How to add or increment single item of the Python Counter class

There is a more Pythonic way to do what you want:

c = Counter(item.property for item in something if item.has_some_property)

It uses a generator expression instead of open-coding the loop.

Edit: Missed your no-list-comprehensions paragraph. I still think this is the way to actually use Counter in practice. If you have too much code to put into a generator expression or list comprehension, it is often better to factor that into a function and call that from a comprehension.

>>> c = collections.Counter(a=23, b=-9)

You can add a new element and set its value like this:

>>> c['d'] = 8
>>> c
Counter({'a': 23, 'd': 8, 'b': -9})

Increment:

>>> c['d'] += 1
>>> c
Counter({'a': 23, 'd': 9, 'b': -9} 

Note though that c['b'] = 0 does not delete:

>>> c['b'] = 0
>>> c
Counter({'a': 23, 'd': 9, 'b': 0})

To delete use del:

>>> del c['b']
>>> c
Counter({'a': 23, 'd': 9})

Counter is a dict subclass

Well, you don't really need to use methods of Counter in order to count, do you? There's a += operator for that, which also works in conjunction with Counter.

c = Counter()
for item in something:
    if item.has_some_property:
        c[item.property] += 1
    elif item.has_some_other_property:
        c[item.other_property] += 1
    elif item.has_some.third_property:
        c[item.third_property] += 1