PERF: DataFrame.__setitem__ (pandas-dev#44796)

pandas/core/internals/blocks.py

@@ -1457,7 +1457,8 @@ def iget(self, col):
     def set_inplace(self, locs, values) -> None:
         # NB: This is a misnomer, is supposed to be inplace but is not,
         #  see GH#33457
-        assert locs.tolist() == [0]
+        # When an ndarray, we should have locs.tolist() == [0]
+        # When a BlockPlacement we should have list(locs) == [0]
         self.values = values
         try:
             # TODO(GH33457) this can be removed

pandas/core/internals/managers.py

@@ -1066,22 +1066,12 @@ def iset(
 
         # Note: we exclude DTA/TDA here
         value_is_extension_type = is_1d_only_ea_dtype(value.dtype)
-
-        # categorical/sparse/datetimetz
-        if value_is_extension_type:
-
-            def value_getitem(placement):
-                return value
-
-        else:
+        if not value_is_extension_type:
             if value.ndim == 2:
                 value = value.T
             else:
                 value = ensure_block_shape(value, ndim=2)
 
-            def value_getitem(placement):
-                return value[placement.indexer]
-
             if value.shape[1:] != self.shape[1:]:
                 raise AssertionError(
                     "Shape of new values must be compatible with manager shape"
@@ -1092,11 +1082,37 @@ def value_getitem(placement):
             # In this case, get_blkno_placements will yield only one tuple,
             #  containing (self._blknos[loc], BlockPlacement(slice(0, 1, 1)))
 
+            # Check if we can use _iset_single fastpath
+            blkno = self.blknos[loc]
+            blk = self.blocks[blkno]
+            if len(blk._mgr_locs) == 1:  # TODO: fastest way to check this?
+                return self._iset_single(
+                    # error: Argument 1 to "_iset_single" of "BlockManager" has
+                    # incompatible type "Union[int, slice, ndarray[Any, Any]]";
+                    # expected "int"
+                    loc,  # type:ignore[arg-type]
+                    value,
+                    inplace=inplace,
+                    blkno=blkno,
+                    blk=blk,
+                )
+
             # error: Incompatible types in assignment (expression has type
             # "List[Union[int, slice, ndarray]]", variable has type "Union[int,
             # slice, ndarray]")
             loc = [loc]  # type: ignore[assignment]
 
+        # categorical/sparse/datetimetz
+        if value_is_extension_type:
+
+            def value_getitem(placement):
+                return value
+
+        else:
+
+            def value_getitem(placement):
+                return value[placement.indexer]
+
         # Accessing public blknos ensures the public versions are initialized
         blknos = self.blknos[loc]
         blklocs = self.blklocs[loc].copy()
@@ -1172,6 +1188,29 @@ def value_getitem(placement):
             # Newly created block's dtype may already be present.
             self._known_consolidated = False
 
+    def _iset_single(
+        self, loc: int, value: ArrayLike, inplace: bool, blkno: int, blk: Block
+    ) -> None:
+        """
+        Fastpath for iset when we are only setting a single position and
+        the Block currently in that position is itself single-column.
+
+        In this case we can swap out the entire Block and blklocs and blknos
+        are unaffected.
+        """
+        # Caller is responsible for verifying value.shape
+
+        if inplace and blk.should_store(value):
+            iloc = self.blklocs[loc]
+            blk.set_inplace(slice(iloc, iloc + 1), value)
+            return
+
+        nb = new_block_2d(value, placement=blk._mgr_locs)
+        old_blocks = self.blocks
+        new_blocks = old_blocks[:blkno] + (nb,) + old_blocks[blkno + 1 :]
+        self.blocks = new_blocks
+        return
+
     def insert(self, loc: int, item: Hashable, value: ArrayLike) -> None:
         """
         Insert item at selected position.
@@ -1197,8 +1236,13 @@ def insert(self, loc: int, item: Hashable, value: ArrayLike) -> None:
         bp = BlockPlacement(slice(loc, loc + 1))
         block = new_block_2d(values=value, placement=bp)
 
-        self._insert_update_mgr_locs(loc)
-        self._insert_update_blklocs_and_blknos(loc)
+        if not len(self.blocks):
+            # Fastpath
+            self._blklocs = np.array([0], dtype=np.intp)
+            self._blknos = np.array([0], dtype=np.intp)
+        else:
+            self._insert_update_mgr_locs(loc)
+            self._insert_update_blklocs_and_blknos(loc)
 
         self.axes[0] = new_axis
         self.blocks += (block,)