Lists vs. StringIO vs. Regular String Concat for building strings in Python

results-python3.11.txt

=================================
Python 3.11
=================================
Concatenating 5 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.629
build_ul_concat     : 1.013
Concatenating 32 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.356
build_ul_concat     : 1.074
Concatenating 302 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.253
build_ul_concat     : 1.375
Concatenating 3,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.296
build_ul_concat     : 1.398
Concatenating 30,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.23
build_ul_concat     : 1.567
Concatenating 300,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.296
build_ul_concat     : 1.615
Concatenating 3,000,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.22
build_ul_concat     : 1.6

results-python3.6.txt

Results are pretty noisy as each test only runs for ~1.5sec; use as a rough guide only
=================================
Python 3.6
=================================
Concatenating 5 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.702
build_ul_concat     : 1.045
Concatenating 32 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.342
build_ul_concat     : 1.021
Concatenating 302 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.215
build_ul_concat     : 1.257
Concatenating 3,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.237
build_ul_concat     : 1.48
Concatenating 30,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.255
build_ul_concat     : 1.672
Concatenating 300,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.015
build_ul_concat     : 1.141
Concatenating 3,000,002 strings...
build_ul_list       : 1.0
build_ul_stringio   : 1.333
build_ul_concat     : 1.811

string_building_perf.py

#!/usr/bin/env python3
from io import StringIO
from itertools import permutations
import string
import random
import sys
import timeit


word = 'x' * 8
words = [''.join(p) for p in permutations(string.ascii_letters[:8])]


def get_8x():
    return word


next_word = -1
def get_random_word():
    global next_word
    next_word += 1
    next_word %= len(words)
    return words[next_word]
    #return random.choice(words)


def build_ul_list(linect, get_word=get_8x):
    buffer = ['<ul>\n']
    for i in range(linect):
        buffer.extend([
            '<li>', get_word(), '</li>',
        ])
    buffer.append('</ul>')
    return ''.join(buffer)


def build_ul_stringio(linect, get_word=get_8x):
    buffer = StringIO()
    buffer.write('<ul>\n')
    for i in range(linect):
        buffer.write('<li>')
        buffer.write(get_word())
        buffer.write('</li>')
    buffer.write('</ul>')
    return buffer.getvalue()


def build_ul_concat(linect, get_word=get_8x):
    buffer = '<ul>\n'
    for i in range(linect):
        buffer += '<li>'
        buffer += get_word()
        buffer += '</li>'
    buffer += '</ul>'


def run_tests(size, iter_count):
    string_count = 2 + 3 * size
    print(f"Concatenating {string_count:,} strings", end="", flush=True)
    fastest = None
    results = {}
    for fn in (build_ul_list, build_ul_stringio, build_ul_concat):
        elapsed = timeit.timeit(lambda: fn(size, get_word=get_random_word), number=iter_count)
        if fastest is None or fastest > elapsed:
            fastest = elapsed
        results[fn] = elapsed
        print(".", end="", flush=True)
    print()

    for fn, elapsed in results.items():
        print(f"{fn.__name__:20}: {elapsed/fastest:.4}")

if __name__ == "__main__":
    print("=================================")
    print(f"Python {'.'.join(str(x) for x in sys.version_info[:2])}")
    print("=================================")
    run_tests(1, 5000000)
    run_tests(10, 500000)
    run_tests(100, 50000)
    run_tests(1000, 5000)
    run_tests(10000, 500)
    run_tests(100000, 50)
    run_tests(1000000, 5)