How to use mypy with nixpkgs? (Cannot find implementation or library stub for module)

Help
1

I’m unable to get mypy to discover site packages in Python environments created with Nixpkgs. What am I missing here?

Given a simple example.py:

from pydantic import BaseModel

class User(BaseModel):
    id: int
    name = 'Jane Doe'

If I create a normal Python virtual env, mypy --strict example.py passes as expected:

$ nix-shell -p python3 --run "python -m venv mypyenv"
$ mypyenv/bin/pip install mypy pydantic
$ mypyenv/bin/mypy --strict example.py
Success: no issues found in 1 source file

But if I use a Nixpkgs’ Python environment, mypy is unable to find third party packages, here pydantic:

$ nix-shell -p "python3.withPackages(ps: with ps; [ mypy pydantic ])" --run "mypy --strict example.py"
example.py:1: error: Cannot find implementation or library stub for module named 'pydantic'
example.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
example.py:3: error: Class cannot subclass 'BaseModel' (has type 'Any')
Found 2 errors in 1 file (checked 1 source file)
2

What commit of nixpkgs are you on? I believe mypy was broken until
recently (Python: introduce NIX_PYTHONPREFIX in order to set site.PREFIXES by adisbladis · Pull Request #82453 · NixOS/nixpkgs · GitHub).

1 Like
3

Thanks! I was on NixOS 20.03 and completely unaware that mypy was known to be broken.

My example worked on the latest master:

$ nix-shell -I "nixpkgs=https://github.com/NixOS/nixpkgs/archive/4bf632f495a7bdd5fe46575b36d8e772ed804f8b.tar.gz" -p "python3.withPackages(ps: with ps; [ mypy pydantic ])" --run "mypy --strict example.py"
Success: no issues found in 1 source file