contains

Learn the in operator with __contains__.

The contains method

__contains__(self, item) defines the in operator. Writing item in obj calls __contains__; the result is True or False.

Example: class Box:\n def __contains__(self, item):\n return item in self.items. Writing 2 in b returns whether 2 is in b.items. This makes membership checks natural for collection classes.

Membership check

If __contains__ is not defined, Python falls back to __iter__ or __getitem__ for membership checks; but __contains__ is faster and more direct.

Example: in the Box class, writing 2 in b calls __contains__. This method searches the inner list and returns True/False. not in also works automatically; no separate method is needed.

Exercise Add contains to Box

Define the Box class. Add def __init__(self, items): with self.items = items in the body. Add def __contains__(self, item):; in the body write return item in self.items. Then call b = Box([1, 2, 3]) and print(2 in b).

Show example solution

# Define the Box class
class Box:
    def __init__(self, items):
        self.items = items
    def __contains__(self, item):
        return item in self.items

b = Box([1, 2, 3])
print(2 in b)

Test yourself

Match the __contains__ concepts

__contains__

in operator
item in self.items

Inner check
True/False

Return value
Which method does x in obj call?
- __contains__
- __iter__
- __getitem__
- __in__
Show answer

__contains__ — The in operator calls the __contains__ method.

Complete the __contains__ signature

class Box:
    def __contains__(self, ____):
        return item in self.items

Show answer

class Box:
    def __contains__(self, item):
        return item in self.items

Next lesson: Summary - Magic methods

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