CODEWITHSUNDEEP
pythonpython-3.xexceptiontype-hintingmypy
SariO
SariO

Reputation: 469

Correct typing of tuple of exceptions/errors in python when using mypy

I have written my own decorator add_warning in order to print costom error messages when certain errors occur. The decorator takes a message and the type of errors for which to print this message. I would also like to add typing to this decorator and check it using mypy. This works well in the case where I am just throwing a plain Exception when I use Type[Exception]. However, mypy complains when I use other errors such as OSError or AttributeError by saying:

error: Argument "errors" to "add_warning" has incompatible type "Tuple[Type[OSError], Type[AttributeError]]"; expected "Union[str, Type[Exception], Tuple[Type[Any]]]".

Does anybody know if there is a better way than to use Any or Tuple[Type[OSError], Type[AttributeError]] here? Specifically, is there a more general type for all Python errors?

Below is the code: 

from functools import wraps
from typing import Union, Tuple, Callable, Type


def add_warning(message: str, flag: str = 'Info',
                errors: Union[str, Type[Exception], Tuple[Type[Exception]]] = 'all') -> Callable:
    """
    Add a warning message to a function, when certain error types occur.
    """
    if errors == 'all':
        errors = Exception

    def decorate(func: Callable):
        @wraps(func)
        def wrapper(*args, **kwargs):
            try:
                result = func(*args, **kwargs)
            except errors:
                warn(message, flag)
                return []
            else:
                return result
        return wrapper
    return decorate


def warn(message: str, flag: str = 'Info') -> None:
    """Print the colored warning message."""
    print(f"{flag}: {message}")


if __name__ == '__main__':

    @add_warning('This is another test warning.', flag='Error')
    def test_func1():
        raise Exception

    @add_warning('This is a test warning.', flag='Error', errors=(OSError, AttributeError))
    def test_func2():
        raise OSError

    test_func1()
    test_func2()

Upvotes: 2

Views: 2008

Answers (1)

juanpa.arrivillaga
juanpa.arrivillaga

Reputation: 96236

The issue is that Tuple[Type[Exception] means a tuple with a single value. You want a variable-sized tuple, so use ellipses: Tuple[Type[Exception], ...] The following works without mypy complaining:

from functools import wraps
from typing import Union, Tuple, Callable, Type


def add_warning(message: str, flag: str = 'Info',
                errors: Union[str, Type[Exception], Tuple[Type[Exception], ...]] = 'all') -> Callable:
    """
    Add a warning message to a function, when certain error types occur.
    """
    if errors == 'all':
        errors = Exception

    def decorate(func: Callable):
        @wraps(func)
        def wrapper(*args, **kwargs):
            try:
                result = func(*args, **kwargs)
            except errors:
                warn(message, flag)
                return []
            else:
                return result
        return wrapper
    return decorate


def warn(message: str, flag: str = 'Info') -> None:
    """Print the colored warning message."""
    print(f"{flag}: {message}")


if __name__ == '__main__':

    @add_warning('This is another test warning.', flag='Error')
    def test_func1():
        raise Exception

    @add_warning('This is a test warning.', flag='Error', errors=(OSError, AttributeError))
    def test_func2():
        raise OSError

    test_func1()
    test_func2()

Upvotes: 2

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