typecast.py

from typing import get_type_hints


def cast(argname, value, hints):
    """ Only cast arguments if type hints are available for them.
    """
    if argname in hints:
        expected_type = hints[argname]
        if not issubclass(type(value), expected_type):
            # Will throw type error if argument cannot be cast
            value = expected_type(value)
    return value


def typecast(function):
    """ Cast the argument and keyword argument to
    the type annotations when they are provided.
    """
    def type_caster(*args, **kwargs):
        hints = get_type_hints(function)
        # This accesses function positional argument names
        arg_names = function.__code__.co_varnames
        args_casted =  [cast(argname, val, hints)
                        for (argname, val) in zip(arg_names, args)]
        kwargs_casted = {argname: cast(argname, val, hints)
                         for (argname, val) in kwargs.items()}

        result = function(*args_casted, **kwargs_casted)

        if 'return' in hints:
            result = cast('return', result, hints)
        return result
    return type_checker


if __name__ == '__main__':
    @typecast
    def func(x: str, y, z: bool=True):
        return (x, y, z)

    # We give the func an int but expect a str since types will be casted
    x, y, z = func(3, 4)
    assert type(x) is str
    assert type(y) is int
    assert type(z) is bool
    x, y, z = func(3, 4, z=1)
    assert type(z) is bool