BUG/API: Indexes on empty frames/series should be RangeIndex (#49637)

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

* fix black

* fix window stuff

* Add docs

* double ticks

* unneeded line

* update thatsnew text

* update whatsnew text

* fix rst

* Update doc/source/whatsnew/v2.0.0.rst

Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

Co-authored-by: Terji Petersen <terjipetersen@Terjis-Air.fritz.box>
Co-authored-by: Matthew Roeschke <10647082+mroeschke@users.noreply.github.com>

Author: 3 people

doc/source/whatsnew/v2.0.0.rst

@@ -312,6 +312,35 @@ The new behavior, as for datetime64, either gives exactly the requested dtype or
    ser.astype("timedelta64[s]")
    ser.astype("timedelta64[D]")
 
+.. _whatsnew_200.api_breaking.zero_len_indexes:
+
+Empty DataFrames/Series will now default to have a ``RangeIndex``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Before, constructing an empty (where ``data`` is ``None`` or an empty list-like argument) :class:`Series` or :class:`DataFrame` without
+specifying the axes (``index=None``, ``columns=None``) would return the axes as empty :class:`Index` with object dtype.
+
+Now, the axes return an empty :class:`RangeIndex`.
+
+*Previous behavior*:
+
+.. code-block:: ipython
+
+   In [8]: pd.Series().index
+   Out[8]:
+   Index([], dtype='object')
+
+   In [9] pd.DataFrame().axes
+   Out[9]:
+   [Index([], dtype='object'), Index([], dtype='object')]
+
+*New behavior*:
+
+.. ipython:: python
+
+   pd.Series().index
+   pd.DataFrame().axes
+
 .. _whatsnew_200.api_breaking.deps:
 
 Increased minimum versions for dependencies
@@ -373,6 +402,7 @@ Other API changes
 - Changed behavior of :class:`Index` constructor with an object-dtype ``numpy.ndarray`` containing all-``bool`` values or all-complex values, this will now retain object dtype, consistent with the :class:`Series` behavior (:issue:`49594`)
 - Changed behavior of :meth:`DataFrame.shift` with ``axis=1``, an integer ``fill_value``, and homogeneous datetime-like dtype, this now fills new columns with integer dtypes instead of casting to datetimelike (:issue:`49842`)
 - Files are now closed when encountering an exception in :func:`read_json` (:issue:`49921`)
+- Changed behavior of :func:`read_csv`, :func:`read_json` & :func:`read_fwf`, where the index will now always be a :class:`RangeIndex`, when no index is specified. Previously the index would be a :class:`Index` with dtype ``object`` if the new DataFrame/Series has length 0 (:issue:`49572`)
 - :meth:`DataFrame.values`, :meth:`DataFrame.to_numpy`, :meth:`DataFrame.xs`, :meth:`DataFrame.reindex`, :meth:`DataFrame.fillna`, and :meth:`DataFrame.replace` no longer silently consolidate the underlying arrays; do ``df = df.copy()`` to ensure consolidation (:issue:`49356`)
 -
 

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,
                 )
@@ -2309,8 +2313,7 @@ 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

@@ -582,7 +582,7 @@ def _extract_index(data) -> Index:
     """
     index: Index
     if len(data) == 0:
-        return Index([])
+        return default_index(0)
 
     raw_lengths = []
     indexes: list[list[Hashable] | Index] = []

pandas/core/reshape/merge.py

@@ -1084,8 +1084,8 @@ 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/core/window/common.py

@@ -30,6 +30,8 @@ def dataframe_from_int_dict(data, frame_template):
             result = DataFrame(data, index=frame_template.index)
             if len(result.columns) > 0:
                 result.columns = frame_template.columns[result.columns]
+            else:
+                result.columns = frame_template.columns.copy()
             return result
 
         results = {}

pandas/io/parsers/base_parser.py

@@ -84,6 +84,7 @@
 from pandas.core.indexes.api import (
     Index,
     MultiIndex,
+    default_index,
     ensure_index_from_sequences,
 )
 from pandas.core.series import Series
@@ -1093,8 +1094,9 @@ 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)