ENH: Styler.apply accept ndarray return with axis=None for consistency

[attack68]

• closes ENH: Styler.apply with axis=None can accept ndarray #39359
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

Also minor edits to docs to extend information and show examples.

cc @TomAugspurger

[attack68]

this is ready for review.

It fixes the functionality and edits the docs where necessary, including some better examples.

[jreback]

not sure we should change this. why can't the user just wrap the return value inside the apply.

[attack68]

not sure we should change this. why can't the user just wrap the return value inside the apply.

Dont know why this restriction was placed in the first place. If you use axis in [0,1] then your function must return 1-dim array like (numpy array allowed), but axis=None must need a DataFrame. This makes it quite painful to implement simple numpy functions like nanmax cause the user has to deal with the different cases and match the return type.

The builtin functions always have, for example, logic like this:

        if data.ndim == 1:  # Series from .apply
            return [attr if v else "" for v in extrema]
        else:  # DataFrame from .tee
            return pd.DataFrame(
                np.where(extrema, attr, ""), index=data.index, columns=data.columns
            )

I dont really understand the downside to just handling an array function output in all cases for axis.

[jreback]

ok @attack68 this is ok, yeah consistency is better here. small comments, ping on green.

[jreback]

same shape as input

[jreback]

add versionchanged 1.3

[attack68]

@jreback changes done. mostly green. fails unrelated it seems

[jreback]

thanks @attack68