Python 中的命名常量 class 属性

Question

I have a class attribute, which is constant in a class but has a different value in its subclasses or sibling classes . And the attribute is used in a method in the base class.

In this case, should I express the attribute address as constant, that is, with SCREAMING_SNAKE_CASE like ADDRESS ?

For example,

class BaseClass:
    address = ''

    @classmethod
    def print_address(cls):
        print(cls.address)

class SubClass1(BaseClass):
    address = 'sub1'

class SubClass2(BaseClass):
    address = 'sub2'        

Or, is there a better way to do the same thing?

Answer 1

Constants are usually defined on a module level and written in all capital letters with underscores separating words. Examples include MAX_OVERFLOW and TOTAL.

• https://www.python.org/dev/peps/pep-0008/#constants

So, if it's a class attribute, it's not considered as a constant in terms of PEP-8.

However, there are few real-world exceptions.

Constants for Django models

In examples from Django documentation, constants for choices of fields are encapsulated into the model itself:

class Student(models.Model):
    FRESHMAN = 'FR'
    SOPHOMORE = 'SO'
    JUNIOR = 'JR'
    SENIOR = 'SR'
    GRADUATE = 'GR'
    YