REF, TYP: factor out _mark_right from _add_legend_handle

[MarcoGorelli]

• Ensure all linting tests pass, see here for how to run them

This PR factors out adding (right) to labels which are meant to go on the right y-axis. This does not change anything user-facing, and is simply a precursor to fixing #40044 and #39522 .

The change is that on master, only leg.get_texts() shows b (right) while ax.right_ax.get_legend_handles_labels() shows b. The former is used for constructing the user-facing legend. The reason I think it needs doing is that I believe that to address #40044 and #39522, we need to use .get_legend_handles_labels() for both handles and labels, while currently .get_legend_handles_labels is used for handles and leg.get_texts() is used for labels.

Putting a breakpoint after

pandas/pandas/plotting/_matplotlib/core.py

Line 591 in ab687ae

ax, leg, handle = self._get_ax_legend_handle(self.axes[0])

results in , on this branch:

(Pdb) ax.right_ax.get_legend_handles_labels()
([<matplotlib.lines.Line2D object at 0x7fc94adf1dc0>], ['b (right)'])

and on master:

(Pdb) ax.right_ax.get_legend_handles_labels()
([<matplotlib.lines.Line2D object at 0x7ff2892abbe0>], ['b'])

[MarcoGorelli]

codecov/patch shows the lines from _mark_right_label as not being covered - however, running pytest pandas/tests/plotting/frame/test_frame_subplots.py -k test_subplots_dup_columns hits them

[jreback]

can you add a doc-string

[MarcoGorelli]

@fangchenli I think the coverage check here is failing because the slow plotting tests' coverage isn't currently uploaded to codecov, any suggestions for how to deal with that? Should posix.yml be edited so there's a step at the end which uploads to codecov, like there is in database.yml?

[fangchenli]

@fangchenli I think the coverage check here is failing because the slow plotting tests' coverage isn't currently uploaded to codecov, any suggestions for how to deal with that? Should posix.yml be edited so there's a step at the end which uploads to codecov, like there is in database.yml?

Yes, enable coverage for the build that actually run those tests and upload the report. Codecov'll automatically merge multiple reports. xref #39758

[MarcoGorelli]

codecov check passes now @jreback

[charlesdong1991]

a minor comment on naming, could also keep as is

[charlesdong1991]

thanks @MarcoGorelli I think it's fine to merge!