Justification in Format Strings

You can pad a string on the left or right with the string methods rjust and ljust.

If you want to do the formatting with a format string, and particularly if you are setting float accuracy at the same time, the format string syntax is useful.

You should have seen how to set float precision in a format string:

>>> x = 21.2468
>>> print('x = {:.2f}'.format(x))
21.25

This is one form of formatting that comes after the : inside braces in a format string.

Even simpler is right justification:

>>> print('|{:9}|'.format(21.2468))
|  21.2468|

This can be combined with setting float precision:

>>> print('|{:9.3f}|'.format(21.2468))
|   21.247|

Note that the . character is counted in the width!

Justification with padding works with the string conversion of any data:

>>> print('|{:4}|'.format('yes'))
| yes|
>>> print('|{:7}|'.format('yes'))
|    yes|
>>> print('|{:1}|'.format('yes'))
|yes|

The last example illustrates that the field width is ignored if it is too small.

While right justification is the default, any of the symbols <, > and ^ can be placed right after the colon to choose left, right, or centered justification:

>>> '|{:<7.1f}|'.format(21.2468)
'|21.2   |'
>>> '|{:>7.1f}|'.format(21.2468)
'|   21.2|'
>>> '|{:^7.1f}|'.format(21.2468)
'| 21.2  |'

Of course centering with an odd number of blanks cannot be perfect.

There is a whole mini-language for format strings. We have covered some of the most common parts, but far from all.

New F-string Formatting

There is a new (Python 3.6+) concise formatting scheme that works only with literal (quote-delimited) strings by preceding the first quote with f. The substitutions inside are for local variables, whose names are embedded just inside the curly braces of individual substitutions:

>>> word  = 'best'
>>> x = 21.2468
>>> f'The {word} value is {x:.2f}!!'
'The best value is 21.25!!'
>>> print(f'|{word:<6}|\n|{word:>6}|\n|{word:^6}|')
|best  |
|  best|
| best |

There are more restrictions with the formatting in f-strings than with format strings used with the format method, but they work with all the basic editing modifications discussed here.