Pytest Mypy Plugin

1.0.1 · active · verified Mon Apr 13

pytest-mypy is a pytest plugin that integrates the Mypy static type checker into your pytest test runs. It allows you to run Mypy checks on your Python source files as part of your existing test suite, similar to how pytest-flake8 works for Flake8. The current version is 1.0.1, and it is actively maintained, with releases as needed.

Warnings

gotcha Mypy configuration flags like `--ignore-missing-imports` or not using strict checking (`--strict`, `--disallow-untyped-defs`) can cause Mypy to silently miss type errors. Users might mistakenly believe their code is fully type-checked when issues are being suppressed.
Fix: Always aim for strict Mypy settings in CI/CD pipelines (e.g., using `--strict`). Review your `mypy.ini` or `pyproject.toml` configuration to ensure no critical errors are being ignored. Consider using `plugin.mypy_argv.append('--strict')` in `conftest.py` if configuring via pytest.
gotcha When Mypy is run as a subprocess (the default behavior of `pytest-mypy`), relative paths specified in environment variables like `MYPYPATH` or `PYTHONPATH` may not resolve correctly because pytest tests can run in temporary working directories.
Fix: To resolve pathing issues, use absolute paths for `MYPYPATH`/`PYTHONPATH` or instruct `pytest-mypy` to run Mypy in the same process using the `--mypy-same-process` flag.
gotcha By default, Mypy does not fully check functions without type annotations, and `Any` types can propagate through your code, masking potential type errors. This can lead to false negatives in your type checking results.
Fix: To ensure comprehensive type checking, explicitly add type annotations to your functions. Alternatively, configure Mypy with flags like `--disallow-untyped-defs` (to report errors for untyped function definitions) or use `--strict` mode, which enables a comprehensive set of strict checks.

Install

pip install pytest-mypy Install stable version

Imports

pytest-mypy

This is a pytest plugin and is automatically discovered upon installation; no explicit import statement is typically needed in your test files.

The plugin is activated via the '--mypy' CLI flag or configured through conftest.py.

Quickstart

After installing `pytest-mypy`, enable it by passing the `--mypy` flag to pytest, specifying the files or directories you want to type check. For more granular control or to enforce stricter Mypy settings, you can add options to `plugin.mypy_argv` within a `pytest_configure` hook in your `conftest.py` file.

# my_project/my_module.py
def add_numbers(a: int, b: int) -> int:
    return a + b

def incorrect_call():
    # Mypy should flag this type mismatch
    return add_numbers('hello', 5)

# my_project/test_example.py (empty or with other tests)
# This file just needs to exist for pytest to discover the project.

# To run mypy with pytest (from my_project directory):
# pip install pytest pytest-mypy mypy
# pytest --mypy my_module.py

# Example conftest.py to add extra mypy arguments (e.g., --strict):
# # my_project/conftest.py
# def pytest_configure(config):
#     plugin = config.pluginmanager.getplugin('mypy')
#     if plugin:
#         # Example: make mypy run in strict mode
#         plugin.mypy_argv.append('--strict')
#         # Example: check untyped definitions
#         plugin.mypy_argv.append('--disallow-untyped-defs')

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