Python – String formatting: % vs. .format vs. f-string literal


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

Python 3.6 has now introduced another string formatting format of string literals (aka "f" strings) via the syntax f"my string". Is this formatting option better than the others?

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

     sub1 = "python string!"
     sub2 = "an arg"
     sub_a = "i am a %s" % sub1
     sub_b = "i am a {0}".format(sub1)
     sub_c = f"i am a {sub1}"
     arg_a = "with %(kwarg)s!" % {'kwarg':sub2}
     arg_b = "with {kwarg}!".format(kwarg=sub2)
     arg_c = f"with {sub2}!"
     print(sub_a)    # "i am a python string!"
     print(sub_b)    # "i am a python string!"
     print(sub_c)    # "i am a python string!"
     print(arg_a)    # "with an arg!"
     print(arg_b)    # "with an arg!"
     print(arg_c)    # "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)

Best Solution

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

To answer your second question, string formatting happens at the same time as any other operation - when the string formatting expression is evaluated. And Python, not being a lazy language, evaluates expressions before calling functions, so in your log.debug example, the expression "some debug info: %s"%some_infowill first evaluate to, e.g. "some debug info: roflcopters are active", then that string will be passed to log.debug().