ENH: Add sort parameter to set operations for some Indexes and adjust… by reidy-p · Pull Request #24521 · pandas-dev/pandas (original) (raw)

… tests

• Progress towards ENH: Add sort parameter to other set operations if possible #24471
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

This PR makes some progress towards adding a sort parameter with a default of True to the set operations (union, intersection, difference and symmetric_difference) on Index classes. Adding this parameter into every type of Index would result in a very big PR so I have decided to break it up and try to add it in stages. I have tried to focus on Index, MultiIndex, PeriodIndex and Intervalndex in this PR but have made some very small changes to set operations on other indices where necessary.

Some issues to consider:

• I'm not sure whether it will be possible to control the sorting behaviour of the results of all of the set operations for all of the Index types. For example, because of the way some of the set operations are implemented on some Index types the results may always be sorted even if sort=False is passed (e.g., a union operation on DatetimeIndexs may always return a sorted result in some cases even if sort=False). Perhaps this is not really a problem as long as we document it.
• There are some other corner cases that always ignore the sort parameter at the moment. For example, the intersection of an unsorted Index with itself will return the original unsorted Index even if sort=True is passed because the sort parameter is simply ignored in this case. Similarly, the intersection of an unsorted Index with an empty Index will also return the original unsorted Index even if sort=True. There is similar behaviour for the other set operations.