Fastest Way to Concatenate Two Strings in Python

Question

Given two arbitrary strings:

a = 'start'
b = ' end'

When concatenated they produce 'start end'

What is the fastest way to concatenate the two strings?

Answer 1

Methods:

Method 1:

a += b

Method 2:

a + b

Method 3:

''.join((a, b))

Method 4:

'{0}{1}'.format(a, b)

Method 5:

f'{a}{b}'

Method 6:

'%s%s' % (a, b)

Test Program:

We can utilize the timeit library to test each method.

import timeit

setup: str = 'a = \'start\'\nb = \' end\''
tests: tuple = (
    'a += b',
    'a + b',
    '\'\'.join((a, b))',
    '\'{0}{1}\'.format(a, b)',
    'f\'{a}{b}\'',
    '\'%s%s\' % (a, b)')
number: int = 10000

print(f'Setup:\n{setup}')
times: list = []
for test in tests:
    print(f'\nTest: {test}')
    time: float = timeit.Timer(test, setup=setup).timeit(number=number)
    times.append(str(time))
    print(f'Time: {time}')
print('\n\nTimes Side by Side:\n{}'.format('\n'.join(times)))

With this example code, we run each method 10000 times and return the average time it takes.

Results:

Setup:
a = 'start'
b = ' end'

Test: a += b
Time: 0.0009844999999999993

Test: a + b
Time: 0.00042200000000003346

Test: ''.join((a, b))
Time: 0.0008056000000000174

Test: '{0}{1}'.format(a, b)
Time: 0.002130199999999971

Test: f'{a}{b}'
Time: 0.00047549999999996206

Test: '%s%s' % (a, b)
Time: 0.0015849000000000002


Times Side by Side:
0.0009844999999999993
0.00042200000000003346
0.0008056000000000174
0.002130199999999971
0.00047549999999996206
0.0015849000000000002

As you can see, Method 2, a + b is the fastest concatenation method.