Could operator.methodcaller be optimized using LOAD_METHOD?

[anntzer]

BPO	44850
Nosy	@markshannon, @anntzer, @sweeneyde
PRs	bpo-44850: Improve the performance of methodcaller. #27782

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-08-06.10:42:14.018>
labels = ['3.11', 'library', 'performance']
title = 'Could operator.methodcaller be optimized using LOAD_METHOD?'
updated_at = <Date 2021-08-19.10:07:47.016>
user = 'https://github.com/anntzer'

bugs.python.org fields:

activity = <Date 2021-08-19.10:07:47.016>
actor = 'Mark.Shannon'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-08-06.10:42:14.018>
creator = 'Antony.Lee'
dependencies = []
files = []
hgrepos = []
issue_num = 44850
keywords = ['patch']
message_count = 4.0
messages = ['399066', '399178', '399184', '399903']
nosy_count = 3.0
nosy_names = ['Mark.Shannon', 'Antony.Lee', 'Dennis Sweeney']
pr_nums = ['27782']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue44850'
versions = ['Python 3.11']

Linked PRs

• gh-89013: Improve the performance of methodcaller #106960
• gh-89013: Improve the performance of methodcaller (lazy version) #107201

[anntzer]

Currently, methodcaller is not faster than a plain lambda:

In [1]: class T:
   ...:     a = 1
   ...:     def f(self): pass
   ...:     

In [2]: from operator import *

In [3]: %%timeit t = T(); mc = methodcaller("f")
   ...: mc(t)
   ...: 
   ...: 
83.1 ns ± 0.862 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [4]: %%timeit t = T(); mc = lambda x: x.f()
   ...: mc(t)
   ...: 
   ...: 
81.4 ns ± 0.0508 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

(on some machines, I find that it is even slower).

Compare with attrgetter, which *is* faster:

In [5]: %%timeit t = T(); ag = attrgetter("a")
   ...: ag(t)
   ...: 
   ...: 
33.7 ns ± 0.0407 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [6]: %%timeit t = T(); ag = lambda x: x.a
   ...: ag(t)
   ...: 
   ...: 
50.1 ns ± 0.057 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

Given that the operator module explicitly advertises itself as being "efficient"/"fast", it seems reasonable to try to optimize methodcaller. Looking at its C implementation, methodcaller currently uses PyObject_GetAttr followed by PyObject_Call; I wonder whether this can be optimized using a LOAD_METHOD-style approach to avoid the construction of the bound method (when applicable)?

[sweeneyde]

Using _PyObject_GetMethod similarly to the way that LOAD_METHOD/CALL_METHOD does seems like a reasonable idea to me -- do you want to make a pull request? It would also be nice to see some microbenchmarks for the change once it's ready.

[sweeneyde]

For what it's worth, in my benchmarks on 3.11, methodcaller was already a bit faster than lambda:

#################### Builtin calls ####################

PS > .\python.bat -m pyperf timeit -s "from operator import methodcaller as mc" -s "reverse_it = mc('reverse')" -s "arr = []" "reverse_it(arr)"
Running Release|x64 interpreter...
.....................
Mean +- std dev: 84.3 ns +- 1.9 ns

PS >.\python.bat -m pyperf timeit -s "reverse_it = lambda x: x.reverse()" -s "arr = []" "reverse_it(arr)"
Running Release|x64 interpreter...
.....................
Mean +- std dev: 95.5 ns +- 2.9 ns

#################### Python calls ####################

PS > .\python.bat -m pyperf timeit -s "from operator import methodcaller as mc" -s "reverse_it = mc('reverse')" -s "class A: reverse = lambda self: None" -s "arr=A()" "reverse_it(arr)"
Running Release|x64 interpreter...
.....................
Mean +- std dev: 140 ns +- 4 ns
PS > .\python.bat -m pyperf timeit -s "reverse_it = lambda x: x.reverse()" -s "class A: reverse = lambda self: None" -s "arr=A()" "reverse_it(arr)"
Running Release|x64 interpreter...
.....................
Mean +- std dev: 159 ns +- 4 ns

[markshannon]

If the problem is that methodcaller is not faster than a lambda, then why not use a lambda?

Lambdas are just Python functions and calling them will get faster.

We aren't going to spend time optimizing calls to methodcaller, so you might as well use a lambda.