#!/usr/local/bin/python2
class ExtendableFunction(object):
    def __init__(self, func):
        self._base = func
        self._head = []
        self._tail = []

    def __call__(self, *args, **kwargs):
        for func in self._head:
            func(*args, **kwargs)
        result = self._base(*args, **kwargs)
        for func in self._tail:
            func(*args, **kwargs)
        return result

    @property
    def head(self):
        return self._head

    @property
    def tail(self):
        return self._tail

    @property
    def replica(self):
        base = self._base
        head = self._head[:]
        tail = self._tail[:]
        def f(*args, **kwargs):
            for func in head:
                func(*args, **kwargs)
            result = self._base(*args, **kwargs)
            for func in tail:
                func(*args, **kwargs)
            return result
        return f

def p(*args): print 123
def q(*args): print 456

e = ExtendableFunction(bool)
e.head.append(p)
r = e.replica
print r(1)
e._base = str
e.head.append(q)
print e(1)
print r(1)