where prototype

where.py

from functools import wraps


class where(object):

    def __init__(self, function, predicate):
        self._alternatives = []
        self.where(predicate)(function)

    def where(self, predicate):
        def decorator(function):
            self._alternatives.append((function, predicate))
            return wraps(function)(self)
        return decorator

    def otherwise(self, function):
        self._alternatives.append((function, lambda *args, **kwargs: True))
        return self

    def __call__(self, *args, **kwargs):
        for function, predicate in self._alternatives:
            try:
                valid = predicate(*args, **kwargs)
            except TypeError:
                valid = False

            if valid:
                return function(*args, **kwargs)
        raise ValueError(
            'Invalid arguments args: %s, kwargs: %s' % (args, kwargs)
        )


def carga(predicate):
    def decorator(function):
        return where(function, predicate)
    return decorator


@carga(lambda x: x > 2)
def cosa(x):
    print('X mayor 2')


@cosa.where(lambda x: x == 1)
def cosa(x):
    print('X es 1')


@cosa.otherwise
def cosa(x):
    print('X es una cosa')


cosa(3)
cosa(1)
cosa('Otjer')