import functools
 class piecewise_function(object):
   def __init__(self, fallback):
     self._conds = []
     self._fallback = fallback
 #    self.__call__ = functools.wrap(fallback)(self.__call__)
   def add_piece(self, cond):
     def _inner(f):
       self._conds.append((cond,f))
       return self
     return _inner
   def __call__(self, *args, **kwargs):
     result = self._fallback
     for cond, func in self._conds:
       if cond(*args, **kwargs): result = func
     return result(*args, **kwargs)