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Summary

deque(maxlen=0) keeps everything that is appended to it:

>>> d = deque(maxlen=0)
>>> d.append(1)
>>> d.append(2)
>>> list(d)
[1, 2]                    # CPython: []
>>> d.maxlen
0

A container that reports a bound of zero grows without one, so the idiom of using a zero length deque as a sink holds on to every object handed to it.

append and appendleft in crates/vm/src/stdlib/_collections.rs trim before pushing, on self.maxlen == Some(deque.len()). For an empty deque with maxlen zero that comparison is true, pop_front on an empty deque does nothing, and the push goes through anyway. CPython appends and then trims while the deque is longer than its bound, so the same two lines are enough here.

The rest of the deque already handles the case. I ran every mutating entry point with maxlen=0 against CPython 3.14 and only these two disagreed:

Why the suite is green

Lib/test/test_deque.py has test_maxlen_zero, and it exercises the constructor, extend and extendleft, which are the three paths that were already correct. It never calls append on a deque built with maxlen=0.

Test Plan

Built in a Debian container on rustc 1.98.0.

• extra_tests/snippets/stdlib_collections_deque.py gains the zero bound cases, plus the neighbouring behaviour it would be easy to break: a bounded deque still drops from the far end and only once it is full. On a build without the change the file stops at assert list(d) == [] right after the first append.
• pytest test_snippets.py -k collections_deque, both legs green, so the file holds under CPython 3.14.7 as well.
• cargo run --release -- -m test test_deque: 81 tests, 3 skipped, SUCCESS.
• cargo clippy with the flags CI uses, clean, and cargo fmt --check clean. ruff format --check and ruff check --select I clean on the snippet.
• cargo test --workspace --exclude rustpython_wasm --exclude rustpython-venvlauncher --exclude rustpython-capi: no failures.

The three clippy jobs and the WASM check are red for the reason in #8564, unrelated to this change.

Summary by CodeRabbit

• Bug Fixes

  • Corrected bounded deque behavior when adding items at capacity.
  • Ensured zero-capacity deques remain empty across append, extension, concatenation, rotation, and construction operations.
  • Fixed item eviction and insertion handling for bounded deques.

• Tests

  • Added coverage for zero-capacity and full-capacity deque scenarios.