Fix `itertools.pairwise` #5884

ShaharNaveh · 2025-07-01T20:03:00Z

Summary by CodeRabbit

Refactor
- Improved internal handling of the pairwise iterator for better code clarity and maintainability. No changes to user-facing behavior.

coderabbitai · 2025-07-01T20:03:06Z

Walkthrough

The update refactors the PyItertoolsPairwise iterator in Rust. The constructor now uses a more concise syntax with Self { ... }. The next method's handling of the lock for the old value is made more explicit by acquiring the read lock into a local variable before cloning and updating the stored value with a write lock. No changes were made to public signatures or control flow.

Changes

File(s)	Change Summary
vm/src/stdlib/itertools.rs	Refactored `PyItertoolsPairwise` constructor to use `Self { ... }`; improved lock handling in `next` by storing read lock in a local variable before cloning and explicitly updating stored value with write lock.

Poem

A pairwise hop, a tuple leap,
With locks now held, not just a peep.
The code refined, the flow made clear,
No change in shape, just handling dear.
In itertools, we stride anew,
With Rusty paws, the path we hew. 🐇

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vm/src/stdlib/itertools.rs (2 hunks)

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vm/src/stdlib/itertools.rs

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ShaharNaveh · 2025-07-01T20:12:56Z

Lib/test/test_itertools.py

@@ -1183,7 +1183,7 @@ def test_pairwise(self):
            pairwise(None)                                  # non-iterable argument

    # TODO: RUSTPYTHON
-    @unittest.skip("TODO: RUSTPYTHON, hangs")
+    @unittest.expectedFailure
    def test_pairwise_reenter(self):


I've tried to make this test to pass as well, but for some reason when it's reentering the iter old gets reset to None, causing the next old to advance the iter one more time than needed.
So for the first assert:

Expected: ([1], ([1], [3])) Got: ([1], ([3], [4]))

If someone can guide me where to investigate it would be great:)

when new returned Err, the value of old need to be stored in zelf.old to prevent not to be lost

youknowone · 2025-07-02T00:29:19Z

vm/src/stdlib/itertools.rs

@@ -1959,18 +1959,25 @@ mod decl {

    impl IterNext for PyItertoolsPairwise {
        fn next(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<PyIterReturn> {
-            let old = match zelf.old.read().clone() {
+            let old_guard = {


This is not a clone of old_guard, but clone of old, right?

Suggested change

let old_guard = {

let old = {

or

Suggested change

let old_guard = {

let old_cloned = {

yup, that's a more fitting name

youknowone · 2025-07-02T11:48:30Z

vm/src/stdlib/itertools.rs

                    PyIterReturn::StopIteration(v) => return Ok(PyIterReturn::StopIteration(v)),
                },
                Some(obj) => obj,
            };
+
            let new = match zelf.iterator.next(vm)? {
                PyIterReturn::Return(obj) => obj,
                PyIterReturn::StopIteration(v) => return Ok(PyIterReturn::StopIteration(v)),


old seems like to be stored in this case.

youknowone · 2025-07-02T11:48:49Z

vm/src/stdlib/itertools.rs

-                    PyIterReturn::Return(obj) => obj,
+                    PyIterReturn::Return(obj) => {
+                        // Needed for when we reenter
+                        *zelf.old.write() = Some(obj.clone());


instead of immediately after getting the value

IIRC I tried to do it here first but then the tests failed, will recheck

oh, you are right. I was misunderstanding the spec of pairwise. Thank you!

I tried to save it in the new block, then the tests won't pass:/

ShaharNaveh added 3 commits July 1, 2025 22:56

Fix pairwise hangs

23241b3

Don't skip failing tests;)

083a6f5

Drop lock of read, not write

e6024f1

ShaharNaveh commented Jul 1, 2025

View reviewed changes

Don't set flags

5acdfe8

youknowone requested changes Jul 2, 2025

View reviewed changes

ShaharNaveh added 2 commits July 2, 2025 10:19

Rename old_gaurd -> old_clone

1c0a125

Fix test_pairwise_reenter

193b8a1

ShaharNaveh changed the title ~~Fix pairwise hang~~ Fix itertools.pairwise Jul 2, 2025

youknowone reviewed Jul 2, 2025

View reviewed changes

youknowone approved these changes Jul 2, 2025

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youknowone merged commit 18fdf85 into RustPython:main Jul 2, 2025
12 checks passed

ShaharNaveh deleted the fix-pairwise-hang branch July 2, 2025 12:12

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Fix `itertools.pairwise` #5884

Fix `itertools.pairwise` #5884

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Fix itertools.pairwise #5884

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Fix `itertools.pairwise` #5884

Fix `itertools.pairwise` #5884

ShaharNaveh commented Jul 1, 2025 •

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