String concatenation in Python

文字列連結の効率の話
addのほうが早かったよなと思って、色々試してみた。

def str_mul():
    return name * count

def str_join():
    lst = []
    append = lst.append
    for i in xrange(count):
        append(name)
    return ''.join(lst)

def str_map():
    lst = map(lambda x:name, xrange(count))
    return ''.join(lst)

def list_comp():
    return ''.join(name for i in xrange(count))

def str_add():
    s = ''
    for i in xrange(count):
        s += name
    return s

import cStringIO
def cstring_io():
    io = cStringIO.StringIO()
    write = io.write
    for i in xrange(count):
        write(name)
    return io.getvalue()

import StringIO
def string_io():
    io = StringIO.StringIO()
    write = io.write
    for i in xrange(count):
        write(name)
    return io.getvalue()

from array import array
def str_array():
    a = array('c')
    add = a.fromstring
    for i in xrange(count):
        add(name)
    return a.tostring()

from mmap import mmap
def str_mmap():
    m = mmap(-1, count * len(name))
    write = m.write
    for i in xrange(count):
        write(name)
    m.seek(0)
    return m.read(count * len(name))

def main():
    global count, name
    func_list = (str_mul, str_join, str_add, string_io, cstring_io,
                 str_map, list_comp, str_array, str_mmap)

    # test
    count = 2
    name = 'hello'
    assert all((name * count).__eq__(f()) for f in func_list)

    import timeit
    for c in (10, 1000):
        for b in (1, 1000):
            for f in func_list:
                count = c
                name = 'hello' * b
                print '%10s x %4s, "hello"*%4s: %9.4fms'%(
                    f.func_name, c, b, timeit.timeit(f, number=100)/100*1000)

if __name__ == '__main__':
    main()

   str_mul x   10, "hello"*   1:    0.0006ms
  str_join x   10, "hello"*   1:    0.0042ms
   str_add x   10, "hello"*   1:    0.0033ms
 string_io x   10, "hello"*   1:    0.0312ms
cstring_io x   10, "hello"*   1:    0.0072ms
   str_map x   10, "hello"*   1:    0.0059ms
 list_comp x   10, "hello"*   1:    0.0077ms
 str_array x   10, "hello"*   1:    0.0081ms
  str_mmap x   10, "hello"*   1:    0.0180ms
   str_mul x   10, "hello"*1000:    0.0108ms
  str_join x   10, "hello"*1000:    0.0131ms
   str_add x   10, "hello"*1000:    0.0126ms
 string_io x   10, "hello"*1000:    0.0411ms
cstring_io x   10, "hello"*1000:    0.0364ms
   str_map x   10, "hello"*1000:    0.0146ms
 list_comp x   10, "hello"*1000:    0.0167ms
 str_array x   10, "hello"*1000:    0.0377ms
  str_mmap x   10, "hello"*1000:    0.1105ms
   str_mul x 1000, "hello"*   1:    0.0017ms
  str_join x 1000, "hello"*   1:    0.2352ms
   str_add x 1000, "hello"*   1:    0.2235ms
 string_io x 1000, "hello"*   1:    2.4272ms
cstring_io x 1000, "hello"*   1:    0.5111ms
   str_map x 1000, "hello"*   1:    0.3591ms
 list_comp x 1000, "hello"*   1:    0.2296ms
 str_array x 1000, "hello"*   1:    0.5338ms
  str_mmap x 1000, "hello"*   1:    0.4778ms
   str_mul x 1000, "hello"*1000:    8.5971ms
  str_join x 1000, "hello"*1000:    8.3908ms
   str_add x 1000, "hello"*1000:   29.4590ms
 string_io x 1000, "hello"*1000:   10.6351ms
cstring_io x 1000, "hello"*1000:   28.7235ms
   str_map x 1000, "hello"*1000:    8.8407ms
 list_comp x 1000, "hello"*1000:    8.2300ms
 str_array x 1000, "hello"*1000:   38.9228ms
  str_mmap x 1000, "hello"*1000:   18.6053ms

addは50kB程度までは、文字列長や回数にかかわらず早い。
'str' * nをjoinが逆転するのは見てびっくり。
StringIOは1MBを超えたあたりで突然加速、理由不明。
cStringIOを余裕で追い抜き、100MBあたりではjoinに並ぶ。
ただし、1MBを超えるようなものはPythonで扱うべきではないかも。
for, map, list内包は勝ったり負けたり。
disってみたところ、早そうなのはfor-join。
psycoも使ってみたけど、速度はあまりかわらず。

結論:
addでもjoinでも扱いやすいほうを使おう。
速度はPythonを使う上では気にしない。