guard.py

#Play with dispatch ideas and guards in Python.
class Guard:

	dispatch_table = []

	@classmethod
	def register(cls, func, condition):
	
		cls.dispatch_table += [(func, condition)]

	@classmethod
	def satisfies_predicate(cls, predicate, params):
		return eval(predicate, params) if predicate else True


def when(condition=""):

	def decorator(func):

		Guard.register(func, condition)

		def dispatcher(**args):

			possibles = [method for (method, predicate) in Guard.dispatch_table if method.__name__ == func.__name__ and Guard.satisfies_predicate(predicate, args)]

			return possibles[0](**args) if possibles else func(**args)

		return dispatcher

	return decorator