pre-commit not using virtual environment

Question

I added this to pre-commit-config.yml

-   repo: local
    hooks:
    -   id: python-hook
        name: python-hook
        entry: python python_script.py
        language: python
        pass_filenames: false

Where python_script.py has the following content

#!/usr/bin/env python

import os

print(os.environ["VIRTUAL_ENV"])

import requests

The package requests is installed inside the active virtual environment, when I run pre-commit I get this output

/path/to/home/.cache/pre-commit/repouecs3sp4/py_env-python3.7
Traceback (most recent call last):
  File "python_script.py", line 7, in <module>
    import requests
ModuleNotFoundError: No module named 'requests'

The issue here is that the path to the virtual environment is apparently switched to a different one /path/to/home/.cache/pre-commit/repouecs3sp4/py_env-python3.7.

How can I solve this?

Answer 1

kind of the point of pre-commit is it installs tools in isolated environments so they don't interfere with whatever state your current development environment is in -- language: python tells pre-commit to create its own python virtualenv

if you don't want that you can use language: system -- but this is the unsupported escape hatch for when you don't want managed tools (and note that pre-commit won't provision those environments for your contributors so they will need to make sure they have the particular environment set up and activated)

the better way is to declare the dependencies your local hook needs to pre-commit such that it can provision an environment for you using additional_dependencies -- for example if you needed packaging in your tool:

repos:
-   repo: local
    hooks:
    -   id: python-hook
        name: python-hook
        entry: python python_script.py
        language: python
        pass_filenames: false
        additional_dependencies: [packaging]

note again that pre-commit does not install from the repository under test (if it did, caching would be difficult-to-intractable) so you cannot use things like -r requirements.txt in additional_dependencies)

---

disclaimer: I created pre-commit

Answer 2

Try this:

- repo: local
  hooks:
    - id: python-hook
      name: python-hook
      entry: .venv/bin/python python_script.py
      pass_filenames: false
      language: system

I use it as follows:

- repo: local
  hooks:
    - id: mypy
      name: mypy
      entry: .venv/bin/mypy src
      pass_filenames: false
      language: system
    - id: basedpyright
      name: basedpyright
      entry: .venv/bin/basedpyright src
      pass_filenames: false
      language: system
    - id: pytest
      name: pytest
      entry: .venv/bin/pytest
      pass_filenames: false
      language: system

The tools need project dependencies, which are nicely listed in the lock file. The additional_dependencies config key does not support the lock file, nor pyproject.toml, nor any other requirements.txt

I think pre-commit will install the latest dependency version with that key, but I don't need the latest version, I need to check/test the source with actual dependencies from the lock file.