eq_plugin.py: mypy plugin to check that types are only compared to themselves

README.md

How to Use

1. Put eq_plugin.py somewhere in your project
2. Add plugins=path/to/eq_plugin.py to the mypy section of your mypy.ini (see example in this gist)
3. Run mypy

Example

$ mypy example.py  # Also in this gist
example.py:7: warning: Equality comparison between "example.MyClass" and "def () -> example.MyClass"
example.py:10: warning: Equality comparison between "example.MyClass" and "builtins.int"

eq_plugin.py

from typing import Callable, Optional, Type

from mypy.plugin import MethodContext, Plugin
from mypy.subtypes import is_subtype


class CheckEqTypesMatch(Plugin):

    def get_method_hook(
            self, fullname: str) -> Optional[Callable[[MethodContext], Type]]:
        if fullname.endswith('__eq__'):
            return eq_callback


def eq_callback(ctx: MethodContext) -> Type:
    if len(ctx.arg_types) == 1 and len(ctx.arg_types[0]) == 1:
        arg_type = ctx.arg_types[0][0]
        if ctx.type != arg_type and not is_subtype(arg_type, ctx.type):
            ctx.api.msg.warn(
                'Equality comparison between "{}" and "{}"'.format(
                    ctx.type, ctx.arg_types[0][0]), ctx.context)
    return ctx.default_return_type


plugin = lambda _: CheckEqTypesMatch

example.py

from typing import *

class MyClass:

    def __eq__(self, other) -> bool: ...

class MySubClass(MyClass): ...

if MyClass() == MyClass:
    print('foo')

if MyClass() == 2:
    pass

if MyClass() == MyClass():
    pass

if MyClass() == MySubClass():
    pass

mypy.ini

[mypy]
plugins=
    eq_plugin.py