I am learning the Python string format() method. Though I understand that {} is a placeholder for arguments, I am not sure what : represent in the following code snippet from Programiz tutorial:

import datetime
# datetime formatting
date = datetime.datetime.now()
print("It's now: {:%Y/%m/%d %H:%M:%S}".format(date))



# custom __format__() method
class Person:
    def __format__(self, format):
        if(format == 'age'):
            return '23'
        return 'None'

print("Adam's age is: {:age}".format(Person()))

1. Why is there a : in front of %Y in print("It's now: {:%Y/%m/%d...? The code outputs It's now: 2021, and there is no : in front of 2021.
2. Why is there a : in front of age in print("Adam's age is: {:age}...?

Thanks in advance for your valuable input!!

CodePudding user response：

Everything after : is a parameter to the __format__() method of the class of the corresponding arguent. For instance, for a number you can write {:.2f} to format it as a decimal number with 2 digits of precision after the decimal point.

For a datetime value, it's a format string that could be used with datetime.strftime().

And in your Person class, it will be passed as the format argument to Person.__format__(). So if you don't put :age there, the if condition will fail and it will print None instead of 23.

CodePudding user response：

Try f-strings. In them the colon seems to be more reasonable. It delimits the variable name and its formatting options:

import datetime

# datetime formatting

date = datetime.datetime.now()
print(f"It's now: {date:%Y/%m/%d %H:%M:%S}")

# custom __format__() method

class Person:
    def __format__(self, format):
        if(format == 'age'):
            return '23'
        return 'None'

print(f"Adam's age is: {Person():age}")

I think the colon in the format method is something like a 'forward compatibility' thing.

Btw you can have similar functionality with keyword arguments to format():

print("It's now: {d:%Y/%m/%d %H:%M:%S}".format(d=date))

print("Adam's age is: {adam:age}".format(adam=Person()))

CodePudding user response：

Python objects decide for themselves how they should be formatted using the __format__ method. Mostly we just use the defaults that come with the basic types, but much like __str__ and __repr__ we can customize. The stuff after the colon : is the parameter to __format__.

>>> class Foo:
...     def __format__(self, spec):
...             print(repr(spec))
...             return "I will stubbornly refuse your format"
... 
>>> f = Foo()
>>> print("Its now {:myformat}".format(f))
'myformat'
Its now I will stubbornly refuse your format

we can call the formatter ourselves. datetime uses the strftime format rules.

>>> import datetime
>>> # datetime formatting
>>> date = datetime.datetime.now()
>>> print("It's now: {:%Y/%m/%d %H:%M:%S}".format(date))
It's now: 2021/10/04 11:12:23
>>> date.__format__(":%Y/%m/%d %H:%M:%S")
':2021/10/04 11:12:23'

Your custom Person class implemented __format__ and used the format specifier after the colon to return a value.