`secret_key` is set.  Instead of overriding this method
        we recommend replacing the :class:`session_interface`.

        .. deprecated: 1.0
            Will be removed in 1.1. Use ``session_interface.open_session``
            instead.

        :param request: an instance of :attr:`request_class`.
        """

        warnings.warn(DeprecationWarning(
            '"open_session" is deprecated and will be removed in 1.1. Use'
            ' "session_interface.open_session" instead.'
        ))
        return self.session_interface.open_session(self, request)

    def save_session(self, session, response):
        """Saves the session if it needs updates.  For the default
        implementation, check :meth:`open_session`.  Instead of overriding this
        method we recommend replacing the :class:`session_interface`.

        .. deprecated: 1.0
            Will be removed in 1.1. Use ``session_interface.save_session``
            instead.

        :param session: the session to be saved (a
                        :class:`~werkzeug.contrib.securecookie.SecureCookie`
                        object)
        :param response: an instance of :attr:`response_class`
        """

        warnings.warn(DeprecationWarning(
            '"save_session" is deprecated and will be removed in 1.1. Use'
            ' "session_interface.save_session" instead.'
        ))
        return self.session_interface.save_session(self, session, response)

    def make_null_session(self):
        """Creates a new instance of a missing session.  Instead of overriding
        this method we recommend replacing the :class:`session_interface`.

        .. deprecated: 1.0
            Will be removed in 1.1. Use ``session_interface.make_null_session``
            instead.

        .. versionadded:: 0.7
        """

        warnings.warn(DeprecationWarning(
            '"make_null_session" is deprecated and will be removed in 1.1. Use'
            ' "session_interface.make_null_session" instead.'
        ))
        return self.session_interface.make_null_session(self)

    @setupmethod
    def register_blueprint(self, blueprint, **options):
        """Registers a blueprint on the application.

        .. versionadded:: 0.7
        """
        first_registration = False
        if blueprint.name in self.blueprints:
            assert self.blueprints[blueprint.name] is blueprint, \
                'A blueprint\'s name collision occurred between %r and ' \
                '%r.  Both share the same name "%s".  Blueprints that ' \
                'are created on the fly need unique names.' % \
                (blueprint, self.blueprints[blueprint.name], blueprint.name)
        else:
            self.blueprints[blueprint.name] = blueprint
            self._blueprint_order.append(blueprint)
            first_registration = True
        blueprint.register(self, options, first_registration)

    def iter_blueprints(self):
        """Iterates over all blueprints by the order they were registered.

        .. versionadded:: 0.11
        """
        return iter(self._blueprint_order)

    @setupmethod
    def add_url_rule(self, rule, endpoint=None, view_func=None, provide_automatic_options=None, **options):
        """Connects a URL rule.  Works exactly like the :meth:`route`
        decorator.  If a view_func is provided it will be registered with the
        endpoint.

        Basically this example::

            @app.route('/')
            def index():
                pass

        Is equivalent to the following::

            def index():
                pass
            app.add_url_rule('/', 'index', index)

        If the view_func is not provided you will need to connect the endpoint
        to a view function like so::

            app.view_functions['index'] = index

        Internally :meth:`route` invokes :meth:`add_url_rule` so if you want
        to customize the behavior via subclassing you only need to change
        this method.

        For more information refer to :ref:`url-route-registrations`.

        .. versionchanged:: 0.2
           `view_func` parameter added.

        .. versionchanged:: 0.6
           ``OPTIONS`` is added automatically as method.

        :param rule: the URL rule as string
        :param endpoint: the endpoint for the registered URL rule.  Flask
                         itself assumes the name of the view function as
                         endpoint
        :param view_func: the function to call when serving a request to the
                          provided endpoint
        :param provide_automatic_options: controls whether the ``OPTIONS``
            method should be added automatically. This can also be controlled
            by setting the ``view_func.provide_automatic_options = False``
            before adding the rule.
        :param options: the options to be forwarded to the underlying
                        :class:`~werkzeug.routing.Rule` object.  A change
                        to Werkzeug is handling of method options.  methods
                        is a list of methods this rule should be limited
                        to (``GET``, ``POST`` etc.).  By default a rule
                        just listens for ``GET`` (and implicitly ``HEAD``).
                        Starting with Flask 0.6, ``OPTIONS`` is implicitly
                        added and handled by the standard request handling.
        """
        if endpoint is None:
            endpoint = _endpoint_from_view_func(view_func)
        options['endpoint'] = endpoint
        methods = options.pop('methods', None)

