Python etc / join strings

Published: 29 December 2020, 18:00

What is the fastest way to build a string from many substrings in a loop? In other words, how to concatenate fast when we don’t know in advance how many strings we have? There are many discussions about it, and the common advice is that strings are immutable, so it’s better to use a list and then str.join it. Let’s not trust anyone and just check it.

The straightforward solution:

s = ''
for _ in range(10*8):
  s += 'a'
# 4.04 µs ± 256 ns per loop

Using lists:

a = []
for _ in range(10*8):
# 4.06 µs ± 144 ns per loop

So, it’s about the same. But we can go deeper. What about generator expressions?

''.join('a' for _ in range(10*8))
# 3.56 µs ± 95.9 ns per loop

A bit faster. What if we use list comprehensions instead?

''.join(['a' for _ in range(10*8)])
# 2.52 µs ± 42.1 ns per loop

Wow, this is 1.6x faster than what we had before. Can you make it faster?

And there should be a disclaimer:

  1. Avoid premature optimization, value readability over performance when using a bit slower operation is tolerable.

  2. If you think that something is slow, prove it first. It can be different in your case.