Any reason not to use '+' to concatenate t

Question

A common antipattern in Python is to concatenate a sequence of strings using + in a loop. This is bad because the Python interpreter has to create a new string object for each iteration, and it ends up taking quadratic time. (Recent versions of CPython can apparently optimize this in some cases, but other implementations can't, so programmers are discouraged from relying on this.) ''.join is the right way to do this.

However, I've heard it said (including here on Stack Overflow) that you should never, ever use + for string concatenation, but instead always use ''.join or a format string. I don't understand why this is the case if you're only concatenating two strings. If my understanding is correct, it shouldn't take quadratic time, and I think a + b is cleaner and more readable than either ''.join((a, b)) or '%s%s' % (a, b).

Is it good practice to use + to concatenate two strings? Or is there a problem I'm not aware of?

Answer 1

I have done a quick test:

import sys

str = e = "a xxxxxxxxxx very xxxxxxxxxx long xxxxxxxxxx string xxxxxxxxxx\n"

for i in range(int(sys.argv[1])):
    str = str + e

and timed it:

mslade@mickpc:/binks/micks/ruby/tests$ time python /binks/micks/junk/strings.py  8000000
8000000 times

real    0m2.165s
user    0m1.620s
sys     0m0.540s
mslade@mickpc:/binks/micks/ruby/tests$ time python /binks/micks/junk/strings.py  16000000
16000000 times

real    0m4.360s
user    0m3.480s
sys     0m0.870s

There is apparently an optimisation for the a = a + b case. It does not exhibit O(n^2) time as one might suspect.

So at least in terms of performance, using + is fine.