ENH: Add an axis parameter to DataFrame.diff

[evanpw]

No description provided.

[TomAugspurger]

+1 on adding this parameter. I think the axis should be flipped though, right? axis=0 is a difference along the index, i.e. df.iloc[1] - df.iloc[0]. Similar to how cumsum operates along the index.

See also np.diff.

[jreback]

pls add the issue number (which is the PR number, e.g. 9727)

[evanpw]

@TomAugspurger: You're right about np.diff having a different convention. All of the internal diff machinery (in core.internals.Block, core.common, and C implementations in algos) has this opposite convention as well, but maybe I'll just invert the axis in this public facing DataFrame.diff method.

[jreback]

@evanpw you need to pass the axis as self._get_block_manager_axis(axis) which handles this. frames (only) are represented with inverted axes.

[evanpw]

Ahh, that explains it.

[jreback]

@evanpw thanks!