        # if the methods are not given and the view_func object knows its
        # methods we can use that instead.  If neither exists, we go with
        # a tuple of only ``GET`` as default.
        if methods is None:
            methods = getattr(view_func, 'methods', None) or ('GET',)
        if isinstance(methods, string_types):
            raise TypeError('Allowed methods have to be iterables of strings, '
                            'for example: @app.route(..., methods=["POST"])')
        methods = set(item.upper() for item in methods)

        # Methods that should always be added
        required_methods = set(getattr(view_func, 'required_methods', ()))

        # starting with Flask 0.8 the view_func object can disable and
        # force-enable the automatic options handling.
        if provide_automatic_options is None:
            provide_automatic_options = getattr(view_func,
                'provide_automatic_options', None)

        if provide_automatic_options is None:
            if 'OPTIONS' not in methods:
                provide_automatic_options = True
                required_methods.add('OPTIONS')
            else:
                provide_automatic_options = False

        # Add the required methods now.
        methods |= required_methods

        rule = self.url_rule_class(rule, methods=methods, **options)
        rule.provide_automatic_options = provide_automatic_options

        self.url_map.add(rule)
        if view_func is not None:
            old_func = self.view_functions.get(endpoint)
            if old_func is not None and old_func != view_func:
                raise AssertionError('View function mapping is overwriting an '
                                     'existing endpoint function: %s' % endpoint)
            self.view_functions[endpoint] = view_func

    def route(self, rule, **options):
        """A decorator that is used to register a view function for a
        given URL rule.  This does the same thing as :meth:`add_url_rule`
        but is intended for decorator usage::

            @app.route('/')
            def index():
                return 'Hello World'

        For more information refer to :ref:`url-route-registrations`.

        :param rule: the URL rule as string
        :param endpoint: the endpoint for the registered URL rule.  Flask
                         itself assumes the name of the view function as
                         endpoint
        :param options: the options to be forwarded to the underlying
                        :class:`~werkzeug.routing.Rule` object.  A change
                        to Werkzeug is handling of method options.  methods
                        is a list of methods this rule should be limited
                        to (``GET``, ``POST`` etc.).  By default a rule
                        just listens for ``GET`` (and implicitly ``HEAD``).
                        Starting with Flask 0.6, ``OPTIONS`` is implicitly
                        added and handled by the standard request handling.
        """
        def decorator(f):
            endpoint = options.pop('endpoint', None)
            self.add_url_rule(rule, endpoint, f, **options)
            return f
        return decorator

    @setupmethod
    def endpoint(self, endpoint):
        """A decorator to register a function as an endpoint.
        Example::

            @app.endpoint('example.endpoint')
            def example():
                return "example"

        :param endpoint: the name of the endpoint
        """
        def decorator(f):
            self.view_functions[endpoint] = f
            return f
        return decorator

    @staticmethod
    def _get_exc_class_and_code(exc_class_or_code):
        """Ensure that we register only exceptions as handler keys"""
        if isinstance(exc_class_or_code, integer_types):
            exc_class = default_exceptions[exc_class_or_code]
        else:
            exc_class = exc_class_or_code

        assert issubclass(exc_class, Exception)

        if issubclass(exc_class, HTTPException):
            return exc_class, exc_class.code
        else:
            return exc_class, None

    @setupmethod
    def errorhandler(self, code_or_exception):
        """Register a function to handle errors by code or exception class.

        A decorator that is used to register a function given an
        error code.  Example::

            @app.errorhandler(404)
            def page_not_found(error):
                return 'This page does not exist', 404

        You can also register handlers for arbitrary exceptions::

            @app.errorhandler(DatabaseError)
            def special_exception_handler(error):
                return 'Database connection failed', 500

        .. versionadded:: 0.7
            Use :meth:`register_error_handler` instead of modifying
            :attr:`error_handler_spec` directly, for application wide error
            handlers.

        .. versionadded:: 0.7
           One can now additionally also register custom exception types
           that do not necessarily have to be a subclass of the
           :class:`~werkzeug.exceptions.HTTPException` class.

        :param code_or_exception: the code as integer for the handler, or
                                  an arbitrary exception
        """
        def decorator(f):
            self._register_error_handler(None, code_or_exception, f)
            return f
        return decorator

    @setupmethod
    def register_error_handler(self, code_or_exception, f):
        """Alternative error attach function to the :meth:`errorhandler`
        decorator that is more straightforward to use for non decorator
        usage.

