remove multi subset support

Author: jreback

pandas/core/frame.py

@@ -6237,15 +6237,15 @@ def stack(self, level=-1, dropna=True):
         else:
             return stack(self, level, dropna=dropna)
 
-    def explode(self, subset: Iterable) -> "DataFrame":
+    def explode(self, column: str) -> "DataFrame":
         """
-        Create new DataFrame expanding a list-like columns.
+        Create new DataFrame expanding a specified list-like column.
 
         .. versionadded:: 0.25.0
 
         Parameters
         ----------
-        subset : list-like
+        column : str
 
         Returns
         -------
@@ -6256,9 +6256,7 @@ def explode(self, subset: Iterable) -> "DataFrame":
         Raises
         ------
         ValueError :
-            if columns & subset are not unique.
-        ValueError :
-            subset must be list-like
+            if columns of the frame are not unique.
 
         See Also
         --------
@@ -6285,7 +6283,7 @@ def explode(self, subset: Iterable) -> "DataFrame":
         2         []  1
         3     [3, 4]  1
 
-        >>>  df.explode(['A'])
+        >>>  df.explode('A')
              A  B
         0    1  1
         0    2  1
@@ -6297,24 +6295,16 @@ def explode(self, subset: Iterable) -> "DataFrame":
 
         """
 
-        if not is_list_like(subset):
-            raise ValueError("subset must be a list-like")
-        if not Index(subset).is_unique:
-            raise ValueError("subset must be unique")
+        if not is_scalar(column):
+            raise ValueError("column must be a scalar")
         if not self.columns.is_unique:
             raise ValueError("columns must be unique")
 
-        results = [self[s].explode() for s in subset]
-        result = self.drop(subset, axis=1)
-
-        # recursive merge
-        from pandas.core.reshape.merge import merge
-
-        def merger(left, right):
-            return merge(left, right, left_index=True, right_index=True)
-
-        return functools.reduce(merger, [result] + results).reindex(
-            columns=self.columns, copy=False
+        result = self[column].explode()
+        return (
+            self.drop([column], axis=1)
+            .join(result)
+            .reindex(columns=self.columns, copy=False)
         )
 
     def unstack(self, level=-1, fill_value=None):

pandas/tests/frame/test_explode.py

@@ -9,16 +9,19 @@ def test_error():
     df = pd.DataFrame(
         {"A": pd.Series([[0, 1, 2], np.nan, [], (3, 4)], index=list("abcd")), "B": 1}
     )
+    with pytest.raises(ValueError):
+        df.explode(list("AA"))
+
     df.columns = list("AA")
     with pytest.raises(ValueError):
-        df.explode(subset=list("AA"))
+        df.explode("A")
 
 
 def test_basic():
     df = pd.DataFrame(
         {"A": pd.Series([[0, 1, 2], np.nan, [], (3, 4)], index=list("abcd")), "B": 1}
     )
-    result = df.explode(subset=["A"])
+    result = df.explode("A")
     expected = pd.DataFrame(
         {
             "A": pd.Series(
@@ -30,48 +33,13 @@ def test_basic():
     tm.assert_frame_equal(result, expected)
 
 
-def test_all_columns():
-    df = pd.DataFrame(
-        {"A": pd.Series([[0, 1, 2], np.nan, [], (3, 4)], index=list("abcd")), "B": 1}
-    )
-    result = df.explode(subset=["A", "B"])
-    expected = pd.DataFrame(
-        {
-            "A": pd.Series(
-                [0, 1, 2, np.nan, np.nan, 3, 4], index=list("aaabcdd"), dtype=object
-            ),
-            "B": 1,
-        }
-    )
-    tm.assert_frame_equal(result, expected)
-
-
-def test_multiple_columns():
-    df = pd.DataFrame(
-        {
-            "A": pd.Series([[0, 1, 2], np.nan, [], (3, 4)], index=list("abcd")),
-            "B": pd.Series([[0, 1, 2], np.nan, np.nan, 3], index=list("abcd")),
-        }
-    )
-    result = df.explode(subset=["A", "B"])
-    expected = pd.DataFrame(
-        {
-            "A": [0, 0, 0, 1, 1, 1, 2, 2, 2, np.nan, np.nan, 3, 4],
-            "B": [0, 1, 2, 0, 1, 2, 0, 1, 2, np.nan, np.nan, 3, 3],
-        },
-        dtype=object,
-        index=list("aaaaaaaaabcdd"),
-    )
-    tm.assert_frame_equal(result, expected)
-
-
 def test_usecase():
     # explode a single column
     # gh-10511
     df = pd.DataFrame(
         [[11, range(5), 10], [22, range(3), 20]], columns=["A", "B", "C"]
     ).set_index("C")
-    result = df.explode(["B"])
+    result = df.explode("B")
 
     expected = pd.DataFrame(
         {
@@ -89,7 +57,7 @@ def test_usecase():
         [["2014-01-01", "Alice", "A B"], ["2014-01-02", "Bob", "C D"]],
         columns=["dt", "name", "text"],
     )
-    result = df.assign(text=df.text.str.split(" ")).explode(["text"])
+    result = df.assign(text=df.text.str.split(" ")).explode("text")
     expected = pd.DataFrame(
         [
             ["2014-01-01", "Alice", "A"],