From 48b913fb94b518aa61d77197e2eb071da8368adf Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Thu, 4 Mar 2021 07:59:18 -0800
Subject: [PATCH 1/6] PERF: calling BlockPlacement

---
 pandas/_libs/internals.pyx            | 16 +++++++++-------
 pandas/core/internals/blocks.py       |  2 +-
 pandas/core/internals/construction.py |  2 +-
 pandas/core/internals/managers.py     |  3 +--
 4 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/pandas/_libs/internals.pyx b/pandas/_libs/internals.pyx
index 150b7f62b4b26..9b2db897cdd32 100644
--- a/pandas/_libs/internals.pyx
+++ b/pandas/_libs/internals.pyx
@@ -15,11 +15,13 @@ cimport numpy as cnp
 from numpy cimport (
     NPY_INT64,
     int64_t,
+    ndarray,
 )
 
 cnp.import_array()
 
 from pandas._libs.algos import ensure_int64
+from pandas._libs.util cimport is_integer_object
 
 
 @cython.final
@@ -27,10 +29,10 @@ cdef class BlockPlacement:
     # __slots__ = '_as_slice', '_as_array', '_len'
     cdef:
         slice _as_slice
-        object _as_array
+        ndarray _as_array  # Note: this still allows `None`
         bint _has_slice, _has_array, _is_known_slice_like
 
-    def __init__(self, val):
+    def __cinit__(self, val):
         cdef:
             slice slc
 
@@ -39,7 +41,7 @@ cdef class BlockPlacement:
         self._has_slice = False
         self._has_array = False
 
-        if isinstance(val, int):
+        if is_integer_object(val):
             slc = slice(val, val + 1, 1)
             self._as_slice = slc
             self._has_slice = True
@@ -160,12 +162,12 @@ cdef class BlockPlacement:
             np.concatenate([self.as_array] + [o.as_array for o in others])
         )
 
-    cdef iadd(self, other):
+    cdef BlockPlacement iadd(self, other):
         cdef:
             slice s = self._ensure_has_slice()
             Py_ssize_t other_int, start, stop, step, l
 
-        if isinstance(other, int) and s is not None:
+        if is_integer_object(other) and s is not None:
             other_int = <Py_ssize_t>other
 
             if other_int == 0:
@@ -438,13 +440,13 @@ def get_blkno_placements(blknos, group: bool = True):
     """
     Parameters
     ----------
-    blknos : array of int64
+    blknos : np.ndarray[int64]
     group : bool, default True
 
     Returns
     -------
     iterator
-        yield (BlockPlacement, blkno)
+        yield (blkno, BlockPlacement)
     """
     blknos = ensure_int64(blknos)
 
diff --git a/pandas/core/internals/blocks.py b/pandas/core/internals/blocks.py
index f2b8499a316b7..45e2cb670ef2b 100644
--- a/pandas/core/internals/blocks.py
+++ b/pandas/core/internals/blocks.py
@@ -447,7 +447,7 @@ def _split_op_result(self, result) -> List[Block]:
             nbs = []
             for i, loc in enumerate(self.mgr_locs):
                 vals = result[i]
-                block = self.make_block(values=vals, placement=[loc])
+                block = self.make_block(values=vals, placement=loc)
                 nbs.append(block)
             return nbs
 
diff --git a/pandas/core/internals/construction.py b/pandas/core/internals/construction.py
index 814fd389bf644..5b0a3b35a0a1b 100644
--- a/pandas/core/internals/construction.py
+++ b/pandas/core/internals/construction.py
@@ -300,7 +300,7 @@ def ndarray_to_mgr(
             # TODO: What about re-joining object columns?
             dvals_list = [maybe_squeeze_dt64tz(x) for x in dvals_list]
             block_values = [
-                make_block(dvals_list[n], placement=[n], ndim=2)
+                make_block(dvals_list[n], placement=n, ndim=2)
                 for n in range(len(dvals_list))
             ]
 
diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index 2ad7471d6f086..7b5b3ff03b58a 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -557,8 +557,7 @@ def quantile(
         if transposed:
             new_axes = new_axes[::-1]
             blocks = [
-                b.make_block(b.values.T, placement=np.arange(b.shape[1]))
-                for b in blocks
+                b.make_block(b.values.T, placement=slice(0, b.shape[1])) for b in blocks
             ]
 
         return type(self)(blocks, new_axes)

