Simple Read-only dict python

readonlydict.py

import UserDict

class ReadOnlyDict(UserDict.IterableUserDict):
	def __setitem__(self, key, item): raise TypeError
	def __delitem__(self, key): raise TypeError
	def clear(self): raise TypeError
	def pop(self, key, *args): raise TypeError
	def popitem(self): raise TypeError
	
	def update(self, dict=None):
		if dict is None:
			pass
		elif isinstance(dict, UserDict.UserDict):
			self.data = dict.data
		elif isinstance(dict, type({})):
			self.data = dict
		else:
			raise TypeError