[New-bugs-announce] [issue39801] list.insert is slow due to manual memmove

[Stefan Pochmann]

New submission from Stefan Pochmann <stefan.pochmann at gmail.com>:

Using a list's insert function is much slower than using slice assignment:

> python -m timeit -n 100000 -s "a=[]" "a.insert(0,0)"
100000 loops, best of 5: 19.2 usec per loop

> python -m timeit -n 100000 -s "a=[]" "a[0:0]=[0]"
100000 loops, best of 5: 6.78 usec per loop

(Note that the list starts empty but grows to 100,000 elements.)

At first I thought maybe it's the attribute lookup or function call overhead or so, but inserting near the end shows that that's negligible:

> python -m timeit -n 100000 -s "a=[]" "a.insert(-1,0)"
100000 loops, best of 5: 79.1 nsec per loop

I asked at StackOverflow and someone pointed out that list.insert uses a manual loop instead of memmove: https://stackoverflow.com/a/60466572/12671057

----------
components: Interpreter Core
messages: 362986
nosy: Stefan Pochmann
priority: normal
severity: normal
status: open
title: list.insert is slow due to manual memmove
type: performance
versions: Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39801>
_______________________________________