[MRG+2] MAINT warn of future behaviour change proposed in #2610

[jnothman]

Assuming my proposal in #2610 to make PRF's labels parameter more functional and deprecate pos_label goes through at some future point, it will create a compatibility issue in the case where all of the following apply:

• y_true and y_pred contain at most two labels
• labels specifies exactly two labels
• pos_label is not None (default)
• average is not None (generally default).

I expect this case is rare atm, but since behaviour would change with #2610 or similar (unless labels changed its name), we can warn in this release (0.15) and change behaviour in 0.16 or 0.17.

[coveralls]

Coverage remained the same when pulling 28c8d0b on jnothman:future_warn_two_labels into 6091d7d on scikit-learn:master.

[agramfort]

sklearn/linear_model/cd_fast.c is modified in this PR

[jnothman]

sklearn/linear_model/cd_fast.c is modified in this PR

It was removed immediately after posting.

[ogrisel]

+1 on my side.

[arjoly]

Looks good +1

[jnothman]

I guess this should be merged, and then the real changes come after the release.