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)