Class Methods

I was today years old when I learnt about how to use the classmethod decorator in Python.

I've always thought of a class method as a method in a class - just intuitively. But there's actually a decorator called "@classmethod".

Imagine you have a class C. A class method in this class would be defined as -

class C: 
	@classmethod
	def cl_method(cls):
		print(f"I'm in the class method {cls=}")

The distinctive marks of a class method are :

• the decorator - @classmethod
• the first argument to the method is always the class itself - denoted by "cls" but it needn't have been

The execution and the behaviour of the method are :

>>> print(C.cl_method)
I'm in the class method cls=<class '__main__.C'>

For a class method, even if an object of the class is used to call the method, the first argument passed is still 'cls'

And This, is the 1024 KB question.

For the most part, they are used to create "alternative constructors" . This is the response I saw documented throughout the internet wherever I asked the question. Even LLMs seem to be trained to return this as the only known answer . Here's me asking llama3.2 -

So what does this mean? I went hunting in the standard libraries for usage of this decorator and I didn't have to look far.

In the datetime library, the date class uses this to implement the today() method and this is the definition -

@classmethod  
def today(cls):  
    "Construct a date from time.time()."  
    t = _time.time()  
    return cls.fromtimestamp(t)

So we see it's using the timestamp function from the class date to print the current date after fetching the current time using the builtin time library.

When it's called -

from datetime import date
print(date.today())

We get "2024-11-17" .

I took longer than I'd like to admit to understand how the representation was being printed and I finally realised it's because the __str__ definition for this class controls the display to be sent as the Isoformat of a date which is "year-month-day" . In hindsight, this is perfectly obvious. 🤦

To make things interesting, I decided to create a function of my own by extending the date class and came up with this -

class MyDate(date):  
    @classmethod  
    def yesterday(cls):  
        today = date.today()  
        d = today - timedelta(days=1)  
        return cls.isoformat(d)
        
print(MyDate.yesterday())

When executed, it returns, unsurprisingly - "2024-12-16" , which is yesterday's date.

This feels like it could have been introduced in the built in date library as well, but that's a rather slippery slope.

Anyway, so that's the @classmethod decorator. I'm sure its brother the @staticmethod is more popular knowledge even if it's even lesser used, but that's a topic for another day.