Python tip: default dict

Sometimes you need to initialize dictionary before you use it, for this reason you can use dict() subclass defaultdict(). This is how you would do it normally.

# counter for cars by vendor
cars = dict()
if 'ferrari' not in cars:
    cars['ferrari'] = 1
else:
    cars['ferrari'] += 1

but with default dict this scenario is far easier

# counter for cars by vendor
from collections import defaultdict
cars = defaultdict(int)
cars['ferrari'] += 1

Always remember that defaultdict takes as parameter callable (function) not default value.