Pythonic Programming

Tip 42

Count with defaultdict

★★^{2.7, 3.4+} When you count the number of occurrences of a particular item in an iterable (say, a specific character in a string), you use an integer number as a counter. When the iterable consists of heterogeneous items and you know the number of their types ahead of time, you can use a list of counters, especially if the item types map nicely to list indexes. But if the number of types is large or unknown, a dictionary is the best counting tool. Iterate through the iterable, check if you have seen the next item before. If so, increment its counter in the dictionary. If not, create a new dictionary item for the new item in the iterable and increment it, anyway:

counter = {}
for item in iterable:
    if item ...