The : format specifier is used to format the result of the expression or call.
>>> pi = 3.14159265359
>>> f"Pi is approximately {pi:.3f}."
'Pi is approximately 3.142.'It can also be used to declare how many characters the field should be wide, and to align numbers:
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
... print(f'{name:10} ==> {phone:10d}')
...
Sjoerd ==> 4127
Jack ==> 4098
Dcab ==> 7678