in — Advanced Examples

Membership test operator; also used in for loops to iterate over items

EasyIntermediateAdvancedExpert

Custom __contains__

Making your own objects support 'in'.

python
class DateRange:
    def __init__(self, start, end):
        self.start = start
        self.end = end

    def __contains__(self, date):
        return self.start <= date <= self.end

    def __repr__(self):
        return f"DateRange({self.start}, {self.end})"

from datetime import date

q1 = DateRange(date(2024, 1, 1), date(2024, 3, 31))
print(f"Feb 15 in Q1: {date(2024, 2, 15) in q1}")
print(f"Apr 15 in Q1: {date(2024, 4, 15) in q1}")

# Without __contains__, Python falls back to __iter__
class Odds:
    def __iter__(self):
        n = 1
        while n < 20:
            yield n
            n += 2

odds = Odds()
print(f"7 in odds: {7 in odds}")
print(f"8 in odds: {8 in odds}")
Expected Output
Feb 15 in Q1: True
Apr 15 in Q1: False
7 in odds: True
8 in odds: False

Implement __contains__ for O(1) membership testing. Without it, Python falls back to iterating through __iter__, which is O(n).

Want to try these examples interactively?

Open Advanced Playground
← Intermediate ExamplesExpert Examples →