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Trio is an example of a library that has made use of this technique in its Language, simply by creating a container exception type. Grouping several exceptions together can be done without changes to the The user to catch their own exceptions by their types. Here, it would be possible for _exit_ to raise an ExceptionGroupĬontaining its own errors along with the user’s errors, and this would allow Sidestepping the multiple exception problem. The issue was resolved by making the cleanup code ignore errors, thus While the user’s exception was chained as the context of theĬleanup error, it was not caught by the user’s except clause Had an issue where an exception raised during cleanup in _exit_Įffectively masked an exception that the user’s code raised inside the context Tempfile.TemporaryDirectory context manager Which are due to the loss of context/cause information (communicatedīy Hypothesis Core Developer Zac Hatfield-Dodds). The difficulties with debugging that are mentioned in the link above, and )Īn ExceptionGroup mechanism as we are proposing here can resolve some of (optionally) reports all of them (see the Hypothesis documentation. Process it may find variations that generate different errors, and The Hypothesis library performsĪutomatic bug reduction (simplifying code that demonstrates a bug). Multiple errors in a complex calculation.This can be improved withĮxceptionGroups, as explained in this issue by pytest developer Ran Benita If more than one of these finalizers raises anĮxception, only the first is reported to the user. Similarly, the pytest library allows users to register finalizers whichĪre executed at teardown. To reraise all of them together (see atexit documentation. Them raise exceptions, only the last one is reraised, but it would be better

Python’s atexit.register() functionĪllows users to register functions that are called on system exit. It is an open issue how to aggregate these errors, particularly when Library’s socket.create_connection function may attempt to connect toĭifferent addresses, and if all attempts fail it needs to report that to the

Multiple failures when retrying an operation.

That work is currentlyīlocked on Python not having native language level support for exception Nurseries, was the main motivation for this PEP. Implementing a better task spawning API in asyncio, inspired by Trio That document demonstrates how difficult it is to create an effective APIįor reporting and handling multiple errors without the language changes weĪre proposing (see also the Programming Without ‘except *’ section.) Work on this PEP was initially motivated by theĭifficulties in handling MultiErrors, which are detailed in a designĭocument for an improved version, MultiError2. Library has a MultiError exception type which it raises to report aĬollection of errors. The Python standard library’s asyncio.gather() įunction provides two options: raise the first exception, or return theĮxceptions in the results list. There isn’t currentlyĪ good way for such libraries to handle situations where multiple tasks Multiple tasks and return their results in aggregate. Libraries for async concurrency provide APIs to invoke Several real world use cases are listed below. There are situations where multiple unrelated exceptions need to be propagated The chaining features introduced in PEP 3134 link togetherĮxceptions that are related to each other as the cause or context, but The interpreter is currently able to propagate at most one exception at a

A new syntax except* for handling ExceptionGroups.

Group of unrelated exceptions being propagated together.

A new standard exception type, the ExceptionGroup, which represents a.

Multiple unrelated exceptions simultaneously:

This document proposes language extensions that allow programs to raise and handle Allow mixing except: and except*: in the same try.Not Matching Naked Exceptions in except*.Applying an except* Clause on One Exception at a Time.Extend except to Handle Exception Groups.Make it Impossible to Wrap BaseExceptions in an Exception Group.Make ExceptionGroup Extend BaseException.Guido van Rossum Status : Accepted Type : Standards Track Created : 2 Python-Version : 3.11 Post-History : 2, 2 Toggle light / dark / auto colour theme PEP 654 – Exception Groups and except* Author : Irit Katriel , PEP 654 – Exception Groups and except* | Following system colour scheme Selected dark colour scheme Selected light colour scheme Python Enhancement Proposals