Question

Counter objects are subclasses of dict so they have the method setdefault.

>>> from collections import Counter
>>> c = Counter(houses=5)
>>> print(c.setdefault.__doc__)
D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D

If I do:

>>> c.setdefault('castles')
>>> c.keys()
dict_keys(['castles', 'houses'])
>>> type(c)
<class 'collections.Counter'>

everything seems pretty nice. But:

>>> c
Traceback (most recent call last):
  File "<pyshell#17>", line 1, in <module>
    c
  File "C:\Python32\lib\collections.py", line 586, in __repr__
    items = ', '.join(map('%r: %r'.__mod__, self.most_common()))
  File "C:\Python32\lib\collections.py", line 477, in most_common
    return sorted(self.items(), key=_itemgetter(1), reverse=True)
TypeError: unorderable types: NoneType() < int()
>>> 

Is this a bug?. Should not give c.setdefault('castles') a value/key error instead of silently accept a key without value ? Or maybe a repr method taking into account None values?

Answer 1

Yes, looks like a bug. The problem is that setdefault without a value argument assumes the value is None, while in the case of a Counter it should really insert either one or zero, or fail by raising some exception.

In Python 2.7, your snippet works, btw., although it still inserts a None value, violating Counter‘s invariants.

Mind you, this isn’t the first bug/design flaw that I encounter with collections.Counter.