ENH: allow for top and mid-level assignment to DataFrames with MultIndex columns #7475

pandas/core/frame.py

@@ -3189,10 +3189,20 @@ def _setitem_array(self, key, value):
             self.iloc[indexer] = value
         else:
             if isinstance(value, DataFrame):
-                if len(value.columns) != len(key):
-                    raise ValueError("Columns must be same length as key")
-                for k1, k2 in zip(key, value.columns):
-                    self[k1] = value[k2]
+                columns = value.columns
+                if len(columns) == len(key):
+                    for k1, k2 in zip(key, columns):
+                        self[k1] = value[k2]
+                elif isinstance(columns, MultiIndex):
+                    levels0 = columns.levels[0]
+                    if len(levels0) == len(key):
+                        for k1, k2 in zip(key, levels0):
+                            self[k1] = value[k2]
+                else:
+                    raise ValueError(
+                        "Key must be same length as columns or top level of "
+                        "MultiIndex"
+                    )
             else:
                 self.loc._ensure_listlike_indexer(key, axis=1, value=value)
                 indexer = self.loc._get_listlike_indexer(
@@ -3221,19 +3231,42 @@ def _setitem_frame(self, key, value):
     def _set_item_frame_value(self, key, value: "DataFrame") -> None:
         self._ensure_valid_index(value)
 
-        # align right-hand-side columns if self.columns
-        # is multi-index and self[key] is a sub-frame
-        if isinstance(self.columns, MultiIndex) and key in self.columns:
-            loc = self.columns.get_loc(key)
-            if isinstance(loc, (slice, Series, np.ndarray, Index)):
-                cols = maybe_droplevels(self.columns[loc], key)
-                if len(cols) and not cols.equals(value.columns):
-                    value = value.reindex(cols, axis=1)
-
-        # now align rows
-        value = _reindex_for_setitem(value, self.index)
-        value = value.T
-        self._set_item_mgr(key, value)
+        # standardized key info
+        key_tup = key if isinstance(key, tuple) else (key,)
+        key_len = len(key_tup)
+
+        if key in self.columns or key_len == self.columns.nlevels:
+            # align right-hand-side columns if self.columns
+            # is multi-index and self[key] is a sub-frame
+            if isinstance(self.columns, MultiIndex) and key in self.columns:
+                loc = self.columns.get_loc(key)
+                if isinstance(loc, (slice, Series, np.ndarray, Index)):
+                    cols = maybe_droplevels(self.columns[loc], key)
+                    if len(cols) and not cols.equals(value.columns):
+                        value = value.reindex(cols, axis=1)
+
+            # now align rows
+            value = _reindex_for_setitem(value, self.index)
+            value = value.T
+            self._set_item_mgr(key, value)
+        else:
+            if key_len + value.columns.nlevels != self.columns.nlevels:
+                raise ValueError(
+                    "Must pass key/value pair that conforms with number of column "
+                    "levels"
+                )
+
+            # fill out keys as necessary
+            if value.columns.nlevels > 1:
+                key_list = [key_tup + i for i in value.columns]
+            else:
+                key_list = [key_tup + (i,) for i in value.columns]
+            items = MultiIndex.from_tuples(key_list)
+
+            # align and append block
+            value = _reindex_for_setitem(value, self.index)
+            value = value.T
+            self._mgr.append_block(items, value)
 
     def _iset_item_mgr(self, loc: int, value) -> None:
         self._mgr.iset(loc, value)

pandas/core/internals/managers.py

@@ -1232,6 +1232,26 @@ def insert(self, loc: int, item: Label, value, allow_duplicates: bool = False):
                 stacklevel=5,
             )
 
+    def append_block(self, items, values):
+        base, size = len(self.items), len(items)
+
+        new_axis = self.items.append(items)
+        block = make_block(
+            values=values, ndim=self.ndim, placement=slice(base, base + size)
+        )
+
+        blk_no = len(self.blocks)
+        self._blklocs = np.append(self.blklocs, range(size))
+        self._blknos = np.append(self.blknos, size * (blk_no,))
+
+        self.axes[0] = new_axis
+        self.blocks += (block,)
+
+        self._known_consolidated = False
+
+        if len(self.blocks) > 100:
+            self._consolidate_inplace()
+
     def reindex_axis(
         self,
         new_index,

pandas/tests/frame/indexing/test_indexing.py

@@ -119,7 +119,7 @@ def test_setitem_list(self, float_frame):
         tm.assert_series_equal(float_frame["B"], data["A"], check_names=False)
         tm.assert_series_equal(float_frame["A"], data["B"], check_names=False)
 
