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serhiy-storchaka

opened on Sep 22, 2023

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Feature or enhancement

The purpose of introducing multiple OSError subclasses was to allow user to write except FileNotFoundError: instead of except OSError as e: if e.errno != errno.ENOENT: raise. OSError constructor now determines the actual type of exception by the errno argument. Of course, you can also call the subclass constructor explicitly. Unfortunately, to get a good exception you need to explicitly specify errno argument: FileNotFoundError(errno.ENOENT, "No usable temporary directory found in %s" % dirlist). "Good exception" is an exception which works with code that tests the errno attribute as well as with code that uses isinstance(). There is a lot of code in the stdlib and in the wild which calls OSError subclass constructors with a single argument or even without arguments. See #109601 for example.

I propose to infer the default value for errno (and maybe for strerror) from exception class. In case if several errno's are mapped to the same exception class, one of them is chosen as the default errno. When you omit errno argument, you may still want to provide filename or other optional arguments. For this, the constructor needs to support keyword arguments. The constructor signature will be a union of the following signatures:

(errno, strerror, /, filename=None, winerror=None, filename2=None)
(strerror, /, *, filename=None, winerror=None, filename2=None)
(*, filename=None, winerror=None, filename2=None)

So, you would be able to write FileNotFound('No such file', filename=path) or even simply FileNotFound(filename=path).

cc @pitrou @warsaw

Linked PRs

• gh-109714: Improve the OSError constructor #156430
• gh-109714: Omit redundant arguments when raising OSError subclasses #156440
• gh-109714: Pass the file name and a message when raising OSError subclasses #156441

Reacted by Gregory P. Smith, Jeffrey Bosboom and Avasam

Activity

1. serhiy-storchaka

   added

   type-featureA feature request or enhancementA feature request or enhancement

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   3.13only security fixesonly security fixes

   on Sep 22, 2023
2. pitrou commented on Sep 22, 2023

   

   pitrou

   on Sep 22, 2023

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   Hmm, I wouldn't be against it, but the reason I didn't do it at the time is that errno is the system error indicator at the time was exception was raised. If you synthetize your own errno then it starts meaning something else.

   Perhaps propose this on Discourse to get more opinions?

   Reacted by GalaxySnail
3. serhiy-storchaka commented on Sep 22, 2023

   

   serhiy-storchaka

   on Sep 22, 2023

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   Perhaps propose this on Discourse to get more opinions?

   https://discuss.python.org/t/improve-oserror-constructor/34390

   Reacted by Gregory P. Smith
4. serhiy-storchaka commented on Sep 22, 2023

   

   serhiy-storchaka

   on Sep 22, 2023

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   See also #109720 -- the changes necessary to pass errno and strerror explicitly.
5. srittau commented on Sep 2, 2024

   

   srittau

   on Sep 2, 2024

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   For reference, I think the discussion on Discourse was generally in favor of this (and I agree).
6. serhiy-storchaka commented on Aug 26, 2026

   

   serhiy-storchaka

   on Aug 26, 2026

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   #156430 is the implementation.
7. serhiy-storchaka commented on Aug 27, 2026

   

   serhiy-storchaka

   on Aug 27, 2026

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   There are two follow-ups:

   • gh-109714: Omit redundant arguments when raising OSError subclasses #156440 drops now redundant errno and strerror arguments in existing code. It does not touch the Tools scripts which should be ran by older Python versions.
   • gh-109714: Pass the file name and a message when raising OSError subclasses #156441 is what remains from WIP: Always pass errno argument for OSError subclasses #109720 after adding support of automatic errno and strerror. It adds the filename argument where it is known instead of including it in the error message, and adds explicit error messages whether the default is less appropriate (for example the default for ETIMEDOUT is "Connection timed out", but not all TimeoutErrors are related to connection, and "No such resource" is better than "No such file or directory" in importlib.resources). In some sense it fixes a mild regression in gh-109714: Improve the OSError constructor #156430.
8. serhiy-storchaka

   added

   3.16new features, bugs and security fixesnew features, bugs and security fixes

   and removed

   3.13only security fixesonly security fixes

   on Aug 27, 2026

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