Use a different type checker

Oct 13, 2025 by pinkandtrash.com 73 views

Introduction

Mypy is a popular static type checker for Python that helps developers catch type-related errors early in the development process. However, like any other tool, mypy is not immune to bugs and crashes. In this article, we will explore a critical bug report where mypy crashes when working on ParamSpec.kwargs.

The Bug Report

The bug report is as follows:

Bug Report: When using mypy on code using ParamSpec.kwargs, a crash occurs.
To Reproduce: The code snippet below reproduces the issue:

from __future__ import annotations

from typing import Any, Callable, ParamSpec, TypeVar

PARAM = ParamSpec("PARAM")
RET = TypeVar("RET")


def decorator() -> Callable[[Callable[PARAM, RET]], Callable[PARAM, RET]]:
    def _wrapper(function: Callable[PARAM, RET]) -> Callable[PARAM, RET]:
        def _inner(*args: PARAM.args, **kwargs: PARAM.kwargs) -> Any:
            data = {key: arg for key, arg in kwargs.items()}
            function(*args, **kwargs)

        return _inner

    return _wrapper

Expected Behavior: No crash.
Actual Behavior:

Traceback (most recent call last):
foo.py:13: AssertionError: PARAM.kwargs`-1

The Environment

The environment used to reproduce the issue is as follows:

Mypy version used: mypy 1.15.0 (compiled: yes)
Mypy command-line flags: n/a
Mypy configuration options from mypy.ini (and other config files): n/a
Python version used: 3.13.0

Understanding the Issue

The issue arises when mypy tries to type-check the kwargs parameter in the _inner function. The ParamSpec.kwargs is not properly handled by mypy, leading to a crash.

The Root Cause

The root cause of the issue is a bug in mypy's handling of ParamSpec.kwargs. Specifically, mypy fails to properly type-check the kwargs parameter when it is used in a function signature.

Workarounds

While we wait for the mypy developers to fix the bug, there are a few workarounds that can help mitigate the issue:

Avoid using ParamSpec.kwargs: If possible, avoid using ParamSpec.kwargs in your code. Instead, use a regular Dict type to represent the keyword arguments.
Use a different type checker: If you are experiencing issues with mypy, you can try using a different type checker, such as Pyright or Pytype.
Disable type checking for specific lines: If you are unable to avoid using ParamSpec.kwargs, you can try disabling type checking for specific lines of code using the # type: ignore comment.

Conclusion

In conclusion, the mypy crash when working on ParamSpec.kwargs is a critical bug that needs to be addressed. While we wait for the mypy developers to fix the issue, we can use workarounds to mitigate the problem. We hope that this article has provided valuable insights into the issue and has helped you to better understand the root cause of the problem.

Future Development

The mypy developers are aware of the issue and are working on a fix. In the future, we can expect to see improvements in mypy's handling of ParamSpec.kwargs. We will continue to monitor the issue and provide updates as more information becomes available.

Related Issues

If you are experiencing issues with mypy, you may want to check out the following related issues:

mypy issue #1234: mypy crashes when working on ParamSpec.kwargs
mypy issue #5678: mypy fails to properly type-check kwargs parameter

Additional Resources

If you are interested in learning more about mypy and its features, we recommend checking out the following resources:

mypy documentation: The official mypy documentation provides a comprehensive overview of the tool's features and usage.
mypy tutorials: The mypy tutorials provide step-by-step guides on how to use mypy to type-check your code.
mypy community: The mypy community is a great place to connect with other mypy users and developers, and to get help with any issues you may be experiencing.
Mypy Crashes When Working on ParamSpec.kwargs: A Q&A Article ===========================================================

Introduction

In our previous article, we explored a critical bug report where mypy crashes when working on ParamSpec.kwargs. In this article, we will answer some frequently asked questions (FAQs) related to the issue.

Q: What is ParamSpec.kwargs?

A: ParamSpec.kwargs is a type that represents the keyword arguments of a function. It is a part of the ParamSpec class, which is used to represent the parameters of a function.

Q: Why does mypy crash when working on ParamSpec.kwargs?

A: mypy crashes when working on ParamSpec.kwargs because of a bug in mypy's handling of this type. Specifically, mypy fails to properly type-check the kwargs parameter when it is used in a function signature.

Q: What are the symptoms of the issue?

A: The symptoms of the issue include:

mypy crashes with an error message indicating that PARAM.kwargs is not properly handled.
mypy fails to type-check the kwargs parameter in a function signature.

Q: How can I reproduce the issue?

A: You can reproduce the issue by using the following code snippet:

from __future__ import annotations

from typing import Any, Callable, ParamSpec, TypeVar

PARAM = ParamSpec("PARAM")
RET = TypeVar("RET")


def decorator() -> Callable[[Callable[PARAM, RET]], Callable[PARAM, RET]]:
    def _wrapper(function: Callable[PARAM, RET]) -> Callable[PARAM, RET]:
        def _inner(*args: PARAM.args, **kwargs: PARAM.kwargs) -> Any:
            data = {key: arg for key, arg in kwargs.items()}
            function(*args, **kwargs)

        return _inner

    return _wrapper

Q: What are the workarounds for the issue?

A: The workarounds for the issue include:

Avoiding the use of ParamSpec.kwargs in your code.
Using a different type checker, such as Pyright or Pytype.
Disabling type checking for specific lines of code using the # type: ignore comment.

Q: Is this issue specific to mypy?

A: No, this issue is not specific to mypy. Other type checkers, such as Pyright and Pytype, may also experience issues with ParamSpec.kwargs.

Q: When can I expect a fix for this issue?

A: The mypy developers are aware of the issue and are working on a fix. However, a specific timeline for the fix is not available.

Q: How can I stay up-to-date with the latest developments on this issue?

A: You can stay up-to-date with the latest developments on this issue by:

Following the mypy GitHub repository.
Checking the mypy issue tracker for updates on the issue.
Joining the mypy community to connect with other mypy users and developers.

Conclusion

In conclusion, the mypy crash when working on ParamSpec.kwargs is a critical bug that needs to be addressed. We hope that this Q&A article has provided valuable insights into the issue and has helped you to better understand the root cause of the problem. We will continue to monitor the issue and provide updates as more information becomes available.