What is the most efficient string concatenation method in Python?

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Is there an efficient mass string concatenation method in Python (like StringBuilder in C# or StringBuffer in Java)?

I found following methods here:

  • Simple concatenation using +
  • Using a string list and the join method
  • Using UserString from the MutableString module
  • Using a character array and the array module
  • Using cStringIO from the StringIO module

What should be used and why?

(A related question is here.)

python string
12 Answers

Update: Python3.11 has some optimizations for % formatting yet it maybe still better to stick with f-strings.

For Python 3.8.6/3.9, I had to do some dirty hacks, because perfplot was giving out some errors. Here assume that x[0] is a a and x[1] is b:

Performance

The plot is nearly same for large data. For small data,

Performance 2

Taken by perfplot and this is the code, large data == range(8), small data == range(4).

import perfplot

from random import choice
from string import ascii_lowercase as letters

def generate_random(x):
    data = ''.join(choice(letters) for i in range(x))
    sata = ''.join(choice(letters) for i in range(x))
    return [data,sata]

def fstring_func(x):
    return [ord(i) for i in f'{x[0]}{x[1]}']

def format_func(x):
    return [ord(i) for i in "{}{}".format(x[0], x[1])]

def replace_func(x):
    return [ord(i) for i in "|~".replace('|', x[0]).replace('~', x[1])]

def join_func(x):
    return [ord(i) for i in "".join([x[0], x[1]])]

perfplot.show(
    setup=lambda n: generate_random(n),
    kernels=[
        fstring_func,
        format_func,
        replace_func,
        join_func,
    ],
    n_range=[int(k ** 2.5) for k in range(4)],
)

When medium data is there, and four strings are there x[0], x[1], x[2], x[3] instead of two strings:

def generate_random(x):
    a =  ''.join(choice(letters) for i in range(x))
    b =  ''.join(choice(letters) for i in range(x))
    c =  ''.join(choice(letters) for i in range(x))
    d =  ''.join(choice(letters) for i in range(x))
    return [a,b,c,d]

Performance 3

Better to stick with f-strings. Also the speed of %s is similar to .format().

user_113329990

Probably the "new f-strings in Python 3.6" is the most efficient way of concatenating strings.

Using %s

>>> timeit.timeit("""name = "Some"
... age = 100
... '%s is %s.' % (name, age)""", number = 10000)
0.0029734770068898797

Using .format

>>> timeit.timeit("""name = "Some"
... age = 100
... '{} is {}.'.format(name, age)""", number = 10000)
0.004015227983472869

Using f-strings

>>> timeit.timeit("""name = "Some"
... age = 100
... f'{name} is {age}.'""", number = 10000)
0.0019175919878762215
user_34649710
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