From d801c15f8ab3398dcc231821337d7edc0202ac24 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Thu, 4 Mar 2021 09:31:50 -0800
Subject: [PATCH 2/6] PERF: Index.__getitem__ for slices

---
 pandas/core/indexes/base.py  |  7 ++++---
 pandas/core/indexes/multi.py | 13 +++++++------
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 44c9b33ae51c7..2c4fed50bff95 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -4534,7 +4534,6 @@ def __getitem__(self, key):
         # There's no custom logic to be implemented in __getslice__, so it's
         # not overloaded intentionally.
         getitem = self._data.__getitem__
-        promote = self._shallow_copy
 
         if is_scalar(key):
             key = com.cast_scalar_indexer(key, warn_float=True)
@@ -4543,7 +4542,9 @@ def __getitem__(self, key):
         if isinstance(key, slice):
             # This case is separated from the conditional above to avoid
             # pessimization of basic indexing.
-            return promote(getitem(key))
+            result = getitem(key)
+            # Going through simple_new for performance.
+            return type(self)._simple_new(result, name=self.name)
 
         if com.is_bool_indexer(key):
             key = np.asarray(key, dtype=bool)
@@ -4553,7 +4554,7 @@ def __getitem__(self, key):
             if np.ndim(result) > 1:
                 deprecate_ndim_indexing(result)
                 return result
-            return promote(result)
+            return self._shallow_copy(result)
         else:
             return result
 
diff --git a/pandas/core/indexes/multi.py b/pandas/core/indexes/multi.py
index 88b92c7b304ae..fd0e0ef5fa799 100644
--- a/pandas/core/indexes/multi.py
+++ b/pandas/core/indexes/multi.py
@@ -2076,15 +2076,16 @@ def __getitem__(self, key):
 
             return tuple(retval)
         else:
+            # in general cannot be sure whether the result will be sorted
+            sortorder = None
             if com.is_bool_indexer(key):
                 key = np.asarray(key, dtype=bool)
                 sortorder = self.sortorder
-            else:
-                # cannot be sure whether the result will be sorted
-                sortorder = None
-
-                if isinstance(key, Index):
-                    key = np.asarray(key)
+            elif isinstance(key, slice):
+                if key.step is None or key.step > 0:
+                    sortorder = self.sortorder
+            elif isinstance(key, Index):
+                key = np.asarray(key)
 
             new_codes = [level_codes[key] for level_codes in self.codes]
 

From ebfd3a1f10b0c4a49a7ae9fb6704a30ad01aedb5 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Thu, 4 Mar 2021 09:42:30 -0800
Subject: [PATCH 3/6] PERF: BlockManager.ndim

---
 pandas/core/internals/managers.py | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index 7b5b3ff03b58a..435a9d35301e3 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -146,6 +146,9 @@ class BlockManager(DataManager):
     _blknos: np.ndarray
     _blklocs: np.ndarray
 
+    # Non-trivially faster than a property
+    ndim = 2  # overridden by SingleBlockManager
+
     def __init__(
         self,
         blocks: Sequence[Block],
@@ -230,10 +233,6 @@ def __nonzero__(self) -> bool:
     def shape(self) -> Shape:
         return tuple(len(ax) for ax in self.axes)
 
-    @property
-    def ndim(self) -> int:
-        return len(self.axes)
-
     def set_axis(
         self, axis: int, new_labels: Index, verify_integrity: bool = True
     ) -> None:

From 9c2c4d3175b63bc72e48e14043edbfc44b6ce1bf Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Thu, 4 Mar 2021 15:33:20 -0800
Subject: [PATCH 4/6] _simple_new perf

