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Learn how to fix the `Too Many Open Files` error caused by improper handling of file descriptors in Python's `multiprocessing` module.
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This video is based on the question https://stackoverflow.com/q/76137936/ asked by the user 'DrDom' ( https://stackoverflow.com/u/895544/ ) and on the answer https://stackoverflow.com/a/76153078/ provided by the user 'DrDom' ( https://stackoverflow.com/u/895544/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the Too Many Open Files Error in Python's multiprocessing Module

When working with Python's multiprocessing module to handle parallel tasks, many programmers encounter the dreaded OSError: [Errno 24] Too many open files. This can become particularly frustrating when running scripts that generate numerous temporary files. In this post, we will explore how to successfully resolve this issue by properly managing file descriptors.

Understanding the Problem

While executing an external script using multiprocessing, many users create temporary files to store intermediate data. The problem arises when these files are not properly managed, leading to a situation where file descriptors, which track open files, become exhausted.

The Error Scenario

In our case, we are processing multiple items by executing a Python script from a subprocess for each item. The typical workflow includes:

Creating two temporary files.

Writing data to these files.

Running a specified script that relies on these files.

Parsing the output to obtain results.

However, after processing a substantial number of items (approximately 3880-3920), we encountered the following error:

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

This error indicates that the operating system's limit for open files has been reached, preventing further file creation.

Root Cause of the Error

The culprit in this situation was identified as the improper handling of file descriptors. Temporary files created using tempfile.mkstemp() must not only be deleted but also have their corresponding file descriptors closed. Failing to do so means that each subprocess retains its own open references to these files, eventually leading to an accumulation that exceeds the system's limits.

The Solution

To address the issue, we need to ensure file descriptors are explicitly closed after the temporary files have been used. Here’s how to do that properly:

Updated Code Snippet

Here is the modified code to implement the solution:

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

Key Changes Made

Created File Descriptors: Instead of just creating temporary file names, we also capture their file descriptors using tempfile.mkstemp().

Use os.fdopen(): This ensures that we can manage the file descriptors properly and close them once we are done.

Explicitly Close File Descriptors: Even if a file is deleted, the corresponding file descriptor might still be open if not closed, leading to resource leaks.

Conclusion

By following the demonstrated solution, you can effectively eliminate the Too Many Open Files error in your Python scripts that utilize the multiprocessing module. Always remember to manage your file descriptors carefully—closing and cleaning them up is just as crucial as creating them in the first place. If you ever find yourself running into similar issues, consider reviewing how you handle file operations in your code.

If you found this information helpful, don't hesitate to share it with fellow Python developers who might be facing the same challenge!