Python中实现参数类型检查的简单方法

'''
 > NONE, MEDIUM, STRONG = 0, 1, 2
 >
 > @accepts(int, int, int)
 ... def average(x, y, z):
 ...   return (x + y + z) / 2
 ...
 > average(5.5, 10, 15.0)
 TypeWarning: 'average' method accepts (int, int, int), but was given
 (float, int, float)
 15.25
'''
def accepts(*types, **kw):
  """ Function decorator. Checks that inputs given to decorated function
  are of the expected type.

  Parameters:
  types -- The expected types of the inputs to the decorated function.
       Must specify type for each parameter.
  kw  -- Optional specification of 'debug' level (this is the only valid
       keyword argument, no other should be given).
       debug = ( 0 | 1 | 2 )

  """
  if not kw:
    # default level: MEDIUM
    debug = 1
  else:
    debug = kw['debug']
  try:
    def decorator(f):
      def newf(*args):
        if debug == 0:
          return f(*args)
        assert len(args) == len(types)
        argtypes = tuple(map(type, args))
        if argtypes != types:
          msg = info(f.__name__, types, argtypes, 0)
          if debug == 1:
            print  sys.stderr, 'TypeWarning: ', msg
          elif debug == 2:
            raise TypeError, msg
        return f(*args)
      newf.__name__ = f.__name__
      return newf
    return decorator
  except KeyError, key:
    raise KeyError, key + "is not a valid keyword argument"
  except TypeError, msg:
    raise TypeError, msg

def info(fname, expected, actual, flag):
  """ Convenience function returns nicely formatted error/warning msg. """
  format = lambda types: ', '.join([str(t).split("'")[1] for t in types])
  expected, actual = format(expected), format(actual)
  msg = "'%s' method " % fname      + ("accepts", "returns")[flag] + " (%s), but " % expected     + ("was given", "result is")[flag] + " (%s)" % actual
  return msg