faster way to do .format()

Question

I'm writing a program that needs to do a lot of string formatting and I have noticed that .format() is taking a small but significant amount of cpu time. Here's how I'm using it:

str = 'vn {0:.15f} {1:.15f} {2:.15f}\n'.format(normal_array[i].x, normal_array[i].y, normal_array[i].z)

Does anyone know if there is even a slightly faster way to do this as a small fraction X 100000 can add up

Answer 1

Try to replace .format with % expression and pre-calculate normal_array:

item = normal_array[i]
'vn %.15f %.15f %.15f\n' % (item.x, item.y, item.z)

Also replacing indexes with iteration over values can slightly improve speed:

for item in normal_array:
    'vn %.15f %.15f %.15f\n' % (item.x, item.y, item.z)

Benchmark:

def gen_data(n):
    l = []
    for k in xrange(n):
        l.append(collections.namedtuple('normal', ('x', 'y', 'z'))(random.random(), random.random(), random.random()))
    return l

if __name__ == '__main__':
    times = 1000
    print 'format:'
    print timeit.Timer('for i in xrange(len(normal_array)):\n    str = "vn {0:.15f} {1:.15f} {2:.15f}\\n".format(normal_array[i].x, normal_array[i].y, normal_array[i].z)\n',
            'from __main__ import gen_data; normal_array = gen_data(1000)').timeit(times)
    print '%s:'
    print timeit.Timer('for i in xrange(len(normal_array)):\n    str = "vn %.15f %.15f %.15f\\n".format(normal_array[i].x, normal_array[i].y, normal_array[i].z)\n',
            'from __main__ import gen_data; normal_array = gen_data(1000)').timeit(times)
    print '%s+iteration:'
    print timeit.Timer('for o in normal_array:\n    str = "vn %.15f %.15f %.15f\\n".format(o.x, o.y, o.z)\n',
            'from __main__ import gen_data; normal_array = gen_data(1000)').timeit(times)

Results (lower is better)

format:
5.34718108177
%s:
1.30601406097
%s+iteration:
1.23484301567

Answer 2

Also you can try to migrate to PyPy, there was an article about string formatting comparison in cpython and PyPy.