Possible Duplicate:
capitalization of library class names
Why does the dict subclass defaultdict not follow the CapWords naming convention that the other two dict subclasses from the collections module (Counter, OrderedDict) follow?
PEP 8 recommends the CapWords convention for classes.
Looking throughout CPython source I can mention that collection modules consists of two capitalized collection types: OrderedDict and Counter both implemented in Python and three other types, where namedtuple is a function, deque and defaultdict types are implemented in C. So there’s a whole difference.