* How to use custom string formatter in f-strings. * Default used string formatters in f strings.

custom.py

class Person:
    def __init__(self, name) -> None:
        self.name = name
        
    def __format__(self, __format_spec):
        if __format_spec == "scream":
            return self.name.upper() + '!'
        elif __format_spec == "repeat":
            return self.name * 3
        return self.name
          
p = Person('Patrick')
print(f"{p}") # Patrick
print(f"{p:scream}") # PATRICK!
print(f"{p:repeat}") # PatrickPatrickPatrick

default.py

## ROUNDING
>>> from math import pi
>>> f"The numerical value of PI is {pi}"
# 'The numerical value of PI is 3.141592653589793'

>>> f"The numerical value of PI is {pi:0.2f}"
# 'The numerical value of PI is 3.14'

## Formatting and rendering
name = 'Patrick'
>>> f"Your name is {name:_^20}"
'Your name is ______Patrick_______'

>>> f"Your name is {name:_>20}"
'Your name is _____________Patrick'

>>> f"Your name is {name:_<20}"
'Your name is Patrick_____________'