Real Python: Checking for Membership Using Python's "in" and "not in" Operators

Python’s in and not in operators allow you to quickly check if a given value is or isn’t part of a collection of values. This type of check is generally known as a membership test in Python. Therefore, these operators are known as membership operators.

By the end of this video course, you’ll understand that:

• The in operator in Python is a membership operator used to check if a value is part of a collection.
• You can write not in in Python to check if a value is absent from a collection.
• Python’s membership operators work with several data types like lists, tuples, ranges, and dictionaries.
• You can use operator.contains() as a function equivalent to the in operator for membership testing.
• You can support in and not in in custom classes by implementing methods like .__contains__(), .__iter__(), or .__getitem__().

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https://realpython.com/courses/checking-membership-in-not-in-operators/