Thursday, October 27, 2011

Python string formatting: % vs. .format

Python 2.6 introduced the string.format() method with a slightly different syntax from the existing % operator. Which is better and for what situations?

The following uses each method and has the same outcome, so what is the difference?

sub1 = "python string!"
sub2 = "an arg"

a = "i am a %s"%sub1
b = "i am a {0}".format(sub1)

c = "with %(kwarg)s!"%{'kwarg':sub2}
d = "with {kwarg}!".format(kwarg=sub2)

print a
print b
print c
print d

To answer the question... .format just seems more sophisticated in many ways. You can do stuff like re-use arguments, which you can't do with %. An annoying thing about % is also how it can either take a variable or a tuple. You'd think the following would always work:

"hi there %s" % name

yet, if name happens to be (1, 2, 3), it will throw a TypeError. To guarantee that it always prints, you'd need to do

"hi there %s" % (name,)   # supply the single argument as a single-item tuple

which is just ugly. .format doesn't have those issues. Also in the second example you gave, the .format example is much cleaner looking.

Why would you not use it?

  • not knowing about it (me before reading this)
  • having to be compatible with Python 2.5

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