slicing a slice with an array without expanding the slice #10580
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dcherian
reviewed
Jul 28, 2025
xarray/core/indexing.py
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| # shortcut for the usual case | ||
| return old_indexer | ||
| if isinstance(old_indexer, slice): | ||
| if isinstance(applied_indexer, slice): | ||
| indexer = slice_slice(old_indexer, applied_indexer, size) | ||
| elif isinstance(applied_indexer, integer_types): | ||
| indexer = range(*old_indexer.indices(size))[applied_indexer] # type: ignore[assignment] | ||
| elif is_full_slice(old_indexer): |
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Does this fix point (3) in #10311 (comment)
This is "combined" with the arrayized version of BasicIndexer(slice(None), slice(None), slice(None))
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no, because for vectorized indexing we use _combine_indexers, not _index_indexer_1d (but see also my comment in #10311 about whether we can frame that particular case – fancy indexing along a single dimension – as an outer indexer)
dcherian
reviewed
Jul 28, 2025
dcherian
reviewed
Jul 28, 2025
dcherian
approved these changes
Jul 29, 2025
shoyer
reviewed
Jul 30, 2025
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The lazy indexing machinery currently uses
_index_indexer_1dto keep indexing lazy (for orthogonal indexing only), which tries very hard to combine multiple sequential indexers into a single indexer.However, it currently uses
_expand_slicewhen indexing aslicewith an array, which has the potential to run out of memory for very large dimension sizes of the indexed array.Instead of materializing, we can just combine the slice (
s) with the array (idx) by:Interestingly, this works regardless of whether
stepis negative or positive, but we do have to raise anIndexErrorif there's any value>= size.