fixtures: fix catastrophic performance problem in reorder_items
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This makes some minor clarity and performance improvements to the code.
nicoddemus
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Jun 3, 2024
changelog/12355.bugfix.rst
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| Fix possible catasrophic performance slowdown on a certain parametrization pattern involving many higher-scoped parameters. | |||
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| Fix possible catasrophic performance slowdown on a certain parametrization pattern involving many higher-scoped parameters. | |
| Fix possible catastrophic performance slowdown on a certain parametrization pattern involving many higher-scoped parameters. |
Fix pytest-dev#12355. In the issue, it was reported that the `reorder_items` has quadratic (or worse...) behavior with certain simple parametrizations. After some debugging I found that the problem happens because the "Fix items_by_argkey order" loop keeps adding the same item to the deque, and it reaches epic sizes which causes the slowdown. I don't claim to understand how the `reorder_items` algorithm works, but if as far as I understand, if an item already exists in the deque, the correct thing to do is to move it to the front. Since a deque doesn't have such an (efficient) operation, this switches to `OrderedDict` which can efficiently append from both sides, deduplicate and move to front.
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First commit is a cleanup commit (no functional changes intended).
The second commit fixes #12355.
In the issue, it was reported that the
reorder_itemshas quadratic (or worse...) behavior with certain simple parametrizations. After some debugging I found that the problem happens because the "Fix items_by_argkey order" loop keeps adding the same item to the deque, and it reaches epic sizes which causes the slowdown.I don't claim to understand how the
reorder_itemsalgorithm works, but if as far as I understand, if an item already exists in the deque, the correct thing to do is to move it to the front. Since a deque doesn't have such an (efficient) operation, this switches toOrderedDictwhich can efficiently append from both sides, deduplicate and move to front.