PyCQA/flake8-bugbear

Report on uncatchable exception handlers (?)

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#349 opened on Feb 7, 2023

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Description

Was surprised bugbear didn't report this, but not encountered in live code so I don't know if it's seen much / ever in the wild:

try:
  ...
except BaseException:
  ...
except:
  ...

will report B001/E722, but in a project that uses bare excepts widely and disables those checks the except never being possible to run seems like a separate error.

try:
  ...
except BaseException:
  ...
except Exception:
  ...

Gives no warning, nor does a plausibly more common

try:
  ...
except Exception:
  ...
except ValueError:
  ...

or similar variants. Checking whether a caught exception is a built-in, and if so issubclass of a previously caught exception, seems straightforward. I quickly checked mypy and vulture as well and neither of them sees it. But this might definitely not be common enough that it's worth bothering with.

Contributor guide

Research direction
Study the flake8 bugbear plugin structure, focusing on how existing checks like B001 are implemented. Look at the AST representation of try/except blocks using Python's ast module. Identify the built in exception classes and their hierarchy. Write a new check that iterates over except handlers in a try block and uses issubclass to determine if an exception type is a subclass of any previously caught exception (excluding BaseException for bare except). Consider edge cases like variable binding (except Exception as e) and tuples of exceptions. Add a test case and register the check.
Tech stack
python
Domain
devtools
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Pythonflake8
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65