Python class 属性的正确语法参考

Question

Having known the difference between class and instance attributes in Python, which of these two is the proper way to reference a class attribute?

import requests


class SomePage:
    
    PAGE_URL = 'www.someurl.com'

    def __init__(self) -> None:
        # Option A
        self.request_a = requests.get(self.PAGE_URL)
        # Option B
        self.request_b = requests.get(SomePage.PAGE_URL)

The result would be the same for any option. Is there any of them preferable?

Answer 1

These are not the same, so what you use depends on what you intend. This becomes clear if we consider the following example:

class Foo:
    bar = 42
    def method1(self):
        return self.bar
    def method2(self):
        return Foo.bar

class Baz(Foo):
    bar = 99

baz = Baz()

In the repl:

>>> baz.method1()
99
>>> baz.method2()
42

So, self.PAGE_URL will check the instance namespace, then the class namespace, then all the namespaces in the classes method resolution order .

On the other hand Foo.bar will check Foo 's namespace, then all namespaces in Foo's method resolution order .