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

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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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