        .. versionadded:: 0.7
        """
        self._register_error_handler(None, code_or_exception, f)

    @setupmethod
    def _register_error_handler(self, key, code_or_exception, f):
        """
        :type key: None|str
        :type code_or_exception: int|T<=Exception
        :type f: callable
        """
        if isinstance(code_or_exception, HTTPException):  # old broken behavior
            raise ValueError(
                'Tried to register a handler for an exception instance {0!r}.'
                ' Handlers can only be registered for exception classes or'
                ' HTTP error codes.'.format(code_or_exception)
            )

        try:
            exc_class, code = self._get_exc_class_and_code(code_or_exception)
        except KeyError:
            raise KeyError(
                "'{0}' is not a recognized HTTP error code. Use a subclass of"
                " HTTPException with that code instead.".format(code_or_exception)
            )

        handlers = self.error_handler_spec.setdefault(key, {}).setdefault(code, {})
        handlers[exc_class] = f

    @setupmethod
    def template_filter(self, name=None):
        """A decorator that is used to register custom template filter.
        You can specify a name for the filter, otherwise the function
        name will be used. Example::

          @app.template_filter()
          def reverse(s):
              return s[::-1]

        :param name: the optional name of the filter, otherwise the
                     function name will be used.
        """
        def decorator(f):
            self.add_template_filter(f, name=name)
            return f
        return decorator

    @setupmethod
    def add_template_filter(self, f, name=None):
        """Register a custom template filter.  Works exactly like the
        :meth:`template_filter` decorator.

        :param name: the optional name of the filter, otherwise the
                     function name will be used.
        """
        self.jinja_env.filters[name or f.__name__] = f

    @setupmethod
    def template_test(self, name=None):
        """A decorator that is used to register custom template test.
        You can specify a name for the test, otherwise the function
        name will be used. Example::

          @app.template_test()
          def is_prime(n):
              if n == 2:
                  return True
              for i in range(2, int(math.ceil(math.sqrt(n))) + 1):
                  if n % i == 0:
                      return False
              return True

        .. versionadded:: 0.10

        :param name: the optional name of the test, otherwise the
                     function name will be used.
        """
        def decorator(f):
            self.add_template_test(f, name=name)
            return f
        return decorator

    @setupmethod
    def add_template_test(self, f, name=None):
        """Register a custom template test.  Works exactly like the
        :meth:`template_test` decorator.

        .. versionadded:: 0.10

        :param name: the optional name of the test, otherwise the
                     function name will be used.
        """
        self.jinja_env.tests[name or f.__name__] = f

    @setupmethod
    def template_global(self, name=None):
        """A decorator that is used to register a custom template global function.
        You can specify a name for the global function, otherwise the function
        name will be used. Example::

            @app.template_global()
            def double(n):
                return 2 * n

        .. versionadded:: 0.10

        :param name: the optional name of the global function, otherwise the
                     function name will be used.
        """
        def decorator(f):
            self.add_template_global(f, name=name)
            return f
        return decorator

    @setupmethod
    def add_template_global(self, f, name=None):
        """Register a custom template global function. Works exactly like the
        :meth:`template_global` decorator.

        .. versionadded:: 0.10

        :param name: the optional name of the global function, otherwise the
                     function name will be used.
        """
        self.jinja_env.globals[name or f.__name__] = f

    @setupmethod
    def before_request(self, f):
        """Registers a function to run before each request.

        For example, this can be used to open a database connection, or to load
        the logged in user from the session.

        The function will be called without any arguments. If it returns a
        non-None value, the value is handled as if it was the return value from
        the view, and further request handling is stopped.
        """
        self.before_request_funcs.setdefault(None, []).append(f)
        return f

    @setupmethod
    def before_first_request(self, f):
        """Registers a function to be run before the first request to this
        instance of the application.

        The function will be called without any arguments and its return
        value is ignored.

        .. versionadded:: 0.8
        """
        self.before_first_request_funcs.append(f)
        return f

    @setupmethod
    def after_request(self, f):
        """Register a function to be run after each request.

        Your function must take one parameter, an instance of
        :attr:`response_class` and return a new response object or the
        same (see :meth:`process_response`).

        As of Flask 0.7 this function might not be executed at the end of the
        request in case an unhandled exception occurred.
        """
        self.after_request_funcs.setdefault(None, []).append(f)
        return f

    @setupmethod
    def teardown_request(self, f):
        """Register a function to be run at the end of each request,
        regardless of whether there was an exception or not.  These functions
        are executed when the request context is popped, even if not an
        actual request was performed.

        Example::

            ctx = app.test_request_context()
            ctx.push()
            ...
            ctx.pop()

        When ``ctx.pop()`` is executed in the above example, the teardown
        functions are called just before the request context moves from the
        stack of active contexts.  This becomes relevant if you are using
        such constructs in tests.

