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)
'42'
>>> '{1}'.format(1, 2)
'2'
>>> '{y}'.format(x=1, y=2)
'2'
The conversion let you ask format to use repr() (or ascii()) instead of str() while converting objects to strings:
>>> '{!r}'.format(datetime.now())
'datetime.datetime(2018, 5, 3, 23, 48, 49, 157037)'
>>> '{}'.format(datetime.now())
'2018-05-03 23:49:01.060852'
Finally, the format specification is a way to define how values are presented:
>>> '{:+,}'.format(1234567)
'+1,234,567'
>>> '{:>19}'.format(1234567)
' 1234567'
This specification may be applied to a single object with format function (not the str method):
format(5000000, '+,')
'+5,000,000'
The format function calls __format__ method of the object internally, so you can alter its behavior for your types.