PERF: Index._shallow_copy shares _cache with copies of self

doc/source/whatsnew/v1.2.0.rst

@@ -284,6 +284,7 @@ Performance improvements
 - ``Styler`` uuid method altered to compress data transmission over web whilst maintaining reasonably low table collision probability (:issue:`36345`)
 - Performance improvement in :meth:`pd.to_datetime` with non-ns time unit for ``float`` ``dtype`` columns (:issue:`20445`)
 - Performance improvement in setting values on a :class:`IntervalArray` (:issue:`36310`)
+- Performance improvement for the :meth:`~Index.equals` method on all index classes, when compared to copies of the same index (:issue:`36840`)
 
 .. ---------------------------------------------------------------------------
 
@@ -358,6 +359,7 @@ Indexing
 - Bug in :meth:`Index.sort_values` where, when empty values were passed, the method would break by trying to compare missing values instead of pushing them to the end of the sort order. (:issue:`35584`)
 - Bug in :meth:`Index.get_indexer` and :meth:`Index.get_indexer_non_unique` where int64 arrays are returned instead of intp. (:issue:`36359`)
 - Bug in :meth:`DataFrame.sort_index` where parameter ascending passed as a list on a single level index gives wrong result. (:issue:`32334`)
+- Bug in :meth:`Index.equals`, where it required the name to the be equal. This method should not compare names for equality (:issue:`36840`)
 
 Missing
 ^^^^^^^

pandas/core/indexes/base.py

@@ -561,12 +561,13 @@ def _shallow_copy(self, values=None, name: Label = no_default):
         name : Label, defaults to self.name
         """
         name = self.name if name is no_default else name
-        cache = self._cache.copy() if values is None else {}
-        if values is None:
-            values = self._values
 
-        result = self._simple_new(values, name=name)
-        result._cache = cache
+        if values is not None:
+            return self._simple_new(values, name=name)
+
+        result = self._simple_new(self._values, name=name)
+        result._cache = self._cache.copy()
+        result._id = self._id
         return result
 
     def is_(self, other) -> bool:

pandas/core/indexes/datetimelike.py

@@ -671,17 +671,16 @@ def _with_freq(self, freq):
 
     def _shallow_copy(self, values=None, name: Label = lib.no_default):
         name = self.name if name is lib.no_default else name
-        cache = self._cache.copy() if values is None else {}
 
-        if values is None:
-            values = self._data
-
-        if isinstance(values, np.ndarray):
+        if values is not None:
             # TODO: We would rather not get here
-            values = type(self._data)(values, dtype=self.dtype)
+            if isinstance(values, np.ndarray):
+                values = type(self._data)(values, dtype=self.dtype)
+            return self._simple_new(values, name=name)
 
-        result = type(self)._simple_new(values, name=name)
-        result._cache = cache
+        result = self._simple_new(self._data, name=name)
+        result._cache = self._cache.copy()
+        result._id = self._id
         return result
 
     # --------------------------------------------------------------------

pandas/core/indexes/interval.py

@@ -335,12 +335,12 @@ def _shallow_copy(
         self, values: Optional[IntervalArray] = None, name: Label = lib.no_default
     ):
         name = self.name if name is lib.no_default else name
-        cache = self._cache.copy() if values is None else {}
-        if values is None:
-            values = self._data
+        if values is not None:
+            return self._simple_new(values, name=name)
 
-        result = self._simple_new(values, name=name)
-        result._cache = cache
+        result = self._simple_new(self._data, name=name)
+        result._cache = self._cache.copy()
+        result._id = self._id
         return result
 
     @cache_readonly

pandas/core/indexes/multi.py

@@ -1089,6 +1089,7 @@ def _shallow_copy(
         )
         result._cache = self._cache.copy()
         result._cache.pop("levels", None)  # GH32669
+        result._id = self._id
         return result
 
     def symmetric_difference(self, other, result_name=None, sort=None):
@@ -1192,7 +1193,6 @@ def __array__(self, dtype=None) -> np.ndarray:
     def view(self, cls=None):
         """ this is defined as a copy with the same identity """
         result = self.copy()
-        result._id = self._id
         return result
 
     @doc(Index.__contains__)

pandas/core/indexes/period.py

@@ -260,12 +260,13 @@ def _has_complex_internals(self) -> bool:
 
     def _shallow_copy(self, values=None, name: Label = no_default):
         name = name if name is not no_default else self.name
-        cache = self._cache.copy() if values is None else {}
-        if values is None:
-            values = self._data
 
-        result = self._simple_new(values, name=name)
-        result._cache = cache
+        if values is not None:
+            return self._simple_new(values, name=name)
+
+        result = self._simple_new(self._data, name=name)
+        result._cache = self._cache.copy()
+        result._id = self._id
         return result
 
     def _maybe_convert_timedelta(self, other):

pandas/core/indexes/range.py

@@ -397,13 +397,14 @@ def __iter__(self):
     def _shallow_copy(self, values=None, name: Label = no_default):
         name = self.name if name is no_default else name
 
-        if values is None:
-            result = self._simple_new(self._range, name=name)
-            result._cache = self._cache.copy()
-            return result
-        else:
+        if values is not None:
             return Int64Index._simple_new(values, name=name)
 
+        result = self._simple_new(self._range, name=name)
+        result._cache = self._cache.copy()
+        result._id = self._id
+        return result
+
     @doc(Int64Index.copy)
     def copy(self, name=None, deep=False, dtype=None, names=None):
         name = self._validate_names(name=name, names=names, deep=deep)[0]

pandas/tests/indexes/common.py

@@ -960,3 +960,9 @@ def test_shallow_copy_copies_cache(self):
         # cache values should reference the same object
         for key, val in idx._cache.items():
             assert shallow_copy._cache[key] is val, key
+
+    def test_shallow_copy_copies_id(self):
+        # GH36840
+        idx = self.create_index()
+        shallow_copy = idx._shallow_copy()
+        assert idx._id is shallow_copy._id

pandas/tests/indexes/datetimes/test_constructors.py

@@ -931,7 +931,7 @@ def test_is_(self):
         dti = date_range(start="1/1/2005", end="12/1/2005", freq="M")
         assert dti.is_(dti)
         assert dti.is_(dti.view())
-        assert not dti.is_(dti.copy())
+        assert dti.is_(dti.copy())
 
     def test_index_cast_datetime64_other_units(self):
         arr = np.arange(0, 100, 10, dtype=np.int64).view("M8[D]")

pandas/tests/indexes/multi/test_equivalence.py

@@ -180,7 +180,7 @@ def test_is_():
     assert mi2.is_(mi)
     assert mi.is_(mi2)
 
-    assert not mi.is_(mi.set_names(["C", "D"]))
+    assert mi.is_(mi.set_names(["C", "D"]))
     mi2 = mi.view()
     mi2.set_names(["E", "F"], inplace=True)
     assert mi.is_(mi2)

pandas/tests/indexes/test_base.py

@@ -523,8 +523,8 @@ def test_is_(self):
         assert ind.is_(ind)
         assert ind.is_(ind.view().view().view().view())
         assert not ind.is_(Index(range(10)))
-        assert not ind.is_(ind.copy())
-        assert not ind.is_(ind.copy(deep=False))
+        assert ind.is_(ind.copy())
+        assert ind.is_(ind.copy(deep=False))
         assert not ind.is_(ind[:])
         assert not ind.is_(np.array(range(10)))