Question

I have seen this in one the facebook.py User file.

class User(db.Model):
    username = db.StringProperty(required=True)
    password = db.StringProperty(required=True)
@classmethod
def get_by_email(cls, email):
    return cls.query(cls.email == email).get()

What does cls mean? if you put self, it is referring to the User class normally. But what’s cls refer to?

Thanks in advance.

Answer 1

The name does not matter, but applying classmethod decorator makes the method a class method. In such case the first argument is a class, not an instance.

Please take a look at the comparison of instance, class and static methods using the following code:

>>> class Test(object):
    def instance_method(sth, x):
        return sth, x
    @classmethod
    def class_method(sth, x):
        return sth, x
    @staticmethod
    def static_method(x):
        return x


>>> a = Test()
>>> a.instance_method('abc')
(<__main__.Test object at 0x0000000002C26D30>, 'abc')
>>> a.class_method('abc')
(<class '__main__.Test'>, 'abc')
>>> a.static_method('abc')
'abc'

By convention, when you are referring to the same instance, you name it self. When you are referring to its class, you name it cls. That is only a convention, but it is better to follow it for consistency.