Python operator.contains() Function: Usage and Examples

Test whether a collection contains a given item.

What You Will Learn

contains(container, item) performs item in container. Its argument order is container first, unlike the infix spelling item in container.

Step-by-Step Example

The example below makes the behavior visible with small input values that you can change safely.

from operator import contains
print(contains(['draft', 'ready'], 'ready'))
print(contains('notebook', 'book'))

How the Example Works

The imports provide the standard-library tools used in the demonstration: operator.
The print() calls make the return values or state visible so you can compare them with the explanation and check your prediction.

Expected output from the example:

True
True

When You Would Use This

These tools are useful when composing small reusable functions, adapting callbacks, or eliminating repeated work. Pay attention to whether the operation returns a new object, updates state, or produces lazy results. Test a small success case and a boundary case before integrating it into an application.

Practice: Modify the Code

Check whether each user role is included in an allowed-role set.

Verify the changed output against the behavior described above, including the relevant invalid or boundary case.

For complete parameters and version details, consult the official Python documentation.

Common Beginner Mistakes

Reversing arguments because the Python in expression places the item first.
Assuming the operation has the same behavior for missing values, empty inputs and regular inputs.

Key Takeaways

Test whether a collection contains a given item.
contains(container, item) performs item in container.
Apply this operation only after checking the assumptions and input conditions shown in the example.