BUG/API: ndexes on empty frames/series should be RangeIndex, are Index[object]

Author: Terji Petersen

pandas/core/frame.py

@@ -632,8 +632,6 @@ def __init__(
         copy: bool | None = None,
     ) -> None:
 
-        if data is None:
-            data = {}
         if dtype is not None:
             dtype = self._validate_dtype(dtype)
 
@@ -671,6 +669,12 @@ def __init__(
             else:
                 copy = False
 
+        if data is None:
+            index = index if index is not None else default_index(0)
+            columns = columns if columns is not None else default_index(0)
+            dtype = dtype if dtype is not None else pandas_dtype(object)
+            data = []
+
         if isinstance(data, (BlockManager, ArrayManager)):
             mgr = self._init_mgr(
                 data, axes={"index": index, "columns": columns}, dtype=dtype, copy=copy
@@ -777,7 +781,7 @@ def __init__(
                 mgr = dict_to_mgr(
                     {},
                     index,
-                    columns,
+                    columns if columns is not None else default_index(0),
                     dtype=dtype,
                     typ=manager,
                 )
@@ -2310,7 +2314,7 @@ def maybe_reorder(
             result_index = None
             if len(arrays) == 0 and index is None and length == 0:
                 # for backward compat use an object Index instead of RangeIndex
-                result_index = Index([])
+                result_index = default_index(0)
 
             arrays, arr_columns = reorder_arrays(arrays, arr_columns, columns, length)
             return arrays, arr_columns, result_index

pandas/core/internals/construction.py

@@ -580,9 +580,9 @@ def _extract_index(data) -> Index:
     """
     Try to infer an Index from the passed data, raise ValueError on failure.
     """
-    index = None
+    index: Index | None = None
     if len(data) == 0:
-        index = Index([])
+        index = default_index(0)
     else:
         raw_lengths = []
         indexes: list[list[Hashable] | Index] = []

pandas/core/reshape/merge.py

@@ -1063,8 +1063,8 @@ def _get_join_info(
             else:
                 join_index = default_index(len(left_indexer))
 
-        if len(join_index) == 0:
-            join_index = join_index.astype(object)
+        if len(join_index) == 0 and not isinstance(join_index, MultiIndex):
+            join_index = default_index(0).set_names(join_index.name)
         return join_index, left_indexer, right_indexer
 
     def _create_join_index(

pandas/core/series.py

@@ -385,11 +385,16 @@ def __init__(
         if index is not None:
             index = ensure_index(index)
 
-        if data is None:
-            data = {}
         if dtype is not None:
             dtype = self._validate_dtype(dtype)
 
+        if data is None:
+            index = index if index is not None else default_index(0)
+            if len(index) or dtype is not None:
+                data = na_value_for_dtype(pandas_dtype(dtype), compat=False)
+            else:
+                data = []
+
         if isinstance(data, MultiIndex):
             raise NotImplementedError(
                 "initializing a Series from a MultiIndex is not supported"

pandas/io/parsers/base_parser.py

@@ -80,6 +80,7 @@
 from pandas.core.indexes.api import (
     Index,
     MultiIndex,
+    default_index,
     ensure_index_from_sequences,
 )
 from pandas.core.series import Series
@@ -1085,8 +1086,9 @@ def _get_empty_meta(
         #
         # Both must be non-null to ensure a successful construction. Otherwise,
         # we have to create a generic empty Index.
+        index: Index
         if (index_col is None or index_col is False) or index_names is None:
-            index = Index([])
+            index = default_index(0)
         else:
             data = [Series([], dtype=dtype_dict[name]) for name in index_names]
             index = ensure_index_from_sequences(data, names=index_names)

pandas/tests/apply/test_frame_apply.py

@@ -114,14 +114,14 @@ def test_apply_with_reduce_empty():
     result = empty_frame.apply(x.append, axis=1, result_type="expand")
     tm.assert_frame_equal(result, empty_frame)
     result = empty_frame.apply(x.append, axis=1, result_type="reduce")
-    expected = Series([], index=pd.Index([], dtype=object), dtype=np.float64)
+    expected = Series([], dtype=np.float64)
     tm.assert_series_equal(result, expected)
 
     empty_with_cols = DataFrame(columns=["a", "b", "c"])
     result = empty_with_cols.apply(x.append, axis=1, result_type="expand")
     tm.assert_frame_equal(result, empty_with_cols)
     result = empty_with_cols.apply(x.append, axis=1, result_type="reduce")
-    expected = Series([], index=pd.Index([], dtype=object), dtype=np.float64)
+    expected = Series([], dtype=np.float64)
     tm.assert_series_equal(result, expected)
 
     # Ensure that x.append hasn't been called
@@ -147,7 +147,7 @@ def test_nunique_empty():
     tm.assert_series_equal(result, expected)
 
     result = df.T.nunique()
-    expected = Series([], index=pd.Index([]), dtype=np.float64)
+    expected = Series([], dtype=np.float64)
     tm.assert_series_equal(result, expected)
 
 

pandas/tests/apply/test_str.py

@@ -8,7 +8,6 @@
 
 from pandas import (
     DataFrame,
-    Index,
     Series,
 )
 import pandas._testing as tm
@@ -149,8 +148,8 @@ def test_agg_cython_table_series(series, func, expected):
         tm.get_cython_table_params(
             Series(dtype=np.float64),
             [
-                ("cumprod", Series([], Index([]), dtype=np.float64)),
-                ("cumsum", Series([], Index([]), dtype=np.float64)),
+                ("cumprod", Series([], dtype=np.float64)),
+                ("cumsum", Series([], dtype=np.float64)),
             ],
         ),
         tm.get_cython_table_params(

pandas/tests/extension/base/constructors.py

@@ -119,7 +119,7 @@ def test_construct_empty_dataframe(self, dtype):
         # GH 33623
         result = pd.DataFrame(columns=["a"], dtype=dtype)
         expected = pd.DataFrame(
-            {"a": pd.array([], dtype=dtype)}, index=pd.Index([], dtype="object")
+            {"a": pd.array([], dtype=dtype)}, index=pd.RangeIndex(0)
         )
         self.assert_frame_equal(result, expected)
 

pandas/tests/extension/base/missing.py

@@ -55,7 +55,7 @@ def test_dropna_frame(self, data_missing):
 
         # axis = 1
         result = df.dropna(axis="columns")
-        expected = pd.DataFrame(index=[0, 1])
+        expected = pd.DataFrame(index=pd.RangeIndex(2), columns=pd.Index([]))
         self.assert_frame_equal(result, expected)
 
         # multiple

pandas/tests/frame/indexing/test_xs.py

@@ -84,7 +84,7 @@ def test_xs_corner(self):
         # no columns but Index(dtype=object)
         df = DataFrame(index=["a", "b", "c"])
         result = df.xs("a")
-        expected = Series([], name="a", index=Index([]), dtype=np.float64)
+        expected = Series([], name="a", dtype=np.float64)
         tm.assert_series_equal(result, expected)
 
     def test_xs_duplicates(self):