Typechecking for Python

typecheck.py

"""
Quick 'n dirty hack for a typecheck decorator.
Only tested on functions without kwargs. Use at own peril, etc.

Usage:

@typecheck
def some_function(x: str, y: int) -> str:
    ...
"""

from functools import wraps
from collections import namedtuple

Argument = namedtuple("Argument", "name type")

def typecheck(func):
    @wraps(func)
    def replacement(*args):
        arg_names = func.__code__.co_varnames[:func.__code__.co_argcount]
        requested_types = [Argument(name, func.__annotations__[name])
                           for name in arg_names]

        for arg, requested_type in zip(args, requested_types):
            if not isinstance(arg, requested_type.type):
                raise TypeError("Argument {} is not type {}".format(
                    requested_type.name, requested_type.type))

        retval = func(*args, **kwargs)
        if not isinstance(retval, func.__annotations__['return']):
            raise TypeError("Function must return type {}".format(
                func.__annotations__['return']))
        else:
            return retval
    return replacement