Python's "in" and "not in" Operators: Check for Membership

Python's "in" and "not in" Operators: Check for Membership

by Leodanis Pozo Ramos Dec 19, 2022 basics best-practices python
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Python’s in and not in operators allow you to quickly determine if a given value is or isn’t part of a collection of values. This type of check is common in programming, and it’s generally known as a membership test in Python. Therefore, these operators are known as membership operators.

In this tutorial, you’ll learn how to:

  • Perform membership tests using the in and not in operators
  • Use in and not in with different data types
  • Work with operator.contains(), the equivalent function to the in operator
  • Provide support for in and not in in your own classes

To get the most out of this tutorial, you’ll need basic knowledge of Python, including built-in data types, such as lists, tuples, ranges, strings, sets, and dictionaries. You’ll also need to know about Python generators, comprehensions, and classes.

Getting Started With Membership Tests in Python

Sometimes you need to find out whether a value is present in a collection of values or not. In other words, you need to check if a given value is or is not a member of a collection of values. This kind of check is commonly known as a membership test.

Arguably, the natural way to perform this kind of check is to iterate over the values and compare them with the target value. You can do this with the help of a for loop and a conditional statement.

Consider the following is_member() function:

Python 
>>> def is_member(value, iterable):
...     for item in iterable:
...         if value is item or value == item:
...             return True
...     return False
...

This function takes two arguments, the target value and a collection of values, which is generically called iterable. The loop iterates over iterable while the conditional statement checks if the target value is equal to the current value. Note that the condition checks for object identity with is or for value equality with the equality operator (==). These are slightly different but complementary tests.

If the condition is true, then the function returns True, breaking out of the loop. This early return short-circuits the loop operation. If the loop finishes without any match, then the function returns False:

Python 
>>> is_member(5, [2, 3