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Discover the best practices for handling variables in Python functions, focusing on when to use global variables versus passing parameters.
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Is it Better to Use Global Variables or Pass to Function in Python?

When programming in Python, especially within frameworks like AWS Lambda, developers often face the decision of how to handle variables across functions. A common question that arises is whether to use global variables or to pass variables as parameters. This guide will answer that question in the context of a specific scenario involving a variable known as file_id. We will explore the implications of both approaches and recommend best practices for clean and maintainable code.

The Problem Scenario

Imagine you have an AWS Python Lambda function consisting of seven distinct functions. You extract a variable named file_id from the payload in one function, and later invoke another function where file_id will only be used in the rare case of an error. The question is: should you make file_id global or should you pass it as an additional parameter? Here’s a snippet of the code illustrating the scenario:

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

Solution: Passing the Variable as a Parameter

Why Passing as a Parameter is the Preferred Approach

Maintainability: By passing file_id as a parameter, you make call_external_service function cleaner and more manageable. This approach clearly defines what inputs the function expects and reduces dependencies on external state.

Testability: Functions that operate solely on parameters are easier to test. You can create unit tests for call_external_service where you can provide different file_id values without relying on a global state. This isolation leads to more robust tests and fewer side effects.

Avoid Side Effects: Global variables can introduce hidden side effects in your application. If other parts of your code inadvertently change the value of file_id, it can lead to bugs that are hard to trace.

Implementing the Change

To implement this change, simply modify the call_external_service function to accept an additional parameter file_id, like so:

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

Then, ensure to pass file_id when calling call_external_service from execute:

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

Conclusion

While it may seem tempting to use global variables in situations where a variable is only occasionally needed, the practice of passing variables as parameters is widely considered superior. It enhances the code's readability, maintainability, and testability, making it easier for others (and yourself) to understand and work with the function in the future.

Decisions on how to handle variables in your code can significantly impact the overall quality and robustness of your software projects. Embrace best practices and strive for clean code!