BUG: Fix error for boxplot when using a pre-grouped DataFrame with more than one grouping

doc/source/whatsnew/v3.0.0.rst

@@ -409,7 +409,7 @@ Period
 
 Plotting
 ^^^^^^^^
--
+- Bug in :meth:`.DataFrameGroupBy.boxplot` passes a ``tuple`` instead of a ``string`` when input ``DataFrame`` is pre-grouped using more than one ``column`` (:issue:`14701`)
 -
 
 Groupby/resample/rolling

pandas/plotting/_matplotlib/boxplot.py

@@ -533,14 +533,16 @@ def boxplot_frame_groupby(
  )
  axes = flatten_axes(axes)
 
- ret = pd.Series(dtype=object)
-
+ data = {}
  for (key, group), ax in zip(grouped, axes):
  d = group.boxplot(
  ax=ax, column=column, fontsize=fontsize, rot=rot, grid=grid, **kwds
  )
  ax.set_title(pprint_thing(key))
- ret.loc[key] = d
+ # GH 14701 refactored to allow the 'key' to be passed as a tuple,
+ # which occurs when there is more than one group
+ data[key] = d
+ ret = pd.Series(data)
  maybe_adjust_figure(fig, bottom=0.15, top=0.9, left=0.1, right=0.9, wspace=0.2)
  else:
  keys, frames = zip(*grouped)

pandas/tests/plotting/test_boxplot_method.py

@@ -740,3 +740,17 @@ def test_boxplot_multiindex_column(self):
  expected_xticklabel = ["(bar, one)", "(bar, two)"]
  result_xticklabel = [x.get_text() for x in axes.get_xticklabels()]
  assert expected_xticklabel == result_xticklabel
+
+ @pytest.mark.parametrize("group", ["X", ["X", "Y"]])
+ def test_boxplot_multi_groupby_groups(self, group):
+ # GH 14701
+ rows = 20
+ df = DataFrame(
+ np.random.default_rng(12).normal(size=(rows, 2)), columns=["Col1", "Col2"]
+ )
+ df["X"] = Series(np.repeat(["A", "B"], int(rows / 2)))
+ df["Y"] = Series(np.tile(["C", "D"], int(rows / 2)))
+ grouped = df.groupby(group)
+ _check_plot_works(df.boxplot, by=group, default_axes=True)
+ _check_plot_works(df.plot.box, by=group, default_axes=True)
+ _check_plot_works(grouped.boxplot, default_axes=True)