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)