PEP 760: No more bare excepts

[pablogsal]

Basic requirements (all PEP Types)

• Read and followed PEP 1 & PEP 12
• File created from the latest PEP template
• PEP has next available number, & set in filename (pep-NNNN.rst), PR title (PEP 123: <Title of PEP>) and PEP header
• Title clearly, accurately and concisely describes the content in 79 characters or less
• Core dev/PEP editor listed as Author or Sponsor, and formally confirmed their approval
• Author, Status (Draft), Type and Created headers filled out correctly
• PEP-Delegate, Topic, Requires and Replaces headers completed if appropriate
• Required sections included
  • Abstract (first section)
  • Copyright (last section; exact wording from template required)
• Code is well-formatted (PEP 7/PEP 8) and is in code blocks, with the right lexer names if non-Python
• PEP builds with no warnings, pre-commit checks pass and content displays as intended in the rendered HTML
• Authors/sponsor added to .github/CODEOWNERS for the PEP

Standards Track requirements

• PEP topic discussed in a suitable venue with general agreement that a PEP is appropriate
• Suggested sections included (unless not applicable)
  • Motivation
  • Rationale
  • Specification
  • Backwards Compatibility
  • Security Implications
  • How to Teach This
  • Reference Implementation
  • Rejected Ideas
  • Open Issues
• Python-Version set to valid (pre-beta) future Python version, if relevant
• Any project stated in the PEP as supporting/endorsing/benefiting from the PEP formally confirmed such
• Right before or after initial merging, PEP discussion thread created and linked to in Discussions-To and Post-History

---

📚 Documentation preview 📚: https://pep-previews--4015.org.readthedocs.build/

[warsaw]

There's one pretty common use case that this will break: transaction handling in the face of exceptions. E.g. from Mailman:

@public
@contextmanager
def transaction():
    """Context manager for ensuring the transaction is complete."""
    try:
        yield
    except:                                              # noqa: E722
        config.db.abort()
        raise
    else:
        config.db.commit()

This guarantees that no matter what exception occurs, any open transaction will be aborted, while in the successful condition, the transaction will be committed.

This is a general case of "when anything goes wrong, do this thing and reraise the original exception".

If you want to argue that this should use except BareException:, that's fine but please do at least describe this common-ish pattern. While I don't love except BareException: a possible mitigation would be to create an alias to BareException perhaps called exceptions.AnyException so that the code would be more readable:

@public
@contextmanager
def transaction():
    """Context manager for ensuring the transaction is complete."""
    try:
        yield
    except AnyException:
        config.db.abort()
        raise
    else:
        config.db.commit()

Another thought, though probably harder to support, would be to allow bare except only in cases where there is a bare raise in the body of the exception handler.

[JelleZijlstra]

Let's get this merged, especially since people already started discussing the idea on Discourse.

[pablogsal]

Thanks everyone for you suggestions, comments and fixes. You all rock 🤘