Most Efficient Method to Concatenate Strings in Python

Question

At the time of asking this question, I\'m using Python 3.8

When I say efficient, I\'m only referring to the speed at which the strings are concatenated,

Answer 1

For exactly two strings a and b, just use a + b. The alternatives are for joining more than 2 strings, avoiding the temporary str object created by each use of +, as well as the quadratic behavior due to repeatedly copying the contents of earlier operations in the next result.

(There's also f'{a}{b}', but it's syntactically heavier and no faster than a + b.)

Answer 2

Why don't you try it out? You can use timeit.timeit() to run a statement many times and return the overall duration.

Here, we use s to setup the variables a and b (not included in the overall time), and then run the various options 10 million times.

>>> from timeit import timeit
>>>
>>> n = 10 * 1000 * 1000
>>> s = "a = 'start'; b = ' end'"
>>>
>>> timeit("c = a + b",                 setup=s, number=n)
0.4452877212315798
>>>
>>> timeit("c = f'{a}{b}'",             setup=s, number=n)
0.5252049304544926
>>>
>>> timeit("c = '%s%s'.format(a, b)",   setup=s, number=n)
0.6849184390157461
>>>>
>>> timeit("c = ''.join((a, b))",       setup=s, number=n)
0.8546998891979456
>>>
>>> timeit("c = '%s%s' % (a, b)",       setup=s, number=n)
1.1699129864573479
>>>
>>> timeit("c = '{0}{1}'.format(a, b)", setup=s, number=n)
1.5954962372779846

This shows that unless your application's bottleneck is string concatenation, it's probably not worth being too concerned about...

• The best case is ~0.45 seconds for 10 million iterations, or about 45ns per operation.
• The worst case is ~1.59 seconds for 10 million iterations, or about 159ns per operation.

If you're performing literally millions of operations, you'll see a speed improvement of about 1 second.

Note that your results may vary quite drastically depending on the lengths (and number) of the strings you're concatenating, and the hardware you're running on.

Answer 3

from datetime import datetime
a = "start"
b = " end"

start = datetime.now()
print(a+b)
print(datetime.now() - start)

start = datetime.now()
print("".join((a, b)))
print(datetime.now() - start)

start = datetime.now()
print('{0}{1}'.format(a, b))
print(datetime.now() - start)

# Output
# start end
# 0:00:00.000056
# start end
# 0:00:00.000014
# start end
# 0:00:00.000014

Looks like .join() and .format() are basically the same and 4x faster. An F string, eg:

print(f'{a} {b}')

is also a very quick and clean method, especially when working with more complex formats.