BUG: preserve object-dtype index when accessing DataFrame column / PERF: improve perf of Series fastpath constructor

asv_bench/benchmarks/series_methods.py

@@ -3,6 +3,7 @@
 import numpy as np
 
 from pandas import (
+    Index,
     NaT,
     Series,
     date_range,
@@ -12,20 +13,23 @@
 
 
 class SeriesConstructor:
-
-    params = [None, "dict"]
-    param_names = ["data"]
-
-    def setup(self, data):
+    def setup(self):
         self.idx = date_range(
             start=datetime(2015, 10, 26), end=datetime(2016, 1, 1), freq="50s"
         )
-        dict_data = dict(zip(self.idx, range(len(self.idx))))
-        self.data = None if data is None else dict_data
+        self.data = dict(zip(self.idx, range(len(self.idx))))
+        self.array = np.array([1, 2, 3])
+        self.idx2 = Index(["a", "b", "c"])
 
-    def time_constructor(self, data):
+    def time_constructor_dict(self):
         Series(data=self.data, index=self.idx)
 
+    def time_constructor_no_data(self):
+        Series(data=None, index=self.idx)
+
+    def time_constructor_fastpath(self):
+        Series(self.array, index=self.idx, name="name", fastpath=True)
+
 
 class ToFrame:
     params = [["int64", "datetime64[ns]", "category", "Int64"], [None, "foo"]]

pandas/core/series.py

@@ -447,8 +447,12 @@ def __init__(
                     data = SingleArrayManager.from_array(data, index)
 
         generic.NDFrame.__init__(self, data)
-        self.name = name
-        self._set_axis(0, index, fastpath=True)
+        if fastpath:
+            # skips validation of the name
+            object.__setattr__(self, "_name", name)
+        else:
+            self.name = name
+            self._set_axis(0, index)
 
     def _init_dict(
         self, data, index: Index | None = None, dtype: DtypeObj | None = None
@@ -529,15 +533,14 @@ def _constructor_expanddim(self) -> type[DataFrame]:
     def _can_hold_na(self) -> bool:
         return self._mgr._can_hold_na
 
-    def _set_axis(self, axis: int, labels, fastpath: bool = False) -> None:
+    def _set_axis(self, axis: int, labels) -> None:
         """
         Override generic, we want to set the _typ here.
 
         This is called from the cython code when we set the `index` attribute
         directly, e.g. `series.index = [1, 2, 3]`.
         """
-        if not fastpath:
-            labels = ensure_index(labels)
+        labels = ensure_index(labels)
 
         if labels._is_all_dates:
             deep_labels = labels
@@ -549,17 +552,13 @@ def _set_axis(self, axis: int, labels, fastpath: bool = False) -> None:
             ):
                 try:
                     labels = DatetimeIndex(labels)
-                    # need to set here because we changed the index
-                    if fastpath:
-                        self._mgr.set_axis(axis, labels)
                 except (tslibs.OutOfBoundsDatetime, ValueError):
                     # labels may exceeds datetime bounds,
                     # or not be a DatetimeIndex
                     pass
 
-        if not fastpath:
-            # The ensure_index call above ensures we have an Index object
-            self._mgr.set_axis(axis, labels)
+        # The ensure_index call above ensures we have an Index object
+        self._mgr.set_axis(axis, labels)
 
     # ndarray compatibility
     @property