Disallow literal 0 step in slice expressions#18065
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brianschubert wants to merge 3 commits intopython:masterfrom
Draft
Disallow literal 0 step in slice expressions#18065brianschubert wants to merge 3 commits intopython:masterfrom
0 step in slice expressions#18065brianschubert wants to merge 3 commits intopython:masterfrom
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mypy_primer hits look good. All hits are from tests that assert that using a step of |
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Diff from mypy_primer, showing the effect of this PR on open source code: pandas (https://github.com/pandas-dev/pandas)
+ pandas/tests/series/accessors/test_list_accessor.py:100: error: Slice step cannot be 0 [misc]
+ pandas/tests/series/accessors/test_list_accessor.py:103: error: Slice step cannot be 0 [misc]
+ pandas/tests/series/accessors/test_list_accessor.py:118: error: Slice step cannot be 0 [misc]
+ pandas/tests/indexing/test_indexing.py:716: error: Slice step cannot be 0 [misc]
sympy (https://github.com/sympy/sympy)
+ sympy/sets/tests/test_fancysets.py:273: error: Slice step cannot be 0 [misc]
+ sympy/sets/tests/test_fancysets.py:279: error: Slice step cannot be 0 [misc]
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Thinking about this more, we probably want to move mypy away from this sort of hardcoded (and lint-y) checks. Now that slices are generic, I imagine they'll be better ways we can handle this soon. Marking this as a draft for now |
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Follow up to #18063
Using a step of
0when slicing a sequence is often a runtime error:This is true for standard library sequence types (tuple, list, str, memoryview, array.array, etc.), as well as for numpy arrays, pandas series, and many other third-party sequence types (including all that use
slice.indicesinternally).Strictly speaking, creating a slice with a step of zero is possible, similar to how creating a slice with non-integer-like start/stop/step values is possible. However, just like non-integer start/stop/step values, this is usually a mistake (and one that will cause a runtime error) as opposed to an intended API. Mypy currently warns about non-integer start/stop/step values, so also warning about using a step of 0 seems consistent.
This PR is fairly liberal in what it will warn about: it disallows any expressions with a literal-
0type (or unions including a literal0) in the step position of a slice expression. Depending on how significant the mypy_primer hit is, I may relax this a bit.