Initializer not checked against declared type when annotating using self.x

[sotte]

mypy accepts instance variables that are instantiated as None even though the type indicates int. This is not the case for equivalent dataclasses. I'm not sure if this is a bug or intended behavior.

Here is a minimal example:

from dataclasses import dataclass, field


class Foo:
    def __init__(self):
        self.bar: int = None


@dataclass
class Foo2:
    bar: int = None
    bar2: int = field(default=None)

This is how I check the fiel: mypy mypy_demo.py.

Here the resulting error msgs:

mypy_demo.py:11: error: Incompatible types in assignment (expression has type "None", variable has type "int")
mypy_demo.py:12: error: Incompatible types in assignment (expression has type "None", variable has type "int")

I did not find anything helpful in the dataclass section: https://mypy.readthedocs.io/en/latest/additional_features.html#dataclasses

I'm testing with Mypy version 0.701.

[JukkaL]

Thanks for reporting this! The initialization in Foo should be rejected by mypy. This is quite serious, as even a totally bogus initializer is accepted:

class Foo:
    def __init__(self):
        self.bar: int = 'x'  # No error even though this is clearly wrong!

[ilevkivskyi]

Adding -> None helps. Note however --check-untyped-defs is broken for __init__(), see #7309 (there are also couple issues mentioned that we may potentially close as duplicates).

[ilevkivskyi]

#7291 is also related (I think it was discussed whether we should just assume the -> None by default in other issues. IIRC there is no consensus because either option has drawbacks, so status quo wins)

[JukkaL]

Oops, #3948 bites again.

[sotte]

Thanks for the quick response!

Just for completeness sake: my assumption was, that I can do self.bar: int = None because it's also ok to do def bob(a: int = None): print(a). That is that an implicit Optional would be used.

from dataclasses import dataclass, field


class Foo:
    def __init__(self):
        self.bar: int = None

    def bob(self, a: int = None):
        print(a)


@dataclass
class Foo2:
    bar: int = None
    bar2: int = field(default=None)

[JelleZijlstra]

Implicit optional may be allowed in some cases but it's deprecated. It's preferable to explicitly add Optional.

[sotte]

Thanks for clarification!