BUG: Index/Series.to_frame not respecting explicit name=None (#44212)

Author: jbrockmendel

doc/source/whatsnew/v1.4.0.rst

@@ -643,6 +643,7 @@ Other
 - Bug in :meth:`CustomBusinessMonthBegin.__add__` (:meth:`CustomBusinessMonthEnd.__add__`) not applying the extra ``offset`` parameter when beginning (end) of the target month is already a business day (:issue:`41356`)
 - Bug in :meth:`RangeIndex.union` with another ``RangeIndex`` with matching (even) ``step`` and starts differing by strictly less than ``step / 2`` (:issue:`44019`)
 - Bug in :meth:`RangeIndex.difference` with ``sort=None`` and ``step<0`` failing to sort (:issue:`44085`)
+- Bug in :meth:`Series.to_frame` and :meth:`Index.to_frame` ignoring the ``name`` argument when ``name=None`` is explicitly passed (:issue:`44212`)
 
 .. ***DO NOT USE THIS SECTION***
 

pandas/core/indexes/base.py

@@ -1510,7 +1510,9 @@ def to_series(self, index=None, name: Hashable = None) -> Series:
 
         return Series(self._values.copy(), index=index, name=name)
 
-    def to_frame(self, index: bool = True, name: Hashable = None) -> DataFrame:
+    def to_frame(
+        self, index: bool = True, name: Hashable = lib.no_default
+    ) -> DataFrame:
         """
         Create a DataFrame with a column containing the Index.
 
@@ -1561,7 +1563,7 @@ def to_frame(self, index: bool = True, name: Hashable = None) -> DataFrame:
         """
         from pandas import DataFrame
 
-        if name is None:
+        if name is lib.no_default:
             name = self.name or 0
         result = DataFrame({name: self._values.copy()})
 

pandas/core/indexes/multi.py

@@ -1684,7 +1684,7 @@ def unique(self, level=None):
             level = self._get_level_number(level)
             return self._get_level_values(level=level, unique=True)
 
-    def to_frame(self, index: bool = True, name=None) -> DataFrame:
+    def to_frame(self, index: bool = True, name=lib.no_default) -> DataFrame:
         """
         Create a DataFrame with the levels of the MultiIndex as columns.
 
@@ -1736,7 +1736,7 @@ def to_frame(self, index: bool = True, name=None) -> DataFrame:
         """
         from pandas import DataFrame
 
-        if name is not None:
+        if name is not lib.no_default:
             if not is_list_like(name):
                 raise TypeError("'name' must be a list / sequence of column names.")
 

pandas/core/series.py

@@ -1317,7 +1317,7 @@ def repeat(self, repeats, axis=None) -> Series:
         )
 
     @deprecate_nonkeyword_arguments(version=None, allowed_args=["self", "level"])
-    def reset_index(self, level=None, drop=False, name=None, inplace=False):
+    def reset_index(self, level=None, drop=False, name=lib.no_default, inplace=False):
         """
         Generate a new DataFrame or Series with the index reset.
 
@@ -1427,6 +1427,9 @@ def reset_index(self, level=None, drop=False, name=None, inplace=False):
         """
         inplace = validate_bool_kwarg(inplace, "inplace")
         if drop:
+            if name is lib.no_default:
+                name = self.name
+
             new_index = default_index(len(self))
             if level is not None:
                 if not isinstance(level, (tuple, list)):
@@ -1448,6 +1451,14 @@ def reset_index(self, level=None, drop=False, name=None, inplace=False):
                 "Cannot reset_index inplace on a Series to create a DataFrame"
             )
         else:
+            if name is lib.no_default:
+                # For backwards compatibility, keep columns as [0] instead of
+                #  [None] when self.name is None
+                if self.name is None:
+                    name = 0
+                else:
+                    name = self.name
+
             df = self.to_frame(name)
             return df.reset_index(level=level, drop=drop)
 
@@ -1697,7 +1708,7 @@ def to_dict(self, into=dict):
         into_c = com.standardize_mapping(into)
         return into_c((k, maybe_box_native(v)) for k, v in self.items())
 
-    def to_frame(self, name=None) -> DataFrame:
+    def to_frame(self, name: Hashable = lib.no_default) -> DataFrame:
         """
         Convert Series to DataFrame.
 
@@ -1723,7 +1734,7 @@ def to_frame(self, name=None) -> DataFrame:
         2    c
         """
         columns: Index
-        if name is None:
+        if name is lib.no_default:
             name = self.name
             if name is None:
                 # default to [0], same as we would get with DataFrame(self)

pandas/plotting/_matplotlib/core.py

@@ -460,7 +460,11 @@ def _compute_plot_data(self):
             label = self.label
             if label is None and data.name is None:
                 label = "None"
-            data = data.to_frame(name=label)
+            if label is None:
+                # We'll end up with columns of [0] instead of [None]
+                data = data.to_frame()
+            else:
+                data = data.to_frame(name=label)
         elif self._kind in ("hist", "box"):
             cols = self.columns if self.by is None else self.columns + self.by
             data = data.loc[:, cols]

pandas/tests/indexes/datetimes/methods/test_to_frame.py

@@ -1,5 +1,6 @@
 from pandas import (
     DataFrame,
+    Index,
     date_range,
 )
 import pandas._testing as tm
@@ -12,3 +13,14 @@ def test_to_frame_datetime_tz(self):
         result = idx.to_frame()
         expected = DataFrame(idx, index=idx)
         tm.assert_frame_equal(result, expected)
+
+    def test_to_frame_respects_none_name(self):
+        # GH#44212 if we explicitly pass name=None, then that should be respected,
+        #  not changed to 0
+        idx = date_range(start="2019-01-01", end="2019-01-30", freq="D", tz="UTC")
+        result = idx.to_frame(name=None)
+        exp_idx = Index([None], dtype=object)
+        tm.assert_index_equal(exp_idx, result.columns)
+
+        result = idx.rename("foo").to_frame(name=None)
+        tm.assert_index_equal(exp_idx, result.columns)

pandas/tests/series/methods/test_to_frame.py

@@ -1,11 +1,24 @@
 from pandas import (
     DataFrame,
+    Index,
     Series,
 )
 import pandas._testing as tm
 
 
 class TestToFrame:
+    def test_to_frame_respects_name_none(self):
+        # GH#44212 if we explicitly pass name=None, then that should be respected,
+        #  not changed to 0
+        ser = Series(range(3))
+        result = ser.to_frame(None)
+
+        exp_index = Index([None], dtype=object)
+        tm.assert_index_equal(result.columns, exp_index)
+
+        result = ser.rename("foo").to_frame(None)
+        tm.assert_index_equal(result.columns, exp_index)
+
     def test_to_frame(self, datetime_series):
         datetime_series.name = None
         rs = datetime_series.to_frame()