Python etc / `format` syntax

format syntax

The format method of Python string is a mighty tool that supports a lot of things that you are probably not even aware of. Each replacement placeholder ({...}) may contain three parts: field name, conversion and format specification.

The field name is used to specify which argument exactly should be used as a replacement:

>>> '{}'.format(42)
>>> '{1}'.format(1, 2)
>>> '{y}'.format(x=1, y=2)

The conversion let you ask format to use repr() (or ascii()) instead of str() while converting objects to strings:

>>> '{!r}'.format(
'datetime.datetime(2018, 5, 3, 23, 48, 49, 157037)'
>>> '{}'.format(
'2018-05-03 23:49:01.060852'

Finally, the format specification is a way to define how values are presented:

>>> '{:+,}'.format(1234567)
>>> '{:>19}'.format(1234567)
'            1234567'

This specification may be applied to a single object with format function (not the str method):

format(5000000, '+,')

The format function calls __format__ method of the object internally, so you can alter its behavior for your types.