45 – Counting values that satisfy a predicate

(This is my favourite line of Python code. Really.)

To count the values of an iterable that satisfy a given predicate (a function that returns True/False) or a given condition, use the built-in sum and a generator expression:

sum(predicate(value) for value in iterable)

This idiom works in 3 parts:

1. the generator expression goes over all values you want to consider;

2. predicate(value) evaluates the condition, producing True or False; and

3. the built-in sum accumulates all the Boolean values, effectively counting the number of Trues.

If you actually have a predicate function, you might prefer the version sum(map(predicate, iterable)). If you want to use an ad-hoc expression as the condition, then use the generator expression:

ages = [42, 73, 16, 10, 4, 6]

can_vote = sum(age > 18 for age in ages)
print(can_vote)  # 2