**kwargs & relations between int, float & bool

[dsuch]

Hello,

this is a user question.

Python is 3.6.9 and mypy is mypy 0.790+dev.28829fbb684d7535680934b401c05698db5b4d85.

I have a class with methods as below:

from typing import Union

class MyClass(object):

    def func(self, *args:str, **kwargs:Union[int, float]) -> None:
        self.foo = kwargs.pop('foo', 1.0) # type: float

    def func2(self, *args:str, **kwargs:Union[float, int]) -> None:
        self.bar = kwargs.pop('bar', 1) # type: int

    def func3(self, *args:str, **kwargs:Union[bool, int]) -> None:
        self.baz = kwargs.pop('baz', True) # type: bool

In all the functions, **kwargs is a dictionary of str keys with values that may be of several types.

The errors I am getting are:

$ mypy --strict mypy4.py
mypy4.py:9: error: Incompatible types in assignment (expression has type "float", variable has type "int")
mypy4.py:12: error: Incompatible types in assignment (expression has type "int", variable has type "bool")
Found 2 errors in 1 file (checked 1 source file)
$

I would like to clarify if this occurs in relation to this comment in #1850 and the underlying implementation's assumptions of which type is a sub-type of which one?

I can deal with it by using cast, as in the code below, so this is more of a question if the above is expected.

from typing import cast, Union

class MyClass(object):

    def func(self, *args:str, **kwargs:Union[int, float]) -> None:
        self.foo = kwargs.pop('foo', 1.0) # type: float

    def func2(self, *args:str, **kwargs:Union[float, int]) -> None:
        self.bar = cast(int, kwargs.pop('bar', 1)) # type: int

    def func3(self, *args:str, **kwargs:Union[bool, int]) -> None:
        self.baz = cast(bool, kwargs.pop('baz', True)) # type: bool

Please note that the type comments are still needed because otherwise my IDE will not understand the results of casting.

Thanks.

[Akuli]

AFAIK casting is the correct way to extract one of many Unioned types, and cast(int, union_of_int_and_stuff) should return an int even without a type comment (IDE issue).

If #4441 was implemented, then you could do something like this:

class OptionsDict(TypedDict):
    foo: float
    bar: int
    baz: bool

class MyClass:
    def func2(self, *args, str, **kwargs: Expand[OptionsDict]) -> None:
        self.bar = kwargs['bar']
        # self.bar is now bool, without a type comment or cast

[Akuli]

Hmm, this might work too:

def func2(self, *args: str, **kwargs: Union[int, float]) -> None:
    self.bar = kwargs['bar']
    assert isinstance(self.bar, int)
    # what do mypy and your ide say now?

[dsuch]

Hi @Akuli,

from typing import Union

class MyClass:
    def func2(self, *args: str, **kwargs: Union[int, float]) -> None:
        self.bar = kwargs['bar']
        assert isinstance(self.bar, int)

There are no errors from mypy and the IDE considers self.bar to be an int now.

However, with this code ..

from typing import Union

class MyClass:
    def func2(self, *args: str, **kwargs: Union[int, float]) -> None:
        self.bar = kwargs['bar']
        assert isinstance(self.bar, str)

.. there are still no errors from mypy --strict even if the IDE treats self.bar as str.

[Akuli]

I don't know why mypy doesn't complain if you do isinstance(something_that_cant_be_string, str).

[JukkaL]

The original behavior seems reasonable, since int is considered by mypy as a subtype of float, and bool is a subclass of int and so also compatible with float. Union[int, float] is basically the same type as float.