A plugin for flake8 integrating Mypy.




It was created in early 2017 when Mypy performance was often insufficient for in-editor linting. The Flake8 plugin traded correctness for performance, making Mypy only check a bare minimum of problems. These days Mypy is accelerated with mypyc, as well as uses aggressive caching to speed up incremental checks. It's no longer worth it to use hacks such as flake8-mypy.

What was the project anyway?

A plugin for Flake8 integrating mypy. The idea is to enable limited type checking as a linter inside editors and other tools that already support Flake8 warning syntax and config.

List of warnings

flake8-mypy reserves T4 for all current and future codes, T being the natural letter for typing-related errors. There are other plugins greedily reserving the entire letter T. To this I say: ¯\_(ツ)_/¯.

T400: any typing note.

T484: any typing error (after PEP 484, geddit?).

T498: internal mypy error.

T499: internal mypy traceback, stderr output, or an unmatched line.

I plan to support more fine-grained error codes for specific mypy errors in the future.

Two levels of type checking

mypy shines when given a full program to analyze. You can then use options like --follow-imports or --disallow-untyped-calls to exercise the full transitive closure of your modules, catching errors stemming from bad API usage or incompatible types. That being said, those checks take time, and require access to the entire codebase. For some tools, like an editor with an open file, or a code review tool, achieving this is not trivial. This is where a more limited approach inside a linter comes in.

Flake8 operates on unrelated files, it doesn't perform full program analysis. In other words, it doesn't follow imports. This is a curse and a blessing. We cannot find complex problems and the number of warnings we can safely show without risking false positives is lower. In return, we can provide useful warnings with great performance, usable for realtime editor integration.

As it turns out, in this mode of operation, mypy is still able to provide useful information on the annotations within and at least usage of stubbed standard library and third party libraries. However, for best effects, you will want to use separate configuration for mypy's standalone mode and for usage as a Flake8 plugin.


Due to the reasoning above, by default flake8-mypy will operate with options equivalent to the following:

# Specify the target platform details in config, so your developers are
# free to run mypy on Windows, Linux, or macOS and get consistent
# results.

# flake8-mypy expects the two following for sensible formatting

# do not follow imports (except for ones found in typeshed)

# since we're ignoring imports, writing .mypy_cache doesn't make any sense

# suppress errors about unsatisfied imports

# allow untyped calls as a consequence of the options above

# allow returning Any as a consequence of the options above

# treat Optional per PEP 484

# ensure all execution paths are returning

# lint-style cleanliness for typing needs to be disabled; returns more errors
# than the full run.

# The following are off by default since they're too noisy.
# Flip them on if you feel adventurous.

If you disagree with the defaults above, you can specify your own mypy configuration by providing the --mypy-config= command-line option to Flake8 (with the .flake8/setup.cfg equivalent being called mypy_config). The value of that option should be a path to a mypy.ini or setup.cfg compatible file. For full configuration syntax, follow mypy documentation.

For the sake of simplicity and readability, the config you provide will fully replace the one listed above. Values left out will be using mypy's own defaults.

Remember that for the best user experience, your linter integration mode shouldn't generally display errors that a full run of mypy wouldn't. This would be confusing.

Note: chaing the follow_imports option might have surprising effects. If the file you're linting with Flake8 has other files around it, then in "silent" or "normal" mode those files will be used to follow imports. This includes imports from typeshed.


Just run:

python setup.py test

OMG, this is Python 3 only!

Yes, so is mypy. Relax, you can run Flake8 with all popular plugins as a tool perfectly fine under Python 3.5+ even if you want to analyze Python 2 code. This way you'll be able to parse all of the new syntax supported on Python 3 but also effectively all the Python 2 syntax at the same time.

By making the code exclusively Python 3.5+, I'm able to focus on the quality of the checks and re-use all the nice features of the new releases (check out pathlib) instead of wasting cycles on Unicode compatibility, etc.



Change Log


  • avoid raising errors in the default config which don't happen during a full run (disable warn_unused_ignores and warn_redundant_casts)

  • always run type checks from a temporary directory to avoid clashing with unrelated files in the same directory


  • suppress mypy messages about relative imports


  • bugfix: using Flake8 with absolute paths now correctly matches mypy messages

  • bugfix: don't crash on relative imports in the form from . import X


  • switch follow_imports from "silent" to "skip" to avoid name clashing files being used to follow imports within typeshed

  • set MYPYPATH by default to give stubs from typeshed higher priority than local sources


  • performance optimization: skip running mypy over files that contain no annotations or imports from typing

  • bugfix: when running over an entire directory, T484 is now correctly used instead of T499


  • first published version

  • date-versioned


Glued together by Łukasz Langa.

