[BUG] Fix DataFrameGroupBy.boxplot with subplots=False fails for object columns

doc/source/whatsnew/v1.4.0.rst

@@ -542,7 +542,7 @@ Period
 
 Plotting
 ^^^^^^^^
--
+- When given non-numeric data, :meth:`DataFrame.boxplot` now raises a ``ValueError`` rather than a cryptic ``KeyError`` or ``ZeroDivsionError``, in line with other plotting functions like :meth:`DataFrame.hist`. (:issue:`43480`)
 -
 
 Groupby/resample/rolling

pandas/plotting/_matplotlib/boxplot.py

@@ -391,6 +391,11 @@ def plot_group(keys, values, ax: Axes):
             with plt.rc_context(rc):
                 ax = plt.gca()
         data = data._get_numeric_data()
+        naxes = len(data.columns)
+        if naxes == 0:
+            raise ValueError(
+                "boxplot method requires numerical columns, nothing to plot."
+            )
         if columns is None:
             columns = data.columns
         else:

pandas/tests/plotting/test_boxplot_method.py

@@ -543,6 +543,14 @@ def test_groupby_boxplot_subplots_false(self, col, expected_xticklabel):
         result_xticklabel = [x.get_text() for x in axes.get_xticklabels()]
         assert expected_xticklabel == result_xticklabel
 
+    def test_groupby_boxplot_object(self):
+        # GH 43480
+        df = self.hist_df.astype("object")
+        grouped = df.groupby("gender")
+        msg = "boxplot method requires numerical columns, nothing to plot"
+        with pytest.raises(ValueError, match=msg):
+            _check_plot_works(grouped.boxplot, subplots=False)
+
     def test_boxplot_multiindex_column(self):
         # GH 16748
         arrays = [