Unable to deduce/coerce types in Union

[dosisod]

• Are you reporting a bug, or opening a feature request?

Bug

• Please insert below the code you are checking with mypy:

from typing import Union

def convert(num: Union[str, int]) -> int:
	if type(num) is str:
		 return int(num)

	#since we returned above, this can only be an `int`
	return num

print( convert("1") + convert(2) )

• What is the actual behavior/output?

test.py:8: error: Incompatible return value type (got "Union[str, int]", expected "int")

• What is the behavior/output you expect?

No error.

• What are the versions of mypy and Python you are using?

mypy 0.761
Python 3.6.9

• Do you see the same issue after installing mypy from Git master?

Did not check with master.

• What are the mypy flags you are using? (For example --strict-optional)

None.

This is similar to #8264 such that the type is not deduced.

If there is N types in the Union, and one of the if statements return for a given type, there should be N-1 valid types in the Union proceeding the if statement.

This can get annoying when I have to cast the variable when I know the type:

from typing import Union, cast

def convert(num: Union[str, int]) -> int:
	if type(num) is str:
		 return int(num)

	return cast(int, num)

print( convert("1") + 2 ) # prints 3

[JukkaL]

Things should work as expected if you use isinstance(num, str) instead of type(num) is str. The type(...) is check is not recommended since it doesn't work with subclasses (of str, in this case).

[dosisod]

This solved my problem, thanks!