DEPR: Series constructor fastpath keyword

asv_bench/benchmarks/series_methods.py

@@ -28,9 +28,6 @@ def time_constructor_dict(self):
     def time_constructor_no_data(self):
         Series(data=None, index=self.idx)
 
-    def time_constructor_fastpath(self):
-        Series(self.array, index=self.idx2, name="name", fastpath=True)
-
 
 class ToFrame:
     params = [["int64", "datetime64[ns]", "category", "Int64"], [None, "foo"]]

doc/source/whatsnew/v2.2.0.rst

@@ -256,6 +256,7 @@ Other Deprecations
 - Deprecated strings ``H``, ``S``, ``U``, and ``N`` denoting units in :func:`to_timedelta` (:issue:`52536`)
 - Deprecated strings ``H``, ``T``, ``S``, ``L``, ``U``, and ``N`` denoting units in :class:`Timedelta` (:issue:`52536`)
 - Deprecated strings ``T``, ``S``, ``L``, ``U``, and ``N`` denoting frequencies in :class:`Minute`, :class:`Second`, :class:`Milli`, :class:`Micro`, :class:`Nano` (:issue:`52536`)
+- Deprecated the ``fastpath`` keyword in the :class:`Series` constructor (:issue:`20110`)
 - Deprecated the extension test classes ``BaseNoReduceTests``, ``BaseBooleanReduceTests``, and ``BaseNumericReduceTests``, use ``BaseReduceTests`` instead (:issue:`54663`)
 - Deprecated the option ``mode.data_manager`` and the ``ArrayManager``; only the ``BlockManager`` will be available in future versions (:issue:`55043`)
 - Deprecating downcasting the results of :meth:`DataFrame.fillna`, :meth:`Series.fillna`, :meth:`DataFrame.ffill`, :meth:`Series.ffill`, :meth:`DataFrame.bfill`, :meth:`Series.bfill` in object-dtype cases. To opt in to the future version, use ``pd.set_option("future.no_silent_downcasting", True)`` (:issue:`54261`)

pandas/core/frame.py

@@ -5168,12 +5168,7 @@ def _sanitize_column(self, value) -> tuple[ArrayLike, BlockValuesRefs | None]:
 
     @property
     def _series(self):
-        return {
-            item: Series(
-                self._mgr.iget(idx), index=self.index, name=item, fastpath=True
-            )
-            for idx, item in enumerate(self.columns)
-        }
+        return {item: self._ixs(idx, axis=1) for idx, item in enumerate(self.columns)}
 
     # ----------------------------------------------------------------------
     # Reindexing and alignment

pandas/core/series.py

@@ -374,8 +374,18 @@ def __init__(
         dtype: Dtype | None = None,
         name=None,
         copy: bool | None = None,
-        fastpath: bool = False,
+        fastpath: bool | lib.NoDefault = lib.no_default,
     ) -> None:
+        if fastpath is not lib.no_default:
+            warnings.warn(
+                "The 'fastpath' keyword in pd.Series is deprecated and will "
+                "be removed in a future version.",
+                DeprecationWarning,
+                stacklevel=find_stack_level(),
+            )
+        else:
+            fastpath = False
+
         if (
             isinstance(data, (SingleBlockManager, SingleArrayManager))
             and index is None
@@ -1009,7 +1019,8 @@ def _slice(self, slobj: slice, axis: AxisInt = 0) -> Series:
         # axis kwarg is retained for compat with NDFrame method
         #  _slice is *always* positional
         mgr = self._mgr.get_slice(slobj, axis=axis)
-        out = self._constructor(mgr, fastpath=True)
+        out = self._constructor_from_mgr(mgr, axes=mgr.axes)
+        out._name = self._name
         return out.__finalize__(self)
 
     def __getitem__(self, key):

pandas/tests/copy_view/test_constructors.py

@@ -99,7 +99,9 @@ def test_series_from_series_with_reindex(using_copy_on_write):
 def test_series_from_array(using_copy_on_write, idx, dtype, fastpath, arr):
     if idx is None or dtype is not None:
         fastpath = False
-    ser = Series(arr, dtype=dtype, index=idx, fastpath=fastpath)
+    msg = "The 'fastpath' keyword in pd.Series is deprecated"
+    with tm.assert_produces_warning(DeprecationWarning, match=msg):
+        ser = Series(arr, dtype=dtype, index=idx, fastpath=fastpath)
     ser_orig = ser.copy()
     data = getattr(arr, "_data", arr)
     if using_copy_on_write:
@@ -157,7 +159,9 @@ def test_series_from_index_different_dtypes(using_copy_on_write):
 def test_series_from_block_manager(using_copy_on_write, idx, dtype, fastpath):
     ser = Series([1, 2, 3], dtype="int64")
     ser_orig = ser.copy()
-    ser2 = Series(ser._mgr, dtype=dtype, fastpath=fastpath, index=idx)
+    msg = "The 'fastpath' keyword in pd.Series is deprecated"
+    with tm.assert_produces_warning(DeprecationWarning, match=msg):
+        ser2 = Series(ser._mgr, dtype=dtype, fastpath=fastpath, index=idx)
     assert np.shares_memory(get_array(ser), get_array(ser2))
     if using_copy_on_write:
         assert not ser2._mgr._has_no_reference(0)