Python Cookbook

Combining Strings

Credit: Luther Blissett

Problem

You have several small strings that you need to combine into one larger string.

Solution

The + operator concatenates strings and therefore offers seemingly obvious solutions for putting small strings together into a larger one. For example, when you have all the pieces at once, in a few variables:

largeString = small1 + small2 + ' something ' + small3 + ' yet more'

Or when you have a sequence of small string pieces:

largeString = ''
for piece in pieces:
    largeString += piece

Or, equivalently, but a bit more compactly:

import operator
largeString = reduce(operator.add, pieces, '')

However, none of these solutions is generally optimal. To put together pieces stored in a few variables, the string-formatting operator % is often best:

largeString = '%s%s something %s yet more' % (small1, small2, small3)

To join a sequence of small strings into one large string, the string operator join is invariably best:

largeString = ''.join(pieces)

Discussion

In Python, string objects are immutable. Therefore, any operation on a string, including string concatenation, produces a new string object, rather than modifying an existing one. Concatenating N strings thus involves building and then immediately throwing away each of N-1 intermediate results. Performance is therefore quite a bit better for operations that build no intermediate results, but rather produce the desired end result at once. The string-formatting operator % is one such operation, particularly ...