How to format strings in python?

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

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

    #!/usr/bin/python
    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    # "i am a python string!"
    print b    # "i am a python string!"
    print c    # "with an arg!"
    print d    # "with an arg!"
  2. Furthermore when does string formatting occur in Python? For example, if my logging level is set to HIGH will I still take a hit for performing the following % operation? And if so, is there a way to avoid this?

    log.debug("some debug info: %s" % some_info)
Oct 1, 2018 in Python by charlie_brown
• 7,710 points
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To answer your first question... .format just seems more sophisticated in many ways. 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
answered Oct 1, 2018 by ariaholic
• 7,320 points

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