Motivation for this change

The actual code in the currently-packaged ncclient Python package, along with all its dependencies, support Python 3. But currently the way they're packaged breaks this:

$ nix-shell -p python3Packages.ncclient
error: libxslt not supported for interpreter python3.7

The problem is a broken dependency. The (interesting subset of the) actual code dependencies is like this (where P.foo means pythonPackages.foo):

                         +--> libxml2
                         |
P.ncclient --> P.lxml ---+
                         |
                         +--> libxslt

so the ultimate dependency is on the plain old C libraries for libxml2 and libxslt.

Unfortunately, the Python lxml derivation fails to propagate the dependency on these two C libraries, and so ncclient tries to add them back. But it actually depends on the Python package wrappers, like this:

       +--> P.libxml2 -->+-> libxml2
       |                 |
P.ncclient --> P.lxml ---+
       |                 |
       +--> P.libxslt -->+-> libxslt
                 ^
                 |
 Doesn't currently support Python 3

and since P.libxslt doesn't support Python 3, unnecessarily breaks the chain.

There is an issue to enable Python 3 support for P.libxslt. But simply fixing the dependencies seems a quick and correct fix for this case.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Test reproduction

If anyone wants to reproduce a test, then there is a simple standalone netconf server here. If you run that, and use this client.py code:

from ncclient import manager
with manager.connect(host='127.0.0.1', port=8300, username='admin', password='admin', hostkey_verify=False) as m:
  print(m.get_config(source='running').data_xml)

then before this fix, python3 fails as above. With the fix, both

nix-shell --pure -p python2 -p python2Packages.ncclient --command 'python2 client.py'
nix-shell --pure -p python3 -p python3Packages.ncclient --command 'python3 client.py'

give the correct output:

<?xml version="1.0" encoding="UTF-8"?><nc:data xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
    <sys:system xmlns:sys="urn:ietf:params:xml:ns:yang:ietf-system">
      <sys:hostname>ibex</sys:hostname>
      <sys:clock>
        <sys:timezone-utc-offset>0</sys:timezone-utc-offset>
      </sys:clock>
    </sys:system>
  </nc:data>

Notify maintainers

cc @sjourdois @xnaveira