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List:       python-bugs-list
Subject:    [issue39801] list.insert is slow due to manual memmove
From:       Stefan Pochmann <report () bugs ! python ! org>
Date:       2020-02-29 16:11:28
Message-ID: 1582992688.24.0.071544861703.issue39801 () roundup ! psfhosted ! org
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New submission from Stefan Pochmann <stefan.pochmann@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

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