Making MyPy play nice with Pydantic

I ran into a problem at work today that I was sure I had seen before, but had forgotten what the fix was, so this time I'm writing it down.

I'm really enjoying using Pydantic at the moment to help with validating the shape of inputs to some of my functions, and allowing me to define some data structures (called "models") that my brain can think of as types for use in my code. It will also throw errors at runtime if you try to construct a pydantic model from something that doesn't fit the type.

However, the company also tries to use native Python type hints, and is starting to actually check them using MyPy. Turns out, if you have a Pydantic model, i.e. something that inherits from pydantic.BaseModel, MyPy fails the typechecking, with an error that looks like src/my_file.py:12: error: Class cannot subclass 'BaseModel' (has type 'Any'). But I know that BaseModel doesn't have type Any and so does PyCharm, so why doesn't MyPy?

Turns out, there's a plugin for MyPy to fix this. It's documented pretty well in the Pydantic docs (which are awesome!), but I failed to read them, and wasn't pointed there when I tried googling my error messages. So, I needed to add a config file for MyPy, and add this plugin:

# mypy.ini
[mypy]
plugins = pydantic.mypy

[pydantic-mypy]
init_forbid_extra = True
init_typed = True
warn_required_dynamic_aliases = True
warn_untyped_fields = True

The first section adds the plugin, and the second turns on some options to make the checking even more strict. See the Pydantic docs for what these settings actually do.

So now the MyPy settings are in place, if I run the type checker again, I get an error that Pydantic isn't installed. This is true, since we run MyPy from a stripped-down tox environment with only the linter, formatters and MyPy itself installed. Let's fix that. The relevant section of the (really rather long) tox.ini file needs to look like:

[base]
lint_targets = src tests

[testenv:lint]
description = Lint using black, flake8 and isort. Type-check using Mypy.
deps =
    black
    flake8
    isort
    mypy
    pydantic # add this! or put it in a requirements.txt and install from there
commands =
    isort --check-only --diff {[base]lint_targets}
    black --check --diff {[base]lint_targets}
    flake8 {[base]lint_targets}
    mypy --ignore-missing-imports --strict {toxinidir}/src
    mypy --ignore-missing-imports {toxinidir}/tests/unit

And yay! everything works!