How slow is Python's string concatenation vs. str.join?

Question

As a result of the comments in my answer on this thread, I wanted to know what the speed difference is between the += operator and ''.join()

So what is the speed comparison between the two?

Answer 1

Note: This benchmark was informal and is due to be redone because it doesn't show a full picture of how these methods will perform with more realistically long strings. As mentioned in the comments by @Mark Amery, += is not reliably as fast as using f-strings, and str#join isn't as dramatically slower in realistic use cases.

These metrics are also likely outdated by the significant performance improvements introduced by subsequent CPython versions, and most notably, 3.11.

---

The existing answers are very well-written and researched, but here's another answer for the Python 3.6 era, since now we have literal string interpolation (AKA, f-strings):

>>> import timeit
>>> timeit.timeit('f\'{"a"}{"b"}{"c"}\'', number=1000000)
0.14618930302094668
>>> timeit.timeit('"".join(["a", "b", "c"])', number=1000000)
0.23334730707574636
>>> timeit.timeit('a = "a"; a += "b"; a += "c"', number=1000000)
0.14985873899422586

Test performed using CPython 3.6.5 on a 2012 Retina MacBook Pro with an Intel Core i7 at 2.3 GHz.

Answer 2

If I say it algorithmically, if you choose [ += ] then it generates a new object and it will be O(n)**2. But if you use [ .join ] then it will be O(n).

Answer 3

If I expect well, for a list with k string, with n characters in total, time complexity of join should be O(nlogk) while time complexity of classic concatenation should be O(nk).

That would be the same relative costs as merging k sorted list (efficient method is O(nlkg), while the simple one, akin to concatenation is O(nk) ).

Answer 4

From: Efficient String Concatenation

Method 1:

def method1():
  out_str = ''
  for num in xrange(loop_count):
    out_str += 'num'
  return out_str

Method 4:

def method4():
  str_list = []
  for num in xrange(loop_count):
    str_list.append('num')
  return ''.join(str_list)

Now I realise they are not strictly representative, and the 4th method appends to a list before iterating through and joining each item, but it's a fair indication.

String join is significantly faster then concatenation.

Why? Strings are immutable and can't be changed in place. To alter one, a new representation needs to be created (a concatenation of the two).

Answer 5

I rewrote the last answer, could jou please share your opinion on the way i tested?

import time

start1 = time.clock()
for x in range (10000000):
    dog1 = ' and '.join(['spam', 'eggs', 'spam', 'spam', 'eggs', 'spam','spam', 'eggs', 'spam', 'spam', 'eggs', 'spam'])

end1 = time.clock()
print("Time to run Joiner = ", end1 - start1, "seconds")


start2 = time.clock()
for x in range (10000000):
    dog2 = 'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'

end2 = time.clock()
print("Time to run + = ", end2 - start2, "seconds")

NOTE: This example is written in Python 3.5, where range() acts like the former xrange()

The output i got:

Time to run Joiner =  27.086106206103153 seconds
Time to run + =  69.79100515996426 seconds

Personally i prefer ''.join([]) over the 'Plusser way' because it's cleaner and more readable.

Answer 6

My original code was wrong, it appears that + concatenation is usually faster (especially with newer versions of Python on newer hardware)

The times are as follows:

Iterations: 1,000,000       

Python 3.3 on Windows 7, Core i7

String of len:   1 took:     0.5710     0.2880 seconds
String of len:   4 took:     0.9480     0.5830 seconds
String of len:   6 took:     1.2770     0.8130 seconds
String of len:  12 took:     2.0610     1.5930 seconds
String of len:  80 took:    10.5140    37.8590 seconds
String of len: 222 took:    27.3400   134.7440 seconds
String of len: 443 took:    52.9640   170.6440 seconds

Python 2.7 on Windows 7, Core i7

String of len:   1 took:     0.7190     0.4960 seconds
String of len:   4 took:     1.0660     0.6920 seconds
String of len:   6 took:     1.3300     0.8560 seconds
String of len:  12 took:     1.9980     1.5330 seconds
String of len:  80 took:     9.0520    25.7190 seconds
String of len: 222 took:    23.1620    71.3620 seconds
String of len: 443 took:    44.3620   117.1510 seconds

On Linux Mint, Python 2.7, some slower processor

String of len:   1 took:     1.8840     1.2990 seconds
String of len:   4 took:     2.8394     1.9663 seconds
String of len:   6 took:     3.5177     2.4162 seconds
String of len:  12 took:     5.5456     4.1695 seconds
String of len:  80 took:    27.8813    19.2180 seconds
String of len: 222 took:    69.5679    55.7790 seconds
String of len: 443 took:   135.6101   153.8212 seconds

And here is the code:

from __future__ import print_function
import time

def strcat(string):
    newstr = ''
    for char in string:
        newstr += char
    return newstr

def listcat(string):
    chars = []
    for char in string:
        chars.append(char)
    return ''.join(chars)

def test(fn, times, *args):
    start = time.time()
    for x in range(times):
        fn(*args)
    return "{:>10.4f}".format(time.time() - start)

def testall():
    strings = ['a', 'long', 'longer', 'a bit longer', 
               '''adjkrsn widn fskejwoskemwkoskdfisdfasdfjiz  oijewf sdkjjka dsf sdk siasjk dfwijs''',
               '''this is a really long string that's so long
               it had to be triple quoted  and contains lots of
               superflous characters for kicks and gigles
               @!#(*_#)(*$(*!#@&)(*E\xc4\x32\xff\x92\x23\xDF\xDFk^%#$!)%#^(*#''',
              '''I needed another long string but this one won't have any new lines or crazy characters in it, I'm just going to type normal characters that I would usually write blah blah blah blah this is some more text hey cool what's crazy is that it looks that the str += is really close to the O(n^2) worst case performance, but it looks more like the other method increases in a perhaps linear scale? I don't know but I think this is enough text I hope.''']

    for string in strings:
        print("String of len:", len(string), "took:", test(listcat, 1000000, string), test(strcat, 1000000, string), "seconds")

testall()

Answer 7

This is what silly programs are designed to test :)

Use plus

import time

if __name__ == '__main__':
    start = time.clock()
    for x in range (1, 10000000):
        dog = "a" + "b"

    end = time.clock()
    print "Time to run Plusser = ", end - start, "seconds"

Output of:

Time to run Plusser =  1.16350010965 seconds

Now with join....

import time
if __name__ == '__main__':
    start = time.clock()
    for x in range (1, 10000000):
        dog = "a".join("b")

    end = time.clock()
    print "Time to run Joiner = ", end - start, "seconds"

Output Of:

Time to run Joiner =  21.3877386651 seconds

So on python 2.6 on windows, I would say + is about 18 times faster than join :)