        Generally teardown functions must take every necessary step to avoid
        that they will fail.  If they do execute code that might fail they
        will have to surround the execution of these code by try/except
        statements and log occurring errors.

        When a teardown function was called because of an exception it will
        be passed an error object.

        The return values of teardown functions are ignored.

        .. admonition:: Debug Note

           In debug mode Flask will not tear down a request on an exception
           immediately.  Instead it will keep it alive so that the interactive
           debugger can still access it.  This behavior can be controlled
           by the ``PRESERVE_CONTEXT_ON_EXCEPTION`` configuration variable.
        """
        self.teardown_request_funcs.setdefault(None, []).append(f)
        return f

    @setupmethod
    def teardown_appcontext(self, f):
        """Registers a function to be called when the application context
        ends.  These functions are typically also called when the request
        context is popped.

        Example::

            ctx = app.app_context()
            ctx.push()
            ...
            ctx.pop()

        When ``ctx.pop()`` is executed in the above example, the teardown
        functions are called just before the app context moves from the
        stack of active contexts.  This becomes relevant if you are using
        such constructs in tests.

        Since a request context typically also manages an application
        context it would also be called when you pop a request context.

        When a teardown function was called because of an exception it will
        be passed an error object.

        The return values of teardown functions are ignored.

        .. versionadded:: 0.9
        """
        self.teardown_appcontext_funcs.append(f)
        return f

    @setupmethod
    def context_processor(self, f):
        """Registers a template context processor function."""
        self.template_context_processors[None].append(f)
        return f

    @setupmethod
    def shell_context_processor(self, f):
        """Registers a shell context processor function.

        .. versionadded:: 0.11
        """
        self.shell_context_processors.append(f)
        return f

    @setupmethod
    def url_value_preprocessor(self, f):
        """Register a URL value preprocessor function for all view
        functions in the application. These functions will be called before the
        :meth:`before_request` functions.

        The function can modify the values captured from the matched url before
        they are passed to the view. For example, this can be used to pop a
        common language code value and place it in ``g`` rather than pass it to
        every view.

        The function is passed the endpoint name and values dict. The return
        value is ignored.
        """
        self.url_value_preprocessors.setdefault(None, []).append(f)
        return f

    @setupmethod
    def url_defaults(self, f):
        """Callback function for URL defaults for all view functions of the
        application.  It's called with the endpoint and values and should
        update the values passed in place.
        """
        self.url_default_functions.setdefault(None, []).append(f)
        return f

    def _find_error_handler(self, e):
        """Find a registered error handler for a request in this order:
        blueprint handler for a specific code, app handler for a specific code,
        blueprint generic HTTPException handler, app generic HTTPException handler,
        and returns None if a suitable handler is not found.
        """
        exc_class, code = self._get_exc_class_and_code(type(e))

        def find_handler(handler_map):
            if not handler_map:
                return

            for cls in exc_class.__mro__:
                handler = handler_map.get(cls)
                if handler is not None:
                    # cache for next time exc_class is raised
                    handler_map[exc_class] = handler
                    return handler

        # check for any in blueprint or app
        for name, c in ((request.blueprint, code), (None, code),
                        (request.blueprint, None), (None, None)):
            handler = find_handler(self.error_handler_spec.get(name, {}).get(c))

            if handler:
                return handler

    def handle_http_exception(self, e):
        """Handles an HTTP exception.  By default this will invoke the
        registered error handlers and fall back to returning the
        exception as response.

        .. versionadded:: 0.3
        """
        # Proxy exceptions don't have error codes.  We want to always return
        # those unchanged as errors
        if e.code is None:
            return e

        handler = self._find_error_handler(e)
        if handler is None:
            return e
        return handler(e)

    def trap_http_exception(self, e):
        """Checks if an HTTP exception should be trapped or not.  By default
        this will return ``False`` for all exceptions except for a bad request
        key error if ``TRAP_BAD_REQUEST_ERRORS`` is set to ``True``.  It
        also returns ``True`` if ``TRAP_HTTP_EXCEPTIONS`` is set to ``True``.

        This is called for all HTTP exceptions raised by a view function.
        If it returns ``True`` for any exception the error handler for this
        exception is not called and it shows up as regular exception in the
        traceback.  This is helpful for debugging implicitly raised HTTP
        exceptions.

        .. versionchanged:: 1.0
            Bad request errors are not trapped by default in debug mode.

