DataFrameGroupby.boxplot fails when subplots=False (#28102)

doc/source/whatsnew/v1.2.0.rst

@@ -510,6 +510,8 @@ Plotting
 - Bug in :meth:`DataFrame.plot` was rotating xticklabels when ``subplots=True``, even if the x-axis wasn't an irregular time series (:issue:`29460`)
 - Bug in :meth:`DataFrame.plot` where a marker letter in the ``style`` keyword sometimes causes a ``ValueError`` (:issue:`21003`)
 - Twinned axes were losing their tick labels which should only happen to all but the last row or column of 'externally' shared axes (:issue:`33819`)
+- Bug in :meth:`DataFrameGroupBy.boxplot` when ``subplots=False``, a KeyError would raise (:issue:`16748`)
+
 
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/plotting/_matplotlib/boxplot.py

@@ -9,6 +9,7 @@
 from pandas.core.dtypes.missing import remove_na_arraylike
 
 import pandas as pd
+import pandas.core.common as com
 
 from pandas.io.formats.printing import pprint_thing
 from pandas.plotting._matplotlib.core import LinePlot, MPLPlot
@@ -443,6 +444,15 @@ def boxplot_frame_groupby(
                 df = frames[0].join(frames[1::])
             else:
                 df = frames[0]
+
+        # GH 16748, DataFrameGroupby fails when subplots=False and `column` argument
+        # is assigned, and in this case, since `df` here becomes MI after groupby,
+        # so we need to couple the keys (grouped values) and column (original df
+        # column) together to search for subset to plot
+        if column is not None:
+            column = com.convert_to_list_like(column)
+            multi_key = pd.MultiIndex.from_product([keys, column])
+            column = list(multi_key.values)
         ret = df.boxplot(
             column=column,
             fontsize=fontsize,

pandas/tests/plotting/test_boxplot_method.py

@@ -454,3 +454,76 @@ def test_fontsize(self):
         self._check_ticks_props(
             df.boxplot("a", by="b", fontsize=16), xlabelsize=16, ylabelsize=16
         )
+
+    @pytest.mark.parametrize(
+        "col, expected_xticklabel",
+        [
+            ("v", ["(a, v)", "(b, v)", "(c, v)", "(d, v)", "(e, v)"]),
+            (["v"], ["(a, v)", "(b, v)", "(c, v)", "(d, v)", "(e, v)"]),
+            ("v1", ["(a, v1)", "(b, v1)", "(c, v1)", "(d, v1)", "(e, v1)"]),
+            (
+                ["v", "v1"],
+                [
+                    "(a, v)",
+                    "(a, v1)",
+                    "(b, v)",
+                    "(b, v1)",
+                    "(c, v)",
+                    "(c, v1)",
+                    "(d, v)",
+                    "(d, v1)",
+                    "(e, v)",
+                    "(e, v1)",
+                ],
+            ),
+            (
+                None,
+                [
+                    "(a, v)",
+                    "(a, v1)",
+                    "(b, v)",
+                    "(b, v1)",
+                    "(c, v)",
+                    "(c, v1)",
+                    "(d, v)",
+                    "(d, v1)",
+                    "(e, v)",
+                    "(e, v1)",
+                ],
+            ),
+        ],
+    )
+    def test_groupby_boxplot_subplots_false(self, col, expected_xticklabel):
+        # GH 16748
+        df = DataFrame(
+            {
+                "cat": np.random.choice(list("abcde"), 100),
+                "v": np.random.rand(100),
+                "v1": np.random.rand(100),
+            }
+        )
+        grouped = df.groupby("cat")
+
+        axes = _check_plot_works(
+            grouped.boxplot, subplots=False, column=col, return_type="axes"
+        )
+
+        result_xticklabel = [x.get_text() for x in axes.get_xticklabels()]
+        assert expected_xticklabel == result_xticklabel
+
+    def test_boxplot_multiindex_column(self):
+        # GH 16748
+        arrays = [
+            ["bar", "bar", "baz", "baz", "foo", "foo", "qux", "qux"],
+            ["one", "two", "one", "two", "one", "two", "one", "two"],
+        ]
+        tuples = list(zip(*arrays))
+        index = MultiIndex.from_tuples(tuples, names=["first", "second"])
+        df = DataFrame(np.random.randn(3, 8), index=["A", "B", "C"], columns=index)
+
+        col = [("bar", "one"), ("bar", "two")]
+        axes = _check_plot_works(df.boxplot, column=col, return_type="axes")
+
+        expected_xticklabel = ["(bar, one)", "(bar, two)"]
+        result_xticklabel = [x.get_text() for x in axes.get_xticklabels()]
+        assert expected_xticklabel == result_xticklabel