mypy-strict-kwargs

Enforce using keyword arguments where possible.

For example, if we have a function which takes two regular argument, there are three ways to call it. With this plugin, mypy will only accept the form where keyword arguments are used.

"""Showcase errors when calling a function without naming the arguments."""


def add(a: int, b: int) -> int:
    """Add two numbers."""
    return a + b


add(a=1, b=2)  # With this plugin, ``mypy`` will only accept this form
add(1, 2)  # type: ignore[misc]
add(1, b=2)  # type: ignore[misc]

Why?

• In the same spirit as a formatter - think black or ruff format - this lets you stop spending time discussing whether a particular function call should use keyword arguments.
• Sometimes positional arguments are best at first, and then more and more are added and code becomes unclear, without anyone stopping to refactor to keyword arguments.
• The type checker gives better errors when keyword arguments are used. For example, with positional arguments, you may see, Argument 5 to "add" has incompatible type "str"; expected "int". This requires that you count the arguments to see which one is wrong. With named arguments, you get Argument "e" to "add" has incompatible type "str"; expected "int".

Installation

pip install mypy-strict-kwargs

This is tested on Python 3.10+.

Configure mypy to use the plugin

Add the plugin to your mypy configuration file:

.ini files:

[mypy]
plugins = mypy_strict_kwargs

.toml files:

[tool.mypy]

plugins = [
    "mypy_strict_kwargs",
]

Ignoring functions

You can ignore functions by adding configuration to pyproject.toml.

[tool.mypy_strict_kwargs]
ignore_names = ["main.func", "builtins.str"]

This is useful especially for builtins which can look strange with keyword arguments. For example, str(object=1) is not idiomatic.

To find the name of a function to ignore, set the following configuration:

[tool.mypy_strict_kwargs]
debug = true

Then run mypy and look for the debug output.