How to Dynamically Load a Function for Unit Testing in Python with exec()

Learn how to dynamically load functions in unit tests using Python's `exec()` method. This blog covers the common pitfalls and provides a simple solution to resolve the scope issues in your tests.
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How to Dynamically Load a Function for Unit Testing in Python with exec()

Introduction

If you're a Python developer, you may have faced scenarios where you need to dynamically load a function while running unit tests. This can be particularly useful when you want to test various implementations of a function without hardcoding them into your test suite.

In this blog, we will explore how to use the exec() function to load a function as a string during unit testing. We will discuss a common issue that developers encounter with exec() related to variable scoping and present a solution to resolve it.

Understanding the Problem

Let's consider the example below, where we want to define a function on-the-fly to be tested within a unit test scenario.

Here’s how a typical dynamic loading scenario might look in Python:

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

When you execute this code, you may encounter an error:

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

This indicates that the add function is not accessible in the context where it is being called.

Why Does This Happen?

The exec() function without any specific parameters creates the defined functions and variables in a local scope, which ceases to exist once the function call completes. As a result, the add function is undefined when your test tries to use it.

The Solution

To resolve this issue, you can modify your usage of the exec() function to define add in the global scope. This makes the function accessible throughout your test cases.

Here’s how to implement this:

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

Using globals() ensures that everything defined within param is available in the global scope where your tests can access it.

Updated Code Implementation

Below is the revised version of the class ParametrizedTestCase:

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

By applying this change, you ensure that any dynamically loaded functions can be executed in your test methods without scope-related issues.

Conclusion

Dynamically loading functions for testing can be a very powerful tool when utilized correctly. The use of exec() combined with the globals() function ensures that your dynamically created functions are accessible for your unit tests.

With this approach, you can easily adapt your tests to handle various implementations or parameters without cluttering your test code with multiple function definitions.

Happy testing!