        .. versionadded:: 0.8
        """
        if self.config['TRAP_HTTP_EXCEPTIONS']:
            return True

        trap_bad_request = self.config['TRAP_BAD_REQUEST_ERRORS']

        # if unset, trap based on debug mode
        if (trap_bad_request is None and self.debug) or trap_bad_request:
            return isinstance(e, BadRequest)

        return False

    def handle_user_exception(self, e):
        """This method is called whenever an exception occurs that should be
        handled.  A special case are
        :class:`~werkzeug.exception.HTTPException`\s which are forwarded by
        this function to the :meth:`handle_http_exception` method.  This
        function will either return a response value or reraise the
        exception with the same traceback.

        .. versionchanged:: 1.0
            Key errors raised from request data like ``form`` show the the bad
            key in debug mode rather than a generic bad request message.

        .. versionadded:: 0.7
        """
        exc_type, exc_value, tb = sys.exc_info()
        assert exc_value is e
        # ensure not to trash sys.exc_info() at that point in case someone
        # wants the traceback preserved in handle_http_exception.  Of course
        # we cannot prevent users from trashing it themselves in a custom
        # trap_http_exception method so that's their fault then.

        # MultiDict passes the key to the exception, but that's ignored
        # when generating the response message. Set an informative
        # description for key errors in debug mode or when trapping errors.
        if (
            (self.debug or self.config['TRAP_BAD_REQUEST_ERRORS'])
            and isinstance(e, BadRequestKeyError)
            # only set it if it's still the default description
            and e.description is BadRequestKeyError.description
        ):
            e.description = "KeyError: '{0}'".format(*e.args)

        if isinstance(e, HTTPException) and not self.trap_http_exception(e):
            return self.handle_http_exception(e)

        handler = self._find_error_handler(e)

        if handler is None:
            reraise(exc_type, exc_value, tb)
        return handler(e)

    def handle_exception(self, e):
        """Default exception handling that kicks in when an exception
        occurs that is not caught.  In debug mode the exception will
        be re-raised immediately, otherwise it is logged and the handler
        for a 500 internal server error is used.  If no such handler
        exists, a default 500 internal server error message is displayed.

        .. versionadded:: 0.3
        """
        exc_type, exc_value, tb = sys.exc_info()

        got_request_exception.send(self, exception=e)
        handler = self._find_error_handler(InternalServerError())

        if self.propagate_exceptions:
            # if we want to repropagate the exception, we can attempt to
            # raise it with the whole traceback in case we can do that
            # (the function was actually called from the except part)
            # otherwise, we just raise the error again
            if exc_value is e:
                reraise(exc_type, exc_value, tb)
            else:
                raise e

        self.log_exception((exc_type, exc_value, tb))
        if handler is None:
            return InternalServerError()
        return self.finalize_request(handler(e), from_error_handler=True)

    def log_exception(self, exc_info):
        """Logs an exception.  This is called by :meth:`handle_exception`
        if debugging is disabled and right before the handler is called.
        The default implementation logs the exception as error on the
        :attr:`logger`.

        .. versionadded:: 0.8
        """
        self.logger.error('Exception on %s [%s]' % (
            request.path,
            request.method
        ), exc_info=exc_info)

    def raise_routing_exception(self, request):
        """Exceptions that are recording during routing are reraised with
        this method.  During debug we are not reraising redirect requests
        for non ``GET``, ``HEAD``, or ``OPTIONS`` requests and we're raising
        a different error instead to help debug situations.

        :internal:
        """
        if not self.debug \
           or not isinstance(request.routing_exception, RequestRedirect) \
           or request.method in ('GET', 'HEAD', 'OPTIONS'):
            raise request.routing_exception

        from .debughelpers import FormDataRoutingRedirect
        raise FormDataRoutingRedirect(request)

    def dispatch_request(self):
        """Does the request dispatching.  Matches the URL and returns the
        return value of the view or error handler.  This does not have to
        be a response object.  In order to convert the return value to a
        proper response object, call :func:`make_response`.

        .. versionchanged:: 0.7
           This no longer does the exception handling, this code was
           moved to the new :meth:`full_dispatch_request`.
        """
        req = _request_ctx_stack.top.request
        if req.routing_exception is not None:
            self.raise_routing_exception(req)
        rule = req.url_rule
        # if we provide automatic options for this URL and the
        # request came with the OPTIONS method, reply automatically
        if getattr(rule, 'provide_automatic_options', False) \
           and req.method == 'OPTIONS':
            return self.make_default_options_response()
        # otherwise dispatch to the handler for that endpoint
        return self.view_functions[rule.endpoint](**req.view_args)

    def full_dispatch_request(self):
        """Dispatches the request and on top of that performs request
        pre and postprocessing as well as HTTP exception catching and
        error handling.

