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Learn how to properly raise a `FileNotFoundError` in Python when a specified file is not found during the search process.
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This video is based on the question https://stackoverflow.com/q/76610952/ asked by the user 'Muhammad Shaheer Munir' ( https://stackoverflow.com/u/7769333/ ) and on the answer https://stackoverflow.com/a/76611054/ provided by the user 'h4z3' ( https://stackoverflow.com/u/11516012/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving the File Not Found Exception in Python File Handling

When working with file operations in Python, you might encounter situations where your code needs to search for a specific file in a directory and its subdirectories. A common issue arises when you want to raise an exception if the file is not found but the initial attempt does not work as expected. In this guide, we will explore a common problem related to handling file not found exceptions and how to correctly implement a solution.

The Problem

You have a method that takes a path and a file_name as arguments, searching for the specified file within the given directory (including its subdirectories). If the file is found, it is read correctly. However, if the file is not found, your code fails to raise a FileNotFoundError exception. Instead, it ends up returning either None or, worse, no useful feedback to the user.

The initial code structure looks as follows:

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

However, as you may have noticed, this implementation does not effectively raise an exception when the file is not found.

Understanding the Issue

Exception Handling: The try-except block in your original code attempts to handle the FileNotFoundError, but it only catches the error if an attempt to open a file fails (which doesn’t occur in this case since the code skips over files that don’t match).

Loop Logic: The subsequent logic results in a situation where if no matching files are found, you simply exit the loop without any exception being raised—leading to a return of None.

Misplaced Exception: Your second attempt to raise a FileNotFoundError within the loop effectively fails because it is conditioned in such a way that it will rarely execute.

The Solution

To resolve this issue, you should ensure that you raise the exception after all files have been searched and none have been found. Here’s the corrected function:

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

Key Changes

Remove Try-Except Block: The try-except block is not needed for this purpose. Instead, you’ll directly raise the FileNotFoundError after the loop if no file matches.

Clear Exception Message: This provides a more informative error message indicating not only that the file does not exist but also where the search was conducted.

Validating with Pytest

Once you've adjusted your function, ensure to validate it using your pytest framework like so:

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

This test should now pass, confirming that your code effectively raises the expected exception when a file is not found.

Conclusion

In summary, handling file search operations in Python and properly managing exceptions such as FileNotFoundError is essential for writing robust applications. By adopting the suggested corrections and ensuring proper exception handling, you can enhance your code's reliability and improve user feedback when files are missing.

Keep these principles in mind when managing file I/O in Python, and happy coding!