Python's string formatting syntax

As you are surely aware of, you can use python's % operator to do string formatting. It's part of the reasons why the language is so good at text processing.

The left operand is the format string, the right operand is a single format argument, or an iterable of format arguments. In the format string, you can use %s to insert a string, %d to insert an integer, %f for a float, etc. It's much like C's printf family of functions.

    >>> '%s %d' % ('a string', 123)
    'a string 123'

This you almost surely know. You might not know that the string can take a %r argument which calls repr on the arguments. Now that's useful!

    >>> 'a repr: %r' % ['', ""]
    "a repr: ['', '']"
    >>> 

Or that the format parameters can be in a dictionary.

    >>> '%(a_string)s %(an_int)d' % {'a_string':'a string', 'an_int': 123}
    'a string 123'
    >>> 

If the parameters were passed as a dictionary, the format string will not raise exceptions for extra parameters. So you can use locals() on your (trusted) format string, and format it easily. In a class method, self.__dict__ would also be useful.

    >>> a_number = 3
    >>> '%(a_number)d' % locals()
    '3'

String formatting parameters will also take their own parameters. The parameters are decimal characters (and an optional separation dot) between the % sign and the character indicating the type.

Here is an ugly syntax representation. Bear with me.

% [0|-][leading/trailing character count][An optional dot][Truncation](Type character)

That's all, I think. Be careful to add no spaces. The only characters allowed between % and the type indicator are [0-9\.\-]

• First, the usual percentage sign.

• Then, add a single zero if you want to fill the string with zeroes (you specify how many characters you would like to fill the string with in the next argument). If you want the string to have trailing spaces instead of leading spaces, add a minus sign.

• After that, add the number of characters you want to fill the string with. You can skip this.

• If you want to use the truncation argument (explained below), add a dot here.

• Add the truncation argument, which is an integer.

For obvious reasons, you can't have trailing zeroes in anything.

The "Truncation" argument is used to left-truncate a string to that size, to grow (only grow) an integer string by zero-filling the left to that size, or to use that many decimal digits in a float.

Here are some examples with %s. %s doesn't let you put leading zeroes on your string. You can use str.zfill() for that.

    >>> s = '1234'
    >>> '%.1s' % s
    '1'
    >>> '%.4s' % s
    '1234'
    >>> '%.6s' % s
    '1234'
    >>> '%10s' % s
    '      1234'
    >>> '%10.2s' % s
    '        12'
    >>> '%10.4s' % s
    '      1234'
    >>> '%10.6s' % s
    '      1234'

%f examples:

    >>> f = 10.123
    >>> '%f' % f
    '10.123000'
    >>> '%.4f' % f
    '10.1230'
    >>> '%4f' % f
    '10.123000'
    >>> '%20f' % f
    '           10.123000'
    >>> '%20.2f' % f
    '               10.12'
    >>> '%020.2f' % f
    '00000000000000010.12'
    >>> 

%d can also be told to have leading zeroes or leading spaces. As mentioned above the "Truncation" part of the parameter can only make it grow. It wouldn't make sense to let it shrink, since that would shrink the number's value.

    >>> i = 10
    >>> '%d' % i
    '10'
    >>> '%.1d' % i
    '10'
    >>> '%.4d' % i
    '0010'
    >>> '%4d' % i
    '  10'
    >>> '%4.1d' % i
    '  10'
    >>> '%4.3d' % i
    ' 010'
    >>> '%4d' % i
    '  10'
    >>> '%04d' % i
    '0010'
    >>> 

As you know, in python, characters are indeed strings with len() of 1, so if you want to represent a character you just use %s. But you can also use %c. %c will raise an exception if the input string is not a character, and the extra assertion might prove a little useful. You can add leading and trailing spaces to this parameter too.

Finally, it's worth something to note that python has a newer formatting syntax. I haven't seen it used much, and don't use it either. It's not so practical as the one I described.

More information on string formating can be found in the python documentation

This has hopefully been a thorough dissection of python's string formatting syntax. Now go out and enjoy the sun!