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String concatenation vs. string substitution in Python

Asked 2008-12-17T23:39:39.930
99

In Python, the where and when of using string concatenation versus string substitution eludes me. As the string concatenation has seen large boosts in performance, is this (becoming more) a stylistic decision rather than a practical one?

For a concrete example, how should one handle construction of flexible URIs:

DOMAIN = 'http://stackoverflow.com'
QUESTIONS = '/questions'

def so_question_uri_sub(q_num):
    return "%s%s/%d" % (DOMAIN, QUESTIONS, q_num)

def so_question_uri_cat(q_num):
    return DOMAIN + QUESTIONS + '/' + str(q_num)

Edit: There have also been suggestions about joining a list of strings and for using named substitution. These are variants on the central theme, which is, which way is the Right Way to do it at which time? Thanks for the responses!

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2 Answers

12

Be wary of concatenating strings in a loop! The cost of string concatenation is proportional to the length of the result. Looping leads you straight to the land of N-squared. Some languages will optimize concatenation to the most recently allocated string, but it's risky to count on the compiler to optimize your quadratic algorithm down to linear. Best to use the primitive (join?) that takes an entire list of strings, does a single allocation, and concatenates them all in one go.

answered 2008-12-18T04:40:07.623
10

What you want to concatenate/interpolate and how you want to format the result should drive your decision.

  • String interpolation allows you to easily add formatting. In fact, your string interpolation version doesn't do the same thing as your concatenation version; it actually adds an extra forward slash before the q_num parameter. To do the same thing, you would have to write return DOMAIN + QUESTIONS + "/" + str(q_num) in that example.

  • Interpolation makes it easier to format numerics; "%d of %d (%2.2f%%)" % (current, total, total/current) would be much less readable in concatenation form.

  • Concatenation is useful when you don't have a fixed number of items to string-ize.

Also, know that Python 2.6 introduces a new version of string interpolation, called string templating:

def so_question_uri_template(q_num):
    return "{domain}/{questions}/{num}".format(domain=DOMAIN,
                                               questions=QUESTIONS,
                                               num=q_num)

String templating is slated to eventually replace %-interpolation, but that won't happen for quite a while, I think.

answered 2008-12-18T00:10:09.900

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