-        msg = "Columns must be same length as key"
+        msg = "Key must be same length as columns or top level of MultiIndex"
         with pytest.raises(ValueError, match=msg):
             data[["A"]] = float_frame[["A", "B"]]
         newcolumndata = range(len(data.index) - 1)

pandas/tests/indexing/multiindex/test_multiindex.py

@@ -1,4 +1,5 @@
 import numpy as np
+import pytest
 
 import pandas._libs.index as _index
 from pandas.errors import PerformanceWarning
@@ -93,3 +94,102 @@ def test_multiindex_with_datatime_level_preserves_freq(self):
         result = df.loc[0].index
         tm.assert_index_equal(result, dti)
         assert result.freq == dti.freq
+
+    def test_multiindex_get_loc_list_raises(self):
+        # https://github.com/pandas-dev/pandas/issues/35878
+        idx = pd.MultiIndex.from_tuples([("a", 1), ("b", 2)])
+        msg = "unhashable type"
+        with pytest.raises(TypeError, match=msg):
+            idx.get_loc([])
+
+    def test_multiindex_frame_assign(self):
+        df0 = DataFrame({"a": [0, 1, 2, 3], "b": [3, 4, 5, 6]})
+        df1 = pd.concat({"x": df0, "y": df0}, axis=1)
+        df2 = pd.concat({"q": df1, "r": df1}, axis=1)
+
+        # level one assign
+        result = df2.copy()
+        result["m"] = result["q"] + result["r"]
+        expected = pd.concat({"q": df1, "r": df1, "m": 2 * df1}, axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        # level one assign - multiple
+        result = df2.copy()
+        result[["m", "n"]] = 2 * result[["q", "r"]]
+        expected = pd.concat({"q": df1, "r": df1, "m": 2 * df1, "n": 2 * df1}, axis=1)
+        tm.assert_frame_equal(result, expected)
+
+        # level two assign
+        result = df2.copy()
+        result["m", "x"] = df2["q", "x"] + df2["q", "y"]
+        expected = pd.concat(
+            {"q": df1, "r": df1, "m": pd.concat({"x": 2 * df0}, axis=1)}, axis=1
+        )
+        tm.assert_frame_equal(result, expected)
+
+        # level two assign - multiple (seems like getitem is not caught up here)
+        result = df2.copy()
+        result[[("m", "x"), ("n", "y")]] = 2 * df2["q"]
+        expected = pd.concat(
+            {
+                "q": df1,
+                "r": df1,
+                "m": pd.concat({"x": 2 * df0}, axis=1),
+                "n": pd.concat({"y": 2 * df0}, axis=1),
+            },
+            axis=1,
+        )
+        tm.assert_frame_equal(result, expected)
+
+        # level three assign
+        result = df2.copy()
+        result["m", "x", "a"] = df2["q", "x", "a"] + df2["q", "x", "b"]
+        expected = pd.concat(
+            {
+                "q": df1,
+                "r": df1,
+                "m": pd.concat(
+                    {"x": pd.concat({"a": df0["a"] + df0["b"]}, axis=1)}, axis=1
+                ),
+            },
+            axis=1,
+        )
+        tm.assert_frame_equal(result, expected)
+
+        # level three assign - multiple
+        result = df2.copy()
+        result[[("m", "x", "a"), ("n", "y", "b")]] = 2 * df2["q", "x"]
+        expected = pd.concat(
+            {
+                "q": df1,
+                "r": df1,
+                "m": pd.concat({"x": pd.concat({"a": 2 * df0["a"]}, axis=1)}, axis=1),
+                "n": pd.concat({"y": pd.concat({"b": 2 * df0["b"]}, axis=1)}, axis=1),
+            },
+            axis=1,
+        )
+        tm.assert_frame_equal(result, expected)
+
+        # invalid usage
+        msg = "Must pass key/value pair that conforms with number of column levels"
+        msg2 = "Wrong number of items passed 2, placement implies 1"
+
+        # too few levels at level one
+        with pytest.raises(ValueError, match=msg):
+            df2["m"] = df0
+
+        # too few levels at level two - this appears to be desired
+        # with pytest.raises(ValueError, match=msg):
+        #     df2["m", "x"] = df0["a"]
+
+        # too many levels at level one
+        with pytest.raises(ValueError, match=msg):
+            df2["m"] = df2
+
+        # too many levels at level two
+        with pytest.raises(ValueError, match=msg):
+            df2["m", "x"] = df1
+
+        # too many levels at level three
+        with pytest.raises(ValueError, match=msg2):
+            df2["m", "x", "a"] = df0