How to Mock Functions Within Functions in Python

Learn how to effectively use multiple patches to mock function calls in Python unit testing, along with common troubleshooting tips and best practices.
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How to Mock Functions Within Functions in Python: A Comprehensive Guide

When working with unit testing in Python, especially when using the unittest and mock libraries, you may find yourself needing to test a function that depends on the outputs of other functions. This scenario can sometimes lead to confusion, especially when it comes to mocking multiple function calls. In this post, we'll tackle a common question: How can you mock a function call within another function using multiple patches?

The Problem

Suppose you have an API with the following functions:

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

The challenge arises when you want to test the print_contents_of_cwd function by mocking not just one but two functions, get_value_a and get_value_b, which it relies on for its inputs. The code snippet you attempted using multiple patches looked something like this:

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

The Issue Faced

Upon running the test, you might encounter an error like this:

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

This error can be puzzling and raises two important questions:

Is it allowed to use multiple patches like I did?

Why did I get a TypeError since it seems like legal syntax?

Understanding the Solution

Using Multiple Patches

The Cause of the TypeError

The TypeError you encountered arises from the way the mocking was set up. Specifically, when you apply a patch to a function, the mocked function replaces the original function with a MagicMock object. This means:

val_a and val_b are not the integer values you expect; they are MagicMock objects unless you call them.

Correcting Your Code

To fix your test case, you need to call the mocked functions to retrieve their return values. Here's how you can adjust your function call:

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

Renaming for Clarity

For better readability and to indicate your intent, consider renaming your mocks for clarity. Instead of val_a and val_b, it's more descriptive to use:

get_val_a() as the mock for get_value_a

get_val_b() as the mock for get_value_b

Here’s how you would modify the original test:

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

Conclusion

Mocking function calls within functions is a common but potentially tricky aspect of unit testing in Python. By correctly using multiple patches and ensuring you call the mocked functions, you can effectively isolate and test functionality without executing the real functions. This approach not only leads to cleaner and more reliable tests but also enhances your understanding of the mocking mechanism in Python.

For any further questions or tips regarding testing and mocking in Python, feel free to reach out! Happy coding!