MyPy does not work well with Unions

[ayushr2]

MyPy complains with the following code:

def assert_equals(expected: Union[str, bytes], actual: Union[str, bytes]) -> bool:
    if type(expected) is str:
        expected = bytes(expected, "utf-8")
    if type(actual) is str:
        actual = bytes(actual, "utf-8")

    if expected == actual:
        return True
    else:
        logger.error(f"Expected: {expected!r}, Actual: {actual!r}.")
        return False


def assert_in(look: Union[str, bytes], where: Union[str, bytes]) -> bool:
    if type(look) is str:
        look = bytes(look, "utf-8")
    if type(where) is str:
        where = bytes(where, "utf-8")

    if look in where:
        return True
    else:
        logger.error(f"Looking for {look!r}, but contents are {where!r}.")
        return False

The error is:

line 3: error: Argument 1 to "bytes" has incompatible type "Union[str, bytes]"; expected "str"
line 5: error: Argument 1 to "bytes" has incompatible type "Union[str, bytes]"; expected "str"
line 16: error: Argument 1 to "bytes" has incompatible type "Union[str, bytes]"; expected "str"
line 18: error: Argument 1 to "bytes" has incompatible type "Union[str, bytes]"; expected "str"
line 20: error: Unsupported operand types for in ("Union[str, bytes]" and "Union[str, bytes]")

So even though we convert the variables to a specific type, MyPy still thinks that it is of type Union.

[Michael0x2a]

This is because mypy does not support type narrowing based on expressions of the form if type(var) == T. You need to do if isinstance(var, T) instead.

#7260 has some more related discussion.

[ayushr2]

Thank you so much, that fixed the issue.