Wayne Werner Wayne Werner - 2 months ago 8
Python Question

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?

Answer

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).

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