Revert "ENH: Improve performance for df.__setitem__ with list-like indexers (#38148)" (#38208)

This reverts commit 2f41109.

Author: simonjayhawkins

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):