Python中实现参数类型检查的简单方法

Python自然没有类型强制转换一说了，因为它是动态语言。首先，所有对象都从Object继承而来，其次，它有强大的内省，如果调用某个不存在的方法会有异常抛出。大多数情况，我们都不需要做参数类型栓查，除了一些特殊情况。例如，某个函数接受一个str类型，结果在实际调用时传入的是unicode，测试过程中又没有代码覆盖到，这样问题就比较严重了。解决方法也很简单，借助Python的内省，很容易就能判断出参数的类型。但是每个地方都写检查代码会很累赘，何况它带来的实际价值并不高。一个好的解决方法是使用装饰器。

'''
 >>> 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

本质上讲，这也是一种运行时检查，但效果已经不错了。
更多有趣的装饰器的使用，可以参考这篇文章http://wiki.python.org/moin/PythonDecoratorLibrary