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

There are various string formatting methods:

Python <2.6:"Hello %s" % name
Python 2.6+:"Hello {}".format(name) (usesstr.format)
Python 3.6+:f"{name}" (uses f-strings)

Which is better, and for what situations?

The following methods have the same outcome, so what is the difference?

name = "Alice""Hello %s" % name"Hello {0}".format(name)f"Hello {name}"# Using named arguments:"Hello %(kwarg)s" % {'kwarg': name}"Hello {kwarg}".format(kwarg=name)f"Hello {name}"

When does string formatting run, and how do I avoid a runtime performance penalty?

_{If you are trying to close a duplicate question that is just looking for a way to format a string, please useHow do I put a variable’s value inside a string?.}

Answer*

But please be careful, just now I've discovered one issue when trying to replace all `%` with `.format` in existing code: **`'{}'.format(unicode_string)` will try to encode unicode_string and will probably fail.**

Just look at this Python interactive session log:

    Python 2.7.2 (default, Aug 27 2012, 19:52:55) 
    [GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2
    ; s='й'
    ; u=u'й'
    ; s
    '\xd0\xb9'
    ; u
    u'\u0439'

`s` is just a string (called 'byte array' in Python3) and `u` is a Unicode string (called 'string' in Python3):

    ; '%s' % s
    '\xd0\xb9'
    ; '%s' % u
    u'\u0439'

When you give a Unicode object as a parameter to `%` operator it will produce a Unicode string even if the original string wasn't Unicode:

    ; '{}'.format(s)
    '\xd0\xb9'
    ; '{}'.format(u)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    UnicodeEncodeError: 'latin-1' codec can't encode character u'\u0439' in position 0: ordinal not in range(256)

but the `.format` function will raise "UnicodeEncodeError":

    ; u'{}'.format(s)
    u'\xd0\xb9'
    ; u'{}'.format(u)
    u'\u0439'

and it will work with a Unicode argument fine only if the original string was Unicode.

    ; '{}'.format(u'i')
    'i'

or if argument string can be converted to a string (so called 'byte array')

tips on writing great answers.

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14
There is simply no reason to change working code unless the additional features of the newformat method are really needed ...
Tobias
– Tobias
2013-01-28 22:51:44 +00:00
CommentedJan 28, 2013 at 22:51
absolutely agree with you, Tobias, but sometimes it's needed when upgrading to newer versions of Python
wobmene
– wobmene
2013-01-30 13:17:08 +00:00
CommentedJan 30, 2013 at 13:17
3
For instance? AFAIK, it hasnever been needed; I don't consider it likely that the% string interpolation would ever go away.
Tobias
– Tobias
2013-01-31 13:45:02 +00:00
CommentedJan 31, 2013 at 13:45
4
I consider .format() function safer than % for strings. Often I see beginners' mistakes like this"p1=%s p2=%d" % "abc", 2 or"p1=%s p2=%s" % (tuple_p1_p2,). You might think it's the coder's fault but I think it's just weird faulty syntax that looks nice for the quicky-scriptie but is bad for production code.
wobmene
– wobmene
2014-01-06 15:07:03 +00:00
CommentedJan 6, 2014 at 15:07
4
But I don't like the syntax of .format(), I'd be happier with good old%s,%02d like"p1=%s p2=%02d".format("abc", 2). I blame those who invented and approved the curly braces formatting that needs you to escape them like{{}} and looks ugly imho.
wobmene
– wobmene
2014-01-06 15:15:04 +00:00
CommentedJan 6, 2014 at 15:15

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

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