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pythonPackages.sqlalchemy: Add Postgres support #64284

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pythonPackages.sqlalchemy: Add Postgres support #64284

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ruuda
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@ruuda ruuda commented Jul 4, 2019

This adds a propagated dependency on Psycopg2, which is needed for Postgres support in SQLAlchemy. The dependency can be turned off with a flag. (Although I don't know if it is customary to do this for Python packages, please tell me if there is a better way to do this.)

Motivation for this change

Psycopg2 in SQLAlchemy is needed to use Alembic with Postgres, as SQLAlchemy is a dependency of Alembic.

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/)
    I did verify that alemic can run against Postgres after this change, but I did not test any other binaries.
  • 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.

@ruuda
pythonPackages.sqlalchemy: Add Postgres support
6e1c75a 
This adds a propagated dependency on Psycopg2, which is needed for
Postgres support in SQLAlchemy. The dependency can be turned off with
a flag.

Psycopg2 in SQLAlchemy is in turn needed to use Alembic with Postgres.
@ruuda ruuda requested a review from FRidh as a code owner July 4, 2019 08:09
@FRidh
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FRidh commented Jul 4, 2019

Optional dependencies in libraries don't make sense. If you want additional support, you can simply add that package to your environment.

@FRidh FRidh closed this Jul 4, 2019
@ruuda
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ruuda commented Jul 4, 2019

Thanks for taking a look! I tried that at first, but the following does not work:

{ pkgs }:
  pkgs.buildEnv {
    name = "env-with-alembic";
    paths = [
      pkgs.python36
      pkgs.python36Packages.alembic
      pkgs.python36Packages.psycopg2
    ];
  }

If I enter that environment with nix run, or if I run

nix run -f channel:nixos-unstable pkgs.python36Packages.alembic pkgs.python36Packages.psycopg2

then I get

  File "/nix/store/c6hlkdfgax83053hfcq90h787ljkph7n-python3.6-SQLAlchemy-1.2.14/lib/python3.6/site-packages/sqlalchemy/dialects/postgresql/psycopg2.py", line 584, in dbapi
    import psycopg2
ModuleNotFoundError: No module named 'psycopg2'

when running Alembic, whereas with the change in this PR, I can run it just fine.

What would be the proper way to make the psycopg2 backend available then?

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FRidh commented Jul 4, 2019

Please check the python.withPackages function in the Nixpkgs manual.

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ruuda commented Jul 4, 2019

Ah, of course, thanks! Apologies for the noise.

@ruuda ruuda deleted the sqlalchemy-psycopg branch July 4, 2019 09:17
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