How to Mock.patch a Function with Itself in Python

Learn how to effectively mock a function with itself in Python unit testing, utilizing the unittest library and patch decorators to control function parameters.
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How to Mock.patch a Function with Itself in Python

When working with unit tests in Python, you may encounter a scenario where you need to replace a function with itself while changing its parameters. This can be tricky, especially during integration tests where direct access to function parameters may not be available. In this guide, we’ll explore how to achieve this using the unittest.mock library in Python.

The Problem

Imagine you have a function that performs operations based on a parameter called devmode. You’d like to change the behavior of this function in a test, but since you are testing how it integrates within another function, you don't have direct access to modify the calls.

To illustrate, consider a function called myfunction within a module, and you want to replace:

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

with:

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However, when you attempt to patch the function with itself using decorators, you may unintentionally create a situation that leads to infinite recursion, resulting in an error such as "Too many recursions".

The Solution: Keeping a Reference to the Unpatched Function

To avoid the recursive call issue while allowing myfunction to behave differently, you can keep a reference to the unpatched version of the function before the test executes. Here’s how to do this effectively:

Step-by-Step Implementation

Keep a Reference to the Original Function: Prior to applying the patch, save a reference to the original function outside of your test methods.

Create a Patch Decorator: Use the @ patch decorator to redefine the behavior of your function. In this case, you can specify a lambda that calls the unpatched version of the function with the desired parameters.

Example Code

Here's an example to illustrate these steps:

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

Explanation of the Example

The myfunction is defined normally and takes an optional parameter devmode.

Inside MyTest, we store a reference to the original myfunction in the class-level attribute _unpatched.

The @ patch decorator replaces myfunction with a lambda function that calls the unpatched version with devmode=True.

When test_myfunction_with_devmode is executed, the original function logic runs, but it considers devmode=True, thus printing True as expected.

Important Notes

Passing Arguments: Remember that if your original function accepts additional parameters, you might need to pass them along from the lambda.

Integration Testing Considerations: This method is particularly useful for integration tests where you want to simulate different behavior of a function without modifying its internal structure.

Conclusion

Mocking a function with itself in Python unit testing can be accomplished without falling into the trap of infinite recursion by utilizing references properly and modifying the behavior through lambda functions within decorators. This approach can help you effectively test how various parameters influence function behavior without needing to change the functional code, maintaining the integrity of your tests.

With this knowledge, you're now equipped to handle complex testing scenarios with ease!