What is the most efficient string concatenation method in Python?

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.)

Answer

If you know all components beforehand once, use the literal string interpolation, also known as f-strings or formatted strings, introduced in Python 3.6.

Given the test case from mkoistinen’s answer, having strings

domain = 'some_really_long_example.com'
lang = 'en'
path = 'some/really/long/path/'

The contenders and their execution time on my computer using Python 3.6 on Linux as timed by IPython and the timeit module are

• f'http://{domain}/{lang}/{path}' – 0.151 µs

• 'http://%s/%s/%s' % (domain, lang, path) – 0.321 µs

• 'http://' + domain + '/' + lang + '/' + path – 0.356 µs

• ''.join(('http://', domain, '/', lang, '/', path)) – 0.249 µs (notice that building a constant-length tuple is slightly faster than building a constant-length list).

Thus the shortest and the most beautiful code possible is also fastest.

---

^{The speed can be contrasted with the fastest method for Python 2, which is + concatenation on my computer; and that takes 0.203 µs with 8-bit strings, and 0.259 µs if the strings are all Unicode.}

^{(In alpha versions of Python 3.6 the implementation of f'' strings was the slowest possible – actually the generated byte code is pretty much equivalent to the ''.join() case with unnecessary calls to str.__format__ which without arguments would just return self unchanged. These inefficiencies were addressed before 3.6 final.)}