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

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?

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).

alt text

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.

This is by no means any formal benchmark, but it looks like using f-strings is roughly as performant as using += concatenation; any improved metrics or suggestions are, of course, welcome.

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

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