[mypyc] feat: enhance ForSequence with start, stop, step for IndexExpr case #20106
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This will create optimized code paths for: - `for x in seq[1:]` - `for x in seq[1:2]` - `for x in seq[1:-1]` - `for x in seq[:]` # TODO: exempt this, its invalid - `for x in seq[:2]` - `for x in seq[:-1]` - `for x in seq[1::-1]` - `for x in seq[1:2:-1]` - `for x in seq[1:-1:-1]` - `for x in seq[:2:-1]` - `for x in seq[:-1:-1]` There is no reason to actually create a sliced sequence from the original sequence when we can just grab the appropriate items from the original sequence. We are already grabbing the same number of items from the sliced sequence anyway.
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for more information, see https://pre-commit.ci
BobTheBuidler
commented
Oct 24, 2025
| assert self.stop == -1, "implement me!" | ||
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| self.step = 1 if step is None else step | ||
| assert self.step and self.step >= 1, "this should be unreachable for step None and step 0" |
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this is currently reachable
BobTheBuidler
changed the title
BobTheBuidler
commented
Oct 24, 2025
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| if isinstance(expr, IndexExpr) and isinstance(expr.index, SliceExpr): | ||
| # TODO: maybe we must not apply this optimization to list type specifically | ||
| # because the need to check length changes at each iteration? |
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This can probably still be valid for ListComp and ListExpr
def do_thing():
return [x + 4 for x in some_non_reversible_generator][1:-2:-1]not sure how useful that is though
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This will create optimized code paths for:
for x in seq[1:]for x in seq[1:2]for x in seq[1:-1]for x in seq[:]# TODO: exempt this, its invalidfor x in seq[:2]for x in seq[:-1]for x in seq[1::-1]for x in seq[1:2:-1]for x in seq[1:-1:-1]for x in seq[:2:-1]for x in seq[:-1:-1]There is no reason to actually create a sliced sequence from the original sequence when we can just grab the appropriate items from the original sequence. We are already grabbing the same number of items from the sliced sequence anyway.
(Explain how this PR changes mypy.)