DEPR: is_copy

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

[has2k1]

Although is_copy was meant to be used internally, it leaked out because it was a solution to a justifiable problem i.e you slice a dataframe, you know you created independent dataframe and you want no complaints. A copy() operation on the new dataframe is wasteful, especially in library code.

plotnine uses is_copy in about 20 locations, and almost every call from the user will hit is_copy at least 10 times, and the number goes up linearly depending on different factors. The example on the documentation page goes through about 80 of them.

[jorisvandenbossche]

To repeat myself from the issue: I think @amueller use case is valid one that we should try to support. If not through is_copy, then in another way. (btw, @jreback it would be nice to at least answer to my objection on the issue here before merging)

In case of sklearn’s train_test_split, they are using integer positional indexing, which will (as far as I understand fancy indexing in numpy) never return a view, not even in case of DataFrames with single dtypes. So they can be sure that their subset of a frame is a copy (which they want) and a SettingWithCopyWarning should never be raised on the frames returned by that function.

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

Explicitly taking a copy is not mentioned in those docs, so could certainly be added.