Fastest technique for string concatenation

python at bdurham.com python at bdurham.com
Sat Oct 2 21:09:55 CEST 2010


My understanding is that appending to a list and then joining
this list when done is the fastest technique for string
concatenation. Is this true?

The 3 string concatenation techniques I can think of are:

- append to list, join
- string 'addition' (s = s + char)
- cStringIO

The code that follows my signature confirms that the list
append/join technique is indeed the fastest of these 3
approaches, but perhaps there are other techniques I should
consider?

Malcolm

# test various techniques for string concatenation
import cStringIO
import timeit
source = 'x' * 5000000
def testListAppend():
    output = list()
    for char in source:
        output.append( char )
    output = ''.join( output )

def testStringConcat():
    output = ''
    for char in source:
        output += char

def testStringIO():
    output = cStringIO.StringIO()
    for char in source:
        output.write( char )
    output = output.getvalue()

def time( func ):
    timingObject = timeit.Timer( func )
    runtime = timingObject.timeit( 10 )
    print '%s = %.2f sec' % ( func.__name__, runtime )
time( testListAppend )
time( testStringConcat )
time( testStringIO )

Output using Python 2.7 (32-bit) under Windows 7 (64-bit)

testListAppend = 10.38 sec
testStringConcat = 53.01 sec
testStringIO = 13.85 sec
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