Python Type-Checking Decorator

typecheck.py

# Check if a function's arguments are instances of a (set of) type(s):
# >>> @type_checked
# >>> def test(asdf: str):
# >>>     print(type(asdf))
# >>> test(3)
# ValueError: Argument asdf should be of type str but is int

# Using @type_checked_cast instead of @type_checked tries
# casting the argument to the desired type:
# >>> @type_checked_cast
# >>> def test(asdf: str):
# >>>     print(type(asdf))
# >>> test(3)
# <class 'str'>

from collections.abc import Iterable
from functools import wraps
from inspect import signature

def type_checked(func):
	@wraps(func)
	def wrapper(*args, **kwargs):
		sig = signature(func)
		bound = sig.bind(*args, **kwargs)
		for k,v in bound.arguments.items():
			ann = sig.parameters[k].annotation
			if isinstance(ann, type):
				ann = (ann,)
			if isinstance(ann, Iterable):
				for anni in ann:
					if anni is sig.empty:
						continue
					if isinstance(anni, type) and not isinstance(v, anni):
						raise ValueError("Argument {} should be of type {} but is {}"
						.format(k, ann.__name__, type(v).__name__))
					else:
						# ignore non-type annotations
						pass
			else:
				# ignore non-type, non-Container annotations
				pass
		return func(*bound.args, **bound.kwargs)
	return wrapper


def type_checked_cast(func):
	@wraps(func)
	def wrapper(*args, **kwargs):
		sig = signature(func)
		bound = sig.bind(*args, **kwargs)
		for k,v in bound.arguments.items():
			ann = sig.parameters[k].annotation
			if isinstance(ann, type):
				ann = (ann,)
			if isinstance(ann, Iterable):
				for anni in ann:
					if anni is sig.empty:
						continue
					if isinstance(anni, type) and not isinstance(v, anni):
						bound.arguments[k] = anni(v)
					else:
						# ignore non-type annotations
						pass
			else:
				# ignore non-type, non-Container annotations
				pass
		return func(*bound.args, **bound.kwargs)
	return wrapper