Resolving AttributeError on Mocked Class in Python

Learn how to troubleshoot and resolve the `AttributeError` encountered when mocking classes in Python unit tests, specifically using the mockito library with boto3.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: AttributeError on Mocked Class in Python

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Resolving AttributeError on Mocked Class in Python: A Comprehensive Guide

Python’s mock library is a powerful tool for testing, but errors like AttributeError: 'NoneType' object has no attribute 'last_modified' can be frustrating. This article tackles the common issue of mocking classes, particularly when using the mockito library in conjunction with boto3. Let’s break it down step-by-step.

Understanding the Problem

You’re attempting to test a class that interacts with AWS S3, specifically the Cache class which is responsible for downloading cache files. In your unit test, when you try to access last_modified, an AttributeError arises because the mocking framework did not set up the mocked return values correctly.

The Cause of the Error

From the provided test code:

[[See Video to Reveal this Text or Code Snippet]]

It appears you're trying to mock last_modified on mock_bucket, which comes from the mocked client. However, if the mock setup isn't correct, this will lead to retrieving a None, which, as the error indicates, lacks the last_modified attribute.

How to Resolve the Issue

Step 1: Use the patch Decorator

Example Usage

Here’s an adapted version of your testing code using patch:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Ensure Proper Return Values

If you need to ensure the mocked object behaves like the original, such as simulating a method that returns a value, you should specify the return_value directly within the relevant mocked method:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Verify Mock Objects

Make sure to verify that your mocks are configured correctly:

Check if the Bucket method returns the expected mock bucket.

Ensure that the class methods invoked in your main code return the mock attributes you set.

Common Mistakes to Avoid

Not Setting Return Values: Always remember to include return values for your mock objects. Simply creating a mock object and expecting it to behave like a real one without doing so can lead to unexpected errors.

Misconfigurations in Patching: Make sure that the path you provide to patch corresponds exactly to where the class or method is used, not where it is defined.

Conclusion

By using mock techniques like patch, you can eliminate many common issues associated with unit testing in Python. The key outcomes to remember are:

Mock objects need return values to function as expected.

Verify your mocks to ensure the correct attributes and methods are accessible during testing.

With the steps and examples provided, you should be well-equipped to troubleshoot and resolve the AttributeError when mocking classes in your Python tests. Happy coding!