Exploring patterns for validating function arguments in Python.

testing_args_easier_debug_messages.py

"""
$ python testing_args_easier_debug_messages.py.py
Hello world, I can't believe you've have 42 birthdays! I hope you find time for crafting soon!
Hello universe, I can't believe you've have 117.0 birthdays! I hope you find time for crafting soon!
ValueError: Need positive numbers, not: age=whoops
ValueError: Need positive numbers, not: age=-1
ValueError: Need non-empty strings, not: name=
ValueError: Need non-empty strings, not: hobby=None
"""

from typing import Callable, Generic, List, Tuple, Type, TypeVar, Union


T = TypeVar('T')

E = TypeVar('E', bound=Exception)


class conditional(Generic[T, E]):
    def __init__(self, msg: str, catching: Callable[[T], bool], exception: Type[E] = ValueError) -> None:
        self.msg = msg
        self.catching = catching
        self.exception = exception

    def __call__(self, **kwargs) -> None:
        """Alias for `validate`."""
        self.validate(**kwargs)

    def catch(self, **kwargs: T) -> List[Tuple[str, T]]:
        """Returns each named argument where `catching` evaluates to true."""
        passes_test = []
        for arg, value in kwargs.items():
            if self.catching(value):
                passes_test.append((arg, value))
        return passes_test

    def validate(self, **kwargs: T) -> None:
        """Raises a new `exception` if any argument evaluates to true under the `catching` function."""
        passes_test = self.catch(**kwargs)
        if len(passes_test) > 0:
            s = ','.join([f"{a}={v}" for a, v in passes_test])
            raise self.exception(f"{self.msg}, not: {s}")


def require_non_empty_str(**kwargs: str) -> None:
    conditional("Need non-empty strings", lambda v: not isinstance(v, str) or len(v) == 0)(**kwargs)


def require_positive(**kwargs: Union[int, float]) -> None:
    conditional("Need positive numbers", lambda v: not isinstance(v, (float, int)) or v < 1)(**kwargs)


def example_func_for_validation(age: int, name: str, hobby: str):
    require_non_empty_str(name=name, hobby=hobby)
    require_positive(age=age)
    print(f"Hello {name}, I can't believe you've have {age} birthdays! I hope you find time for {hobby} soon!")


if __name__ == "__main__":
    def _test(**kwargs):
        try:
            example_func_for_validation(**kwargs)
        except ValueError as e:
            print(f"ValueError: {e}")

    _test(age=42, name='world', hobby='crafting')        # ok!
    _test(age=117.0, name='universe', hobby='crafting')  # ok!
    _test(age='whoops', name='world', hobby='crafting')  # ValueError: age=whoops
    _test(age=-1, name='world', hobby='crafting')        # ValueError: age=-1
    _test(age=42, name='', hobby='crafting')             # ValueError: name=""
    _test(age=42, name='world', hobby=None)              # ValueError: hobby=None