B909 misses dict.keys() deletion

[skairunner]

Summary

Using this file:

import typing as t

d: t.Dict[str, int] = {
    "a": 1,
    "b": 2,
}

for key in d.keys():
    del d[key]

When I run this command:

ruff check --preview --select B909 testruff.py

I expect to see a B909 failure, but instead all tests pass. I feel like B909 should be able to catch this, especially because running this code raises a RuntimeError:

RuntimeError: dictionary changed size during iteration

For what it's worth, flake8-bugbear also fails to catch this problem.

Modifying the test code to this causes Ruff to correctly point out the B909 error:

import typing as t

d: t.Dict[str, int] = {
    "a": 1,
    "b": 2,
}

for key in d:
    del d[key]

Version

ruff 0.11.0 (2cd25ef 2025-03-14)

[dylwil3]

That seems like a reasonable extension of the rule to me, thanks!

[dylwil3]

We should also add support for detecting .items as in:

for k,v in d.items():
    if cond(v):
        del d[k]

[skairunner]

Ultimately at least for dicts, the problem is deleting any of its keys while iterating over a dictionary:

for <anything> in <some kind of dictionary iterator over d>:
  #...
  del d[<literally anything>]

The deletion doesn't even have to use a value that comes from the iterator, because all deletes are illegal. I don't know if it's difficult to encode this into Ruff, though.

Things that can go in the dictionary iterator slot include:

# Basic iterators
d
d.keys()
d.items()
# Iteration tools
enumerate(<any of the above>)
zip(<any of the above>, ...)
(most of the itertools functions)

The basic iterators and enumerate are probably the most common, though.