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Drop what a deque with maxlen of zero is handed #8567

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Merged
youknowone merged 1 commit into from
Aug 22, 2026
+53 −4
Merged

Drop what a deque with maxlen of zero is handed #8567

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14 changes: 10 additions & 4 deletions crates/vm/src/stdlib/_collections.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -77,6 +77,10 @@ mod _collections {
fn borrow_deque_mut(&self) -> PyRwLockWriteGuard<'_, VecDeque<PyObjectRef>> {
self.deque.write()
}

fn is_over_maxlen(&self, deque: &VecDeque<PyObjectRef>) -> bool {
self.maxlen.is_some_and(|maxlen| deque.len() > maxlen)
}
}

#[pyclass(
Expand All @@ -96,20 +100,22 @@ mod _collections {
fn append(&self, obj: PyObjectRef) {
self.state.fetch_add(1);
let mut deque = self.borrow_deque_mut();
if self.maxlen == Some(deque.len()) {
deque.push_back(obj);
// Trim after pushing, so that a `maxlen` of zero drops what just
// arrived instead of popping from an empty deque and keeping it.
if self.is_over_maxlen(&deque) {
deque.pop_front();
}
deque.push_back(obj);
}

#[pymethod]
fn appendleft(&self, obj: PyObjectRef) {
self.state.fetch_add(1);
let mut deque = self.borrow_deque_mut();
if self.maxlen == Some(deque.len()) {
deque.push_front(obj);
if self.is_over_maxlen(&deque) {
deque.pop_back();
}
deque.push_front(obj);
}

#[pymethod]
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43 changes: 43 additions & 0 deletions extra_tests/snippets/stdlib_collections_deque.py
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Original file line number Diff line number Diff line change
Expand Up @@ -108,3 +108,46 @@ class D(deque):
# until the allocator gives up, so it is left out of this check.
with assert_raises(MemoryError):
deque([0]) * sys.maxsize


# maxlen=0 keeps nothing, whichever end the item arrives at.
d = deque(maxlen=0)
d.append(1)
d.appendleft(2)
assert list(d) == []
assert len(d) == 0
assert d.maxlen == 0

d = deque(maxlen=0)
d.extend("abc")
d.extendleft("abc")
d += "abc"
assert list(d) == []

assert list(deque("abc", maxlen=0)) == []
assert list(deque("ab", maxlen=0) * 3) == []
assert list(deque("ab", maxlen=0) + deque("cd")) == []

d = deque("abc", maxlen=0)
d.rotate(1)
assert list(d) == []

assert_raises(IndexError, deque(maxlen=0).insert, 0, 1)


# A bounded deque still drops from the far end, and only once it is full.
d = deque(maxlen=1)
d.append(1)
assert list(d) == [1]
d.append(2)
assert list(d) == [2]
d.appendleft(3)
assert list(d) == [3]

d = deque("ab", maxlen=3)
d.append("c")
assert list(d) == ["a", "b", "c"]
d.append("d")
assert list(d) == ["b", "c", "d"]
d.appendleft("z")
assert list(d) == ["z", "b", "c"]
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