Type Checker decorator in python

type_checker.py

from functools import wraps

# Definitions
def __precheck_types(func_annotations, *args, **kwargs):
    annotations = dict((argument, value_type)
                       for argument, value_type in func_annotations.items()
                       if argument != 'return')
    for arg, value in kwargs.items():
        if arg not in annotations:
            continue
        if not isinstance(value, annotations[arg]):
            raise TypeError(f'value \'{value}\' is not of '
                            + f'type {annotations[arg]}')
        del annotations[arg]

    for expected_type, value in zip(annotations.values(), args):
        if not isinstance(value, expected_type):
            raise TypeError(f'value \'{value}\' is not of '
                            + f'type {expected_type}')

def type_checked(bound: bool = False):
    def decorator(func):
        has_return_type = 'return' in func.__annotations__
        return_type = func.__annotations__.get('return', None.__class__)
        if bound:
            @wraps(func)
            def func_wrapper(self, *args, **kwargs):
                __precheck_types(func.__annotations__, *args, **kwargs)
                result = func(self, *args, **kwargs)
                if has_return_type:
                    if not isinstance(result, return_type):
                        raise TypeError('return type is not of '
                                        + f'type {return_type}')
                return result
        else:
            @wraps(func)
            def func_wrapper(*args, **kwargs):
                __precheck_types(func.__annotations__, *args, **kwargs)
                result = func(*args, **kwargs)
                if has_return_type:
                    if not isinstance(result, return_type):
                        raise TypeError('return type is not of '
                                        + f'type {return_type}')
                return result
        return func_wrapper
    return decorator

# Usage
if __name__ == '__main__':
    @type_checked()
    def f(a: str, b: int):
        return a * b


    class A:
        @type_checked(bound=True)
        def g(self, a: str):
            return 4 * a


    @type_checked(bound=False)
    def h(a: str, *args) -> tuple:
        s = a
        for arg in args:
            s += arg
        return s, 2


    print(h('a', 'b', 'c', 'd'))

    print(f('a', 2))

    print(f(b=2, a='f'))

    print(f('c', b=2))

    try:
        print(f(2, 3))
    except Exception as e:
        print(str(e))

    x = A()
    print(x.g('23'))
    try:
        print(x.g(23))
    except Exception as e:
        print(str(e))