PERF: avoid unnecessary recoding in CategoricalIndex._create_categorical #21506
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git diff upstream/master -u -- "*.py" | flake8 --diff
This issue was found when looking for a solution to #20395. I've found that
CategoricalIndex._create_categorical
makes an unnecessary call toCategorical._set_dtype
when passed a dtype that is equal to self.dtype:Internally, some operations in Pandas pass self.dtype to
CategoricalIndex._create_categorical
and onwards to_set_dtype
, which is a very slow code path.By avoiding calling
_set_dtype
unnecessarily, some operations become faster. For example:As
Categoricalndex._create_categorical
is called directly or indirectly by various methods (e.g.CategoricalIndex._shallow_copy
), there are probably other places where this speedup is relevant also.