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fixtures: avoid mutable arg2index state in favor of looking up the request chain #12038

Merged
merged 2 commits into from Mar 3, 2024
Merged

fixtures: avoid mutable arg2index state in favor of looking up the request chain #12038

merged 2 commits into from Mar 3, 2024

Commits on Feb 27, 2024

  1. fixtures: add _iter_chain helper method 

    Will be reused in the next commit.
    @bluetech
    bluetech committed Feb 27, 2024
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  2. fixtures: avoid mutable arg2index state in favor of looking up the … 

    …request chain

pytest allows a fixture to request its own name (directly or
indirectly), in which case the fixture with the same name but one level
up is used.

To know which fixture should be used next, pytest keeps a mutable
item-global dict `_arg2index` which maintains this state. This is not
great:

- Mutable state like this is hard to understand and reason about.

- It is conceptually buggy; the indexing is global (e.g. if requesting
  `fix1` and `fix2`, the indexing is shared between them), but actually
  different branches of the subrequest tree should not affect each
  other.

  This is not an issue in practice because pytest keeps a cache of the
  fixturedefs it resolved anyway (`_fixture_defs`), but if the cache is
  removed it becomes evident.

Instead of the `_arg2index` state, count how many `argname`s deep we are
in the subrequest tree ("the fixture stack") and use that for the index.

This way, no global mutable state and the logic is very localized and
easier to understand.

This is slower, however fixture stacks should not be so deep that this
matters much, I hope.
    @bluetech
    bluetech committed Feb 27, 2024
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