Скачать или смотреть Why exec Fails to Return Variables in Python: A Deep Dive into Scope and Local Variables

Discover why `exec` doesn't return variables properly in Python and learn how to manage variable scope with practical solutions!
---
This video is based on the question https://stackoverflow.com/q/77119558/ asked by the user 'Rodney Price' ( https://stackoverflow.com/u/6035735/ ) and on the answer https://stackoverflow.com/a/77119594/ provided by the user 'nokla' ( https://stackoverflow.com/u/20258214/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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: Python import works, read text from file and exec doesn't

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding the Issue with exec in Python

Python is celebrated for its simplicity and readability, but its behavior can sometimes lead to confusion, especially when it comes to the exec function. If you've ever run into a situation where variables defined within an exec call seem to disappear, you're not alone. In this post, we'll explore the case where a variable defined in an external Python file doesn't return as expected when called within another function, along with a clear solution to resolve this issue.

The Problem

In this scenario, you have two files:

data.py: Contains a dictionary definition.

exectest.py: Contains a function meant to read the text from data.py and execute it using exec.

The Problematic Code

Here’s a brief look at what we have:

data.py:

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

exectest.py:

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

When you try to call exectest.read_data(), you encounter a NameError, stating that testd is not defined.

Why Does This Happen?

The reason for this error lies in how variable scoping works in Python. When you execute code using the exec function:

The variables defined inside the exec are local to that call and do not automatically get added to the surrounding scope of the function or global namespace.

Hence, when the function tries to access testd, it raises a NameError since testd does not exist in the local or global scope.

The Solution: Managing Variable Scope

To resolve this issue, you need to explictly define where the variables should be stored after executing the code. Here’s how you can do that:

Step-by-Step Solution

Create a local variable dictionary: You can create an empty dictionary that will store the variables defined inside the exec.

Pass the local variable dictionary to exec: This will allow you to access the variables defined in the executed code.

Here’s the revised version of the read_data function:

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

Explanation of the Solution

local_vars = {}: This initializes an empty dictionary to hold your local variables.

exec(text, {}, local_vars): The second parameter (empty dictionary {}) indicates that no global variables are affected. The local_vars dictionary will capture everything defined in the executed code.

testd = local_vars.get('testd', None): This extracts testd from local_vars, allowing your function to return it successfully.

Conclusion

Now that we've broken down the solution, you can confidently manage the variable scope when using exec. This technique allows you to utilize exec safely and effectively, avoiding common pitfalls associated with variable accessibility.

If you've ever faced similar issues or have questions about variable scopes in Python, feel free to leave a comment below!