[deep1blue]

What does this PR do?

_PrefetchDataFetcher in src/lightning/pytorch/loops/fetchers.py uses self.batches as a FIFO queue for prefetched batches on unsized iterables (appending at the tail and popping from the head on each __next__() call).

This cleanup switches self.batches from a list (pop(0) / = []) to collections.deque (popleft() / clear()) to use the idiomatic FIFO container.

Verification

• pytest tests/tests_pytorch/loops/test_fetchers.py (98 passed)

[deependujha]

Thanks, the change is correct and deque is the more idiomatic structure for a FIFO here, so happy to take it as a cleanup.

Two notes:

• The perf framing overstates it: prefetch_batches defaults to 1 and this path only runs for unsized iterables (sized loaders skip prefetching entirely). With B this small, list.pop(0) vs deque.popleft() is a few ns per step. Could you reword the description as a readability cleanup rather than a perf fix?

• Could you drop assert isinstance(fetcher.batches, deque)? It pins an implementation detail. If you want a test, asserting that reset() empties pending prefetched batches would cover the one behavioural line that changed.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (84df182) to head (60ff6ed).
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HEAD has 16045 uploads less than BASE

Flag	BASE (84df182)	HEAD (60ff6ed)
cpu	3646	51
lightning_fabric	1701	0
pytest	1930	0
python3.12	1516	21
python3.12.7	215	3
python3.13	1481	21
python	213	3
lightning	435	6
python3.10	221	3
pytorch_lightning	1510	45
pytorch2.6	299	9
pytest-full	1716	51
pytorch2.12	205	6
pytorch2.7	103	3
pytorch2.9	199	6
pytorch2.8	196	6
pytorch2.13	317	9
pytorch2.10	201	6
pytorch2.11	196	6

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #22052     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
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  Files         270      267      -3     
  Lines       24015    23957     -58     
=========================================
- Hits        20855    18826   -2029     
- Misses       3160     5131   +1971     

[deep1blue]

Thanks @deependujha! Updated in 60ff6ed:

• Reworded the PR title and description as a readability cleanup using the idiomatic FIFO container.
• Dropped assert isinstance(fetcher.batches, deque) and replaced it with an assertion in test_prefetch_iterator verifying that fetcher.reset() empties pending prefetched batches.

Verified locally with targeted tests (nice pytest tests/tests_pytorch/loops/test_fetchers.py: 98 passed).