Скачать или смотреть How to Fix the name 'list2_array' is not defined Error in Python Multiprocessing

Learn how to resolve the `name 'list2_array' is not defined` issue when working with Python's multiprocessing and shared arrays. This guide provides a clear solution to get your code running smoothly.
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Understanding the name 'list2_array' is not defined Error

If you've ever tried to use Python's multiprocessing for tasks that require shared data, you may have stumbled upon the frustrating error: name 'list2_array' is not defined. This error often prevents your code from running successfully, showcasing the importance of properly defining shared variables in a multiprocessing context. In this guide, we will explore why this error occurs, and how to effectively resolve it by adapting our code to handle shared arrays correctly.

The Problem Explained

In the provided example, you are trying to filter two lists using a multiprocessing approach. Here is the simplified code that replicates the issue:

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

When this code is executed, a NameError is raised because the function func does not have access to the list2_array variable, as it is defined later in the code block. This is a common pitfall when using multiprocessing in Python—each process runs independently and doesn't necessarily recognize variables declared after the start of the process.

Expected Output:

The anticipated output might be the following, if the error were resolved:

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

An Effective Solution

To work around this issue, we need to structure our code using starmap, which allows you to pass multiple arguments into your worker function. Let’s break down the revised code step by step:

Revised Code Structure

Define the Worker Function: Modify the func function to accept two parameters—one for the number and another for the shared list.

Construct the Arguments List: Use a list comprehension to prepare the arguments for starmap—pairing each element in my_list with list2.

Use starmap for Multiprocessing: This function will enable passing both parameters easily.

Here’s the adjusted code:

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

Key Highlights

Using starmap: This approach is crucial when you need to pass multiple arguments to your processing function. Each argument can be included in a tuple that corresponds to the parameters of your function.

Clarification on Scope: Variables defined inside the if _name_ == "__main__": block are not accessible within the worker function unless passed explicitly.

Output Verification

After implementing the above changes, you can expect the output to be:

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

This confirms that each element of my_list was correctly processed in relation to list2, successfully finding matches or returning the 'Not Found' message.

Conclusion

The name 'list2_array' is not defined error can be easily resolved by understanding the scope and how to properly implement multiprocessing with shared data. By adopting the starmap method to pass arguments, you can ensure that the necessary information is accessible within your subprocesses, allowing your multiprocessing tasks to run smoothly. Happy coding!