What is the most efficient string concatenation method in python?

Question

Is there any efficient mass string concatenation method in Python (like StringBuilder in C# or StringBuffer in Java)? I found following methods here:

• Simple concatenation using '+'
• Using UserString from MutableString module
• Using character array and the array module
• Using string list and join method
• Using cStringIO from StringIO module

But what do you experts use or suggest, and why?

Answer 1

''.join(sequenceofstrings) is what usually works best -- simplest and fastest.

Answer 2

You may be interested in this: An optimiztion anecdote by Guido. Although it is worth remembering also that this is an old article and it predates the existence of things like ''.join (although I guess string.joinfields is more-or-less the same)

On the strength of that, the array module may be fastest if you can shoehorn your problem into it. But ''.join is probably fast enough and has the benefit of being idiomatic and thus easier for other python programmers to understand.

Finally, the golden rule of optimization: don't optimize unless you know need to, and measure rather than guessing.

You can measure different methods using the timeit module. That can tell you which is fastest, instead of random strangers on the internet making guesses.

Answer 3

It depends on what you're doing.

After Python 2.5, string concatenation with the + operator is pretty fast. If you're just concatenating a couple of values, using the + operator works best:

>>> x = timeit.Timer(stmt="'a' + 'b'")
>>> x.timeit()
0.039999961853027344

>>> x = timeit.Timer(stmt="''.join(['a', 'b'])")
>>> x.timeit()
0.76200008392333984

However, if you're putting together a string in a loop, you're better off using the list joining method:

>>> join_stmt = """
... joined_str = ''
... for i in xrange(100000):
...   joined_str += str(i)
... """
>>> x = timeit.Timer(join_stmt)
>>> x.timeit(100)
13.278000116348267

>>> list_stmt = """
... str_list = []
... for i in xrange(100000):
...   str_list.append(str(i))
... ''.join(str_list)
... """
>>> x = timeit.Timer(list_stmt)
>>> x.timeit(100)
12.401000022888184

...but notice that you have to be putting together a relatively high number of strings before the difference becomes noticeable.

Answer 4

this url has the comparisons of the different approaches along with some benchmarking:

http://skymind.com/~ocrow/python%5Fstring/