---
 pandas/core/indexes/base.py     | 4 +++-
 pandas/core/indexes/category.py | 2 +-
 pandas/core/indexes/range.py    | 2 +-
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 2c4fed50bff95..d4ee7b1868184 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -4554,7 +4554,9 @@ def __getitem__(self, key):
             if np.ndim(result) > 1:
                 deprecate_ndim_indexing(result)
                 return result
-            return self._shallow_copy(result)
+            # NB: Using _constructor._simple_new would break if MultiIndex
+            #  didn't override __getitem__
+            return self._constructor._simple_new(result, name=self.name)
         else:
             return result
 
diff --git a/pandas/core/indexes/category.py b/pandas/core/indexes/category.py
index 13c53dfafed4d..7a204dcce8a88 100644
--- a/pandas/core/indexes/category.py
+++ b/pandas/core/indexes/category.py
@@ -231,7 +231,7 @@ def _simple_new(cls, values: Categorical, name: Optional[Hashable] = None):
         result = object.__new__(cls)
 
         result._data = values
-        result.name = name
+        result._name = name
         result._cache = {}
 
         result._reset_identity()
diff --git a/pandas/core/indexes/range.py b/pandas/core/indexes/range.py
index 67af4b628e552..40d5596ddb296 100644
--- a/pandas/core/indexes/range.py
+++ b/pandas/core/indexes/range.py
@@ -164,7 +164,7 @@ def _simple_new(cls, values: range, name: Hashable = None) -> RangeIndex:
         assert isinstance(values, range)
 
         result._range = values
-        result.name = name
+        result._name = name
         result._cache = {}
         result._reset_identity()
         return result

From 7b69805055e1b0c61179a15a9a1800f438f0e23f Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 5 Mar 2021 08:52:31 -0800
Subject: [PATCH 5/6] PERF: BlockManger._simple_new

---
 pandas/core/internals/managers.py | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index 6a49f1d396cd2..22c0f4fb74125 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -176,6 +176,21 @@ def __init__(
         self._blknos = None
         self._blklocs = None
 
+    @classmethod
+    def _simple_new(cls, blocks: Tuple[Block], axes: List[Index]):
+        """
+        Fastpath constructor; does NO validation.
+        """
+        obj = cls.__new__(cls)
+        obj.axes = axes
+        obj.blocks = blocks
+
+        # Populate known_consolidate, blknos, and blklocs lazily
+        obj._known_consolidated = False
+        obj._blknos = None
+        obj._blklocs = None
+        return obj
+
     @classmethod
     def from_blocks(cls, blocks: List[Block], axes: List[Index]):
         """
@@ -799,8 +814,7 @@ def get_slice(self, slobj: slice, axis: int = 0) -> BlockManager:
         new_axes = list(self.axes)
         new_axes[axis] = new_axes[axis][slobj]
 
-        bm = type(self)(new_blocks, new_axes, verify_integrity=False)
-        return bm
+        return type(self)._simple_new(tuple(new_blocks), new_axes)
 
     @property
     def nblocks(self) -> int:

From 1dfedc4441ab1ae867b0b45203d3a42cc9d4f672 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 5 Mar 2021 10:49:00 -0800
Subject: [PATCH 6/6] mypy fixup

---
 pandas/core/internals/managers.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/core/internals/managers.py b/pandas/core/internals/managers.py
index 22c0f4fb74125..b656c9e83e1a8 100644
--- a/pandas/core/internals/managers.py
+++ b/pandas/core/internals/managers.py
@@ -177,7 +177,7 @@ def __init__(
         self._blklocs = None
 
     @classmethod
-    def _simple_new(cls, blocks: Tuple[Block], axes: List[Index]):
+    def _simple_new(cls, blocks: Tuple[Block, ...], axes: List[Index]):
         """
         Fastpath constructor; does NO validation.
         """
@@ -1335,7 +1335,7 @@ def reindex_indexer(
 
     def _slice_take_blocks_ax0(
         self, slice_or_indexer, fill_value=lib.no_default, only_slice: bool = False
-    ):
+    ) -> List[Block]:
         """
         Slice/take blocks along axis=0.