Revert "ENH: Improve performance for df.__setitem__ with list-like indexers"

asv_bench/benchmarks/indexing.py

@@ -358,14 +358,6 @@ def time_assign_with_setitem(self):
         for i in range(100):
             self.df[i] = np.random.randn(self.N)
 
-    def time_assign_list_like_with_setitem(self):
-        np.random.seed(1234)
-        self.df[list(range(100))] = np.random.randn(self.N, 100)
-
-    def time_assign_list_of_columns_concat(self):
-        df = DataFrame(np.random.randn(self.N, 100))
-        concat([self.df, df], axis=1)
-
 
 class ChainIndexing:
 

doc/source/whatsnew/v1.1.5.rst

@@ -24,7 +24,6 @@ Fixed regressions
 - Fixed regression in :meth:`DataFrame.groupby` aggregation with out-of-bounds datetime objects in an object-dtype column (:issue:`36003`)
 - Fixed regression in ``df.groupby(..).rolling(..)`` with the resulting :class:`MultiIndex` when grouping by a label that is in the index (:issue:`37641`)
 - Fixed regression in :meth:`DataFrame.fillna` not filling ``NaN`` after other operations such as :meth:`DataFrame.pivot` (:issue:`36495`).
-- Fixed performance regression for :meth:`DataFrame.__setitem__` with list-like indexers (:issue:`37954`)
 - Fixed performance regression in ``df.groupby(..).rolling(..)`` (:issue:`38038`)
 - Fixed regression in :meth:`MultiIndex.intersection` returning duplicates when at least one of the indexes had duplicates (:issue:`36915`)
 

pandas/core/indexing.py

@@ -672,8 +672,17 @@ def _ensure_listlike_indexer(self, key, axis=None, value=None):
             and not com.is_bool_indexer(key)
             and all(is_hashable(k) for k in key)
         ):
-            keys = self.obj.columns.union(key, sort=False)
-            self.obj._mgr = self.obj._mgr.reindex_axis(keys, 0)
+            for i, k in enumerate(key):
+                if k not in self.obj:
+                    if value is None:
+                        self.obj[k] = np.nan
+                    elif is_array_like(value) and value.ndim == 2:
+                        # GH#37964 have to select columnwise in case of array
+                        self.obj[k] = value[:, i]
+                    elif is_list_like(value):
+                        self.obj[k] = value[i]
+                    else:
+                        self.obj[k] = value
 
     def __setitem__(self, key, value):
         if isinstance(key, tuple):