isinstance(X, float) wrongly consumes bool type

[tusharsadhwani]

In the following example, adding an unrelated isinstance check causes the code to raise an error.

To Reproduce

Code:

from typing import Union

def get_type_name(value: Union[bool, str]) -> str:
    if value is True:
        return "Boolean"
    if value is False:
        return "Boolean"
    return "String"

Mypy output:

$ mypy --strict testcode.py
Success: no issues found in 1 source file

Adding an isinstance(value, float) check:

from typing import Union

def get_type_name(value: Union[bool, str]) -> str:
    if isinstance(value, float):  # a completely unrelated check
        return "Number"

    if value is True:
        return "Boolean"
    if value is False:
        return "Boolean"

    return "String"

Mypy output:

$ mypy --strict testcode.py
testcode.py:5: error: Missing return statement
testcode.py:9: error: Non-overlapping identity check (left operand type: "str",
right operand type: "Literal[True]")
testcode.py:11: error: Non-overlapping identity check (left operand type: "str",
 right operand type: "Literal[False]")
Found 3 errors in 1 file (checked 1 source file)

Mypy says value is True and value is False are non overlapping checks, but it works:

$ python -i testcode.py
>>> get_type_name(5.0)
'Number'

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: --strict
• Python version used: Python 3.9 and 3.10
• Operating system and version: Linux, Pop!_OS 21.10

[JelleZijlstra]

This is probably because bool is a subclass of int (in reality) and int is a subclass of float (in the type system).

[hauntsaninja]

mypy could maybe have an option to pretend that bool isn't a subclass of int, in which case this is a duplicate of #8363