        .. versionadded:: 0.7
        """
        self.try_trigger_before_first_request_functions()
        try:
            request_started.send(self)
            rv = self.preprocess_request()
            if rv is None:
                rv = self.dispatch_request()
        except Exception as e:
            rv = self.handle_user_exception(e)
        return self.finalize_request(rv)

    def finalize_request(self, rv, from_error_handler=False):
        """Given the return value from a view function this finalizes
        the request by converting it into a response and invoking the
        postprocessing functions.  This is invoked for both normal
        request dispatching as well as error handlers.

        Because this means that it might be called as a result of a
        failure a special safe mode is available which can be enabled
        with the `from_error_handler` flag.  If enabled, failures in
        response processing will be logged and otherwise ignored.

        :internal:
        """
        response = self.make_response(rv)
        try:
            response = self.process_response(response)
            request_finished.send(self, response=response)
        except Exception:
            if not from_error_handler:
                raise
            self.logger.exception('Request finalizing failed with an '
                                  'error while handling an error')
        return response

    def try_trigger_before_first_request_functions(self):
        """Called before each request and will ensure that it triggers
        the :attr:`before_first_request_funcs` and only exactly once per
        application instance (which means process usually).

        :internal:
        """
        if self._got_first_request:
            return
        with self._before_request_lock:
            if self._got_first_request:
                return
            for func in self.before_first_request_funcs:
                func()
            self._got_first_request = True

    def make_default_options_response(self):
        """This method is called to create the default ``OPTIONS`` response.
        This can be changed through subclassing to change the default
        behavior of ``OPTIONS`` responses.

        .. versionadded:: 0.7
        """
        adapter = _request_ctx_stack.top.url_adapter
        if hasattr(adapter, 'allowed_methods'):
            methods = adapter.allowed_methods()
        else:
            # fallback for Werkzeug < 0.7
            methods = []
            try:
                adapter.match(method='--')
            except MethodNotAllowed as e:
                methods = e.valid_methods
            except HTTPException as e:
                pass
        rv = self.response_class()
        rv.allow.update(methods)
        return rv

    def should_ignore_error(self, error):
        """This is called to figure out if an error should be ignored
        or not as far as the teardown system is concerned.  If this
        function returns ``True`` then the teardown handlers will not be
        passed the error.

        .. versionadded:: 0.10
        """
        return False

    def make_response(self, rv):
        """Convert the return value from a view function to an instance of
        :attr:`response_class`.

        :param rv: the return value from the view function. The view function
            must return a response. Returning ``None``, or the view ending
            without returning, is not allowed. The following types are allowed
            for ``view_rv``:

            ``str`` (``unicode`` in Python 2)
                A response object is created with the string encoded to UTF-8
                as the body.

            ``bytes`` (``str`` in Python 2)
                A response object is created with the bytes as the body.

            ``tuple``
                Either ``(body, status, headers)``, ``(body, status)``, or
                ``(body, headers)``, where ``body`` is any of the other types
                allowed here, ``status`` is a string or an integer, and
                ``headers`` is a dictionary or a list of ``(key, value)``
                tuples. If ``body`` is a :attr:`response_class` instance,
                ``status`` overwrites the exiting value and ``headers`` are
                extended.

            :attr:`response_class`
                The object is returned unchanged.

            other :class:`~werkzeug.wrappers.Response` class
                The object is coerced to :attr:`response_class`.

            :func:`callable`
                The function is called as a WSGI application. The result is
                used to create a response object.

        .. versionchanged:: 0.9
           Previously a tuple was interpreted as the arguments for the
           response object.
        """

        status = headers = None

        # unpack tuple returns
        if isinstance(rv, (tuple, list)):
            len_rv = len(rv)

            # a 3-tuple is unpacked directly
            if len_rv == 3:
                rv, status, headers = rv
            # decide if a 2-tuple has status or headers
            elif len_rv == 2:
                if isinstance(rv[1], (Headers, dict, tuple, list)):
                    rv, headers = rv
                else:
                    rv, status = rv
            # other sized tuples are not allowed
            else:
                raise TypeError(
                    'The view function did not return a valid response tuple.'
                    ' The tuple must have the form (body, status, headers),'
                    ' (body, status), or (body, headers).'
                )

        # the body must not be None
        if rv is None:
            raise TypeError(
                'The view function did not return a valid response. The'
                ' function either returned None or ended without a return'
                ' statement.'
            )

