Why sum function is slower if the 'start' argument is an instance of custom class?-CodePudding

I was playing around with sum function and observed the following behaviour.

case 1:

source = """
class A:
    def __init__(self, a):
        self.a = a
    
    def __add__(self, other):
        return self.a   other;

sum([*range(10000)], start=A(10))
"""

import timeit
print(timeit.timeit(stmt=source))

As you can see I am using an instance of custom class as start argument to the sum function. Benchmarking above code takes around 192.60747704200003 seconds in my system.

case 2:

source = """
class A:
    def __init__(self, a):
        self.a = a
    
    def __add__(self, other):
        return self.a   other;

sum([*range(10000)], start=10).  <- Here
"""

import timeit
print(timeit.timeit(stmt=source))

But if I remove the custom class instance and use int object directly it tooks only 111.48285191600007 seconds. I am curious to understand the reason for this speed difference?

My system info:

>>> import platform
>>> platform.platform()
'macOS-12.5-arm64-arm-64bit'
>>> import sys
>>> sys.version
'3.11.0 (v3.11.0:deaf509e8f, Oct 24 2022, 14:43:23) [Clang 13.0.0 (clang-1300.0.29.30)]'

CodePudding user response：

builtin_sum_impl has 2 implementations inside, one if the start is a number which skips creating python "number objects" and just sums numbers in C.

the other slower implementation when start is not a number, which forces the __add__ method of "number objects" to be called, (because it assumes you are summing some weird classes).

you forced it to use the slower one.

CodePudding user response：

Maybe looking at the byte-code can help understand what happens. If you run

import dis

def test_range():
    class A:
        def __init__(self, a):
            self.a = a

        def __add__(self, other):
            return self.a   other

    sum([*range(10000)], start=10)

dis.dis(test_range)

the version with start=A(10) generates 2 more instructions:

2 LOAD_CONST               1 (<code object A at 0x7ff0bfa25c90, file "/.../main.py", line 5>)
...
26 LOAD_CONST               4 (10)
28 LOAD_CONST               5 (('start',))
30 CALL_FUNCTION_KW         2
32 POP_TOP
34 LOAD_CONST               0 (None)
36 RETURN_VALUE

2 LOAD_CONST               1 (<code object A at 0x7ff0bfa25c90, file "/.../main.py", line 5>)
...
26 LOAD_FAST                0 (A)       <--- here
28 LOAD_CONST               4 (10)
30 CALL_FUNCTION            1           <--- and here
32 LOAD_CONST               5 (('start',))
34 CALL_FUNCTION_KW         2
36 POP_TOP
38 LOAD_CONST               0 (None)
40 RETURN_VALUE

Complete byte-code for version with start=A(10) is here.

My (limited) understanding is that those 2 lines point to the initialization of A. Please, someone confirm.