Types of class member variables are not checked

[kencoken]

Running mypy on the following class, using Python 3.6 support (PEP 526):

class TestClass:

    a: str = 9

    def __init__(self):

        self.b: str = 8
        self.b = 12

    def test_method(self):
        self.b = 50

Results in the following output:

another_test.py: note: In class "TestClass":
another_test.py:3: error: Incompatible types in assignment (expression has type "int", variable has type "str")

It seems as if the type of the member variable b is not checked.

[gvanrossum]

Try adding -> None to the signatures of both methods. Unannotated functions/methods are not checked at all.

[kencoken]

Works great – thanks for the pointer!

Is the behaviour you describe intentional? Wasn't able to find anywhere where this was discussed.

[refi64]

#2215 #1370 #1334

[JukkaL]

It's described in the documentation: https://mypy.readthedocs.io/en/latest/basics.html (maybe we should make the wording somehow more explicit, as users commonly miss this point).

[gvanrossum]

I've added a section on this to common_issues.html.