Extend B019 to alru_cache and async methods - #570
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B019 warns about functools.lru_cache/cache on methods because the cache holds instance references and prevents garbage collection. The same problem applies to async_lru.alru_cache, which is the async equivalent, and to plain lru_cache/cache used on async methods (which the check previously skipped because it only visited sync function defs). Add async_lru.alru_cache and alru_cache to the recognised cache decorators and run the check on async def methods too.
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Pull request overview
This pull request expands flake8-bugbear’s B019 check to also detect cache decorators on async def instance methods, including async_lru.alru_cache, addressing the memory-leak risk described in #488.
Changes:
- Run the B019 check for
ast.AsyncFunctionDefnodes in addition toast.FunctionDef. - Extend B019’s recognized cache decorators to include
async_lru.alru_cache/alru_cache. - Update the B019 eval test file and README/changelog text to cover async-method scenarios.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
bugbear.py |
Runs B019 on async method definitions and recognizes async_lru.alru_cache / alru_cache decorators. |
tests/eval_files/b019.py |
Adds async-method test cases for cache/lru_cache and alru_cache (including called forms) and preserves classmethod/staticmethod negatives. |
README.rst |
Updates B019 documentation and changelog entry to reflect async and alru_cache coverage. |
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cooperlees
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Aug 19, 2026
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Tests pass and it all looks good to me. So thanks!
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| import async_lru | ||
| from async_lru import alru_cache |
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Does this need to be a dep for the test running? How does this not cause an error or am I forgetting we have some import mocking magic?
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This extends B019 to cover
async_lru.alru_cacheand cache decorators onasync defmethods, as suggested in #488.alru_cacheis the async port offunctools.lru_cache(maintained by aio-libs), so when it decorates a method it keeps instance references alive in exactly the same way and leads to the same memory leak B019 is meant to catch. While looking into it I noticed B019 only ran on synchronous function definitions, so even a plain@lru_cache/@cacheon anasync defmethod slipped through. I addedasync_lru.alru_cacheandalru_cacheto the recognised decorators and wired the check intovisit_AsyncFunctionDef, so all of these now report on both sync and async methods.I extended
tests/eval_files/b019.pywith an async class coveringalru_cache,async_lru.alru_cache, their called forms, andlru_cache/cacheon async methods, plus the classmethod/staticmethod negatives to make sure those still don't fire. The README description and change log are updated to match.Closes #488