DEPR: is_copy

[jreback]

this has always been an internal attribute. We can simply replace by ._is_copy and provide a deprecation warning on the property.

[jorisvandenbossche]

is_copy is used in the most popular google hits about SettingWithCopyWarning, eg https://stackoverflow.com/questions/20625582/how-to-deal-with-settingwithcopywarning-in-pandas and https://www.dataquest.io/blog/settingwithcopywarning/
(actually in the first you are using it yourself in an answer :-))

What is the alternative for power users in library code if they want to avoid an extra unnecessary copy?

(I never use it myself, as I just do copy() to avoid if needed, so I am personally not attached to this method, but typically don't handle with data where doing an extra copy() is a problem. But I think the use case mentioned by Andreas in #18799 is a valid one)

[amueller]

So the suggested solution is to use copy() and that introduces an additional copy? Can you please document that? It seems a bit counter-intuitive that a library needs to do an expensive operation to avoid a warning in user-space, but if that's the suggested (and documented) fix I'll live with it.

[jreback]

@amueller not sure there is anything to add to: http://pandas.pydata.org/pandas-docs/stable/indexing.html#returning-a-view-versus-a-copy

you are chain indexing, which violates view semantics. the point is it may work, but there are cases where it won't. without copy-on-write, you must copy.