  • Compatibility with pythons that aren't 3.6

    Compatibility with pythons that aren't 3.6

    Not hard-coding the version of python this depends on

    When running flake8 as a pytest plugin it is not so straightforward to change command-line arguments to specify what version of python you are using, nor should you have to.

    opened by revmischa 2
  • issue with round

    issue with round

    got this error: T484 Argument 1 to "round" has incompatible type "Union[float, Decimal]"; expected "float"

    however that is in fact allowed:

    >>> round(Decimal(0.0))
    opened by thehesiod 1
  •  Make temp file creation Windows compatible

    Make temp file creation Windows compatible

    On Windows the temp file must be closed, otherwise mypy can't open it again. Unfortunately, we then loose the nice to read with sugar and obviously must call os.remove manually.

    This is on top of #17 bc I had no way to test this otherwise.

    opened by kaste 0
  • Adapt test_clash for changes in mypy and typeshed master.

    Adapt test_clash for changes in mypy and typeshed master.

    UserDict is now concrete: https://github.com/python/typeshed/pull/1477

    And error message format changed slightly for non-subscripted generic types.

    Also added test docstring that would have saved me some time understanding what the test was about.

    opened by carljm 0
  • Error running flake8-mypy

    Error running flake8-mypy

    When I run flake8, I just get a bunch of these errors and nothing else from mypy:

    /home/ubuntu/.local/lib/python3.6/site-packages is in the MYPYPATH. Please remove it.
    See https://mypy.readthedocs.io/en/latest/running_mypy.html#how-mypy-handles-imports for more info
    opened by Dreamsorcerer 4
  • Improperly handling mypy summary from mypy 0.730 update

    Improperly handling mypy summary from mypy 0.730 update

    Mypy 0.730 introduced a change which adds a summary line when checking files which is on by default.

    This is picked up as an unmatched line and reported as a T499 error.

    Ideally the --no-error-summary flag would just be passed to mypy, but at the very least, no_error_summary=True needs to be added to the mypy_default.ini when the installed mypy version is >=0.730

    opened by cheeseandcereal 0
  • mypy.ini picked up implicitly and false defaults not overridden

    mypy.ini picked up implicitly and false defaults not overridden

    A mypy.ini in the current directory will be picked up implicitly, even if no config file is specified. This has a bad interaction with how the command line arguments are created, since false values are omitted, so those won't properly override the values in the implicitly picked up mypy.ini

    opened by msullivan 0
  • New release?

    New release?

    Any chance of a new release? The latest in PyPI is 17.8.0 and it struggles with Windows (https://github.com/ambv/flake8-mypy/pull/18).

    BTW there's no 17.8.0 tag in this repo.

    opened by jamesmyatt 2
  • Fails to Recognize

    Fails to Recognize "coding: utf-8-unix"

    Flake8-MyPy fails to recognize Python scripts that use the magic comment containing coding: utf-8-unix as a UTF8 file. This bug has persisted for several versions and I had been fixing it myself ( https://github.com/ambv/flake8-mypy/pull/12). Although, my fix no longer works in the newest version of flake8-mypy. The fix has already been applied to MyPy itself ( https://github.com/python/mypy/pull/5085 ), but this very helpful Flake8 plugin still lacks the fix.

    flake8 --version
    3.7.7 (flake8-mypy: 17.8.0, flake8-pyi: 19.3.0, mccabe: 0.6.1, pycodestyle: 2.5.0, pyflakes: 2.1.1, radon: 3.0.1, warn-symbols: 1.1.1) CPython 3.6.7 on Linux
    mypy --version
    mypy 0.701
    opened by DevynCJohnson 1
  • Cannot override writable attribute with a final one

    Cannot override writable attribute with a final one

    The below code should pass the check:

    class MongoDAOBase(abc.ABC):
        collection_name: str = NotImplemented
        def create_index(cls) -> None:
    class PrivacySettingsDAO(MongoDAOBase):
        collection_name: Final = "privacy_settings" # complains
        def create_index(cls) -> None:

    However, a T484 Cannot override writable attribute "collection_name" with a final oneis raised. This problem seems to be introduced recently. My local version (which is older) of flake8-mypy and mypy does not complain, but it fails in CI environment (which gets later version). I'm not sure if this is a flake8-mypy problem or mypy problem, so I just ask here first.

    opened by fr0der1c 0
Łukasz Langa
Python 3.8 & 3.9 Release Manager. llanga on Twitter. Python core developer, hobbyist musician, dad.
Łukasz Langa
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