        # make sure the body is an instance of the response class
        if not isinstance(rv, self.response_class):
            if isinstance(rv, (text_type, bytes, bytearray)):
                # let the response class set the status and headers instead of
                # waiting to do it manually, so that the class can handle any
                # special logic
                rv = self.response_class(rv, status=status, headers=headers)
                status = headers = None
            else:
                # evaluate a WSGI callable, or coerce a different response
                # class to the correct type
                try:
                    rv = self.response_class.force_type(rv, request.environ)
                except TypeError as e:
                    new_error = TypeError(
                        '{e}\nThe view function did not return a valid'
                        ' response. The return type must be a string, tuple,'
                        ' Response instance, or WSGI callable, but it was a'
                        ' {rv.__class__.__name__}.'.format(e=e, rv=rv)
                    )
                    reraise(TypeError, new_error, sys.exc_info()[2])

        # prefer the status if it was provided
        if status is not None:
            if isinstance(status, (text_type, bytes, bytearray)):
                rv.status = status
            else:
                rv.status_code = status

        # extend existing headers with provided headers
        if headers:
            rv.headers.extend(headers)

        return rv

    def create_url_adapter(self, request):
        """Creates a URL adapter for the given request.  The URL adapter
        is created at a point where the request context is not yet set up
        so the request is passed explicitly.

        .. versionadded:: 0.6

        .. versionchanged:: 0.9
           This can now also be called without a request object when the
           URL adapter is created for the application context.
        """
        if request is not None:
            return self.url_map.bind_to_environ(request.environ,
                server_name=self.config['SERVER_NAME'])
        # We need at the very least the server name to be set for this
        # to work.
        if self.config['SERVER_NAME'] is not None:
            return self.url_map.bind(
                self.config['SERVER_NAME'],
                script_name=self.config['APPLICATION_ROOT'],
                url_scheme=self.config['PREFERRED_URL_SCHEME'])

    def inject_url_defaults(self, endpoint, values):
        """Injects the URL defaults for the given endpoint directly into
        the values dictionary passed.  This is used internally and
        automatically called on URL building.

        .. versionadded:: 0.7
        """
        funcs = self.url_default_functions.get(None, ())
        if '.' in endpoint:
            bp = endpoint.rsplit('.', 1)[0]
            funcs = chain(funcs, self.url_default_functions.get(bp, ()))
        for func in funcs:
            func(endpoint, values)

    def handle_url_build_error(self, error, endpoint, values):
        """Handle :class:`~werkzeug.routing.BuildError` on :meth:`url_for`.
        """
        exc_type, exc_value, tb = sys.exc_info()
        for handler in self.url_build_error_handlers:
            try:
                rv = handler(error, endpoint, values)
                if rv is not None:
                    return rv
            except BuildError as e:
                # make error available outside except block (py3)
                error = e

        # At this point we want to reraise the exception.  If the error is
        # still the same one we can reraise it with the original traceback,
        # otherwise we raise it from here.
        if error is exc_value:
            reraise(exc_type, exc_value, tb)
        raise error

    def preprocess_request(self):
        """Called before the request is dispatched. Calls
        :attr:`url_value_preprocessors` registered with the app and the
        current blueprint (if any). Then calls :attr:`before_request_funcs`
        registered with the app and the blueprint.

        If any :meth:`before_request` handler returns a non-None value, the
        value is handled as if it was the return value from the view, and
        further request handling is stopped.
        """

        bp = _request_ctx_stack.top.request.blueprint

        funcs = self.url_value_preprocessors.get(None, ())
        if bp is not None and bp in self.url_value_preprocessors:
            funcs = chain(funcs, self.url_value_preprocessors[bp])
        for func in funcs:
            func(request.endpoint, request.view_args)

        funcs = self.before_request_funcs.get(None, ())
        if bp is not None and bp in self.before_request_funcs:
            funcs = chain(funcs, self.before_request_funcs[bp])
        for func in funcs:
            rv = func()
            if rv is not None:
                return rv

    def process_response(self, response):
        """Can be overridden in order to modify the response object
        before it's sent to the WSGI server.  By default this will
        call all the :meth:`after_request` decorated functions.

        .. versionchanged:: 0.5
           As of Flask 0.5 the functions registered for after request
           execution are called in reverse order of registration.

        :param response: a :attr:`response_class` object.
        :return: a new response object or the same, has to be an
                 instance of :attr:`response_class`.
        """
        ctx = _request_ctx_stack.top
        bp = ctx.request.blueprint
        funcs = ctx._after_request_functions
        if bp is not None and bp in self.after_request_funcs:
            funcs = chain(funcs, reversed(self.after_request_funcs[bp]))
        if None in self.after_request_funcs:
            funcs = chain(funcs, reversed(self.after_request_funcs[None]))
        for handler in funcs:
            response = handler(response)
        if not self.session_interface.is_null_session(ctx.session):
            self.session_interface.save_session(self, ctx.session, response)
        return response

    def do_teardown_request(self, exc=_sentinel):
        """Called after the actual request dispatching and will
        call every as :meth:`teardown_request` decorated function.  This is
        not actually called by the :class:`Flask` object itself but is always
        triggered when the request context is popped.  That way we have a
        tighter control over certain resources under testing environments.

