PERF: use blk.dtype in where() & _setitem_frame()

[auderson]

• closes PERF: bottleneck in where() #61010 (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

this pr is a one-line change to the code here:

pandas/pandas/core/generic.py

Lines 9735 to 9737 in d1ec1a4

	for _dt in cond.dtypes:
	if not is_bool_dtype(_dt):
	raise TypeError(msg.format(dtype=_dt))

Performance comparison:

test script:

import numpy as np
import pandas as pd
import timeit

for width in [10, 1000, 1000_00, 1000_0000]:
    df = pd.DataFrame(np.random.randn(1, width))
    mask = df > 0.5
    tm = timeit.timeit("df.where(mask)", number=10, globals=globals())
    print(width, tm)

for _dt in cond.dtypes:

10 0.002963045029900968
1000 0.006705133942887187
100000 0.40306550299283117
10000000 46.55275956704281

for _dt in cond.dtypes.unique():

10 0.0028260269900783896
1000 0.002695770002901554
100000 0.042065858957357705
10000000 6.068146598991007

for _dt in [blk.dtype for blk in cond._mgr.blocks]:

10 0.0009857049444690347
1000 0.0011893719201907516
100000 0.003112988080829382
10000000 0.13763279700651765

[auderson]

@mroeschke
Hi, do I need to add this in whatsnew? 2.3.0 or 3.0.0?

[auderson]

This pattern is also found in _setitem_frame:

pandas/pandas/core/frame.py

Lines 4276 to 4289 in 5da9eb7

	def _setitem_frame(self, key, value) -> None:
	# support boolean setting with DataFrame input, e.g.
	# df[df > df2] = 0
	if isinstance(key, np.ndarray):
	if key.shape != self.shape:
	raise ValueError("Array conditional must be same shape as self")
	key = self._constructor(key, **self._construct_axes_dict(), copy=False)
	if key.size and not all(is_bool_dtype(dtype) for dtype in key.dtypes):
	raise TypeError(
	"Must pass DataFrame or 2-d ndarray with boolean values only"
	)
	self._where(-key, value, inplace=True)

Before

After

[mroeschke]

Hi, do I need to add this in whatsnew? 2.3.0 or 3.0.0?

v3.0.0.rst please

[mroeschke]

Thanks @auderson