Message8315

Expected behavior:
defaultdict.get(key, default) should not create entries in the map when no key is found and a default is specified.

Eg:

>>> from collections import defaultdict
>>>
>>> d = defaultdict(set)
>>>
>>> d['one'] = [1]
>>>
>>> d['two']
set([])
>>>
>>> print(d)
defaultdict(<type 'set'>, {'two': set([]), 'one': [1]})
>>>
>>> d.get('three', [])
set([])
>>>
>>> print(d)
defaultdict(<type 'set'>, {'two': set([]), 'one': [1], 'three': set([])})
>>>
>>>

The map now contains an entry 'three', this is a nasty source of memory-leaks (our system had maps with millions of entries caused by this subtle bug).