        .. versionchanged:: 0.9
           Added the `exc` argument.  Previously this was always using the
           current exception information.
        """
        if exc is _sentinel:
            exc = sys.exc_info()[1]
        funcs = reversed(self.teardown_request_funcs.get(None, ()))
        bp = _request_ctx_stack.top.request.blueprint
        if bp is not None and bp in self.teardown_request_funcs:
            funcs = chain(funcs, reversed(self.teardown_request_funcs[bp]))
        for func in funcs:
            func(exc)
        request_tearing_down.send(self, exc=exc)

    def do_teardown_appcontext(self, exc=_sentinel):
        """Called when an application context is popped.  This works pretty
        much the same as :meth:`do_teardown_request` but for the application
        context.

        .. versionadded:: 0.9
        """
        if exc is _sentinel:
            exc = sys.exc_info()[1]
        for func in reversed(self.teardown_appcontext_funcs):
            func(exc)
        appcontext_tearing_down.send(self, exc=exc)

    def app_context(self):
        """Binds the application only.  For as long as the application is bound
        to the current context the :data:`flask.current_app` points to that
        application.  An application context is automatically created when a
        request context is pushed if necessary.

        Example usage::

            with app.app_context():
                ...

        .. versionadded:: 0.9
        """
        return AppContext(self)

    def request_context(self, environ):
        """Creates a :class:`~flask.ctx.RequestContext` from the given
        environment and binds it to the current context.  This must be used in
        combination with the ``with`` statement because the request is only bound
        to the current context for the duration of the ``with`` block.

        Example usage::

            with app.request_context(environ):
                do_something_with(request)

        The object returned can also be used without the ``with`` statement
        which is useful for working in the shell.  The example above is
        doing exactly the same as this code::

            ctx = app.request_context(environ)
            ctx.push()
            try:
                do_something_with(request)
            finally:
                ctx.pop()

        .. versionchanged:: 0.3
           Added support for non-with statement usage and ``with`` statement
           is now passed the ctx object.

        :param environ: a WSGI environment
        """
        return RequestContext(self, environ)

    def test_request_context(self, *args, **kwargs):
        """Creates a WSGI environment from the given values (see
        :class:`werkzeug.test.EnvironBuilder` for more information, this
        function accepts the same arguments plus two additional).

        Additional arguments (only if ``base_url`` is not specified):

        :param subdomain: subdomain to use for route matching
        :param url_scheme: scheme for the request, default
            ``PREFERRED_URL_SCHEME`` or ``http``.
        """

        from flask.testing import make_test_environ_builder

        builder = make_test_environ_builder(self, *args, **kwargs)

        try:
            return self.request_context(builder.get_environ())
        finally:
            builder.close()

    def wsgi_app(self, environ, start_response):
        """The actual WSGI application.  This is not implemented in
        `__call__` so that middlewares can be applied without losing a
        reference to the class.  So instead of doing this::

            app = MyMiddleware(app)

        It's a better idea to do this instead::

            app.wsgi_app = MyMiddleware(app.wsgi_app)

        Then you still have the original application object around and
        can continue to call methods on it.

        .. versionchanged:: 0.7
           The behavior of the before and after request callbacks was changed
           under error conditions and a new callback was added that will
           always execute at the end of the request, independent on if an
           error occurred or not.  See :ref:`callbacks-and-errors`.

        :param environ: a WSGI environment
        :param start_response: a callable accepting a status code,
                               a list of headers and an optional
                               exception context to start the response
        """
        ctx = self.request_context(environ)
        error = None
        try:
            try:
                ctx.push()
                response = self.full_dispatch_request()
            except Exception as e:
                error = e
                response = self.handle_exception(e)
            except:
                error = sys.exc_info()[1]
                raise
            return response(environ, start_response)
        finally:
            if self.should_ignore_error(error):
                error = None
            ctx.auto_pop(error)

    def __call__(self, environ, start_response):
        """Shortcut for :attr:`wsgi_app`."""
        return self.wsgi_app(environ, start_response)

    def __repr__(self):
        return '<%s %r>' % (
            self.__class__.__name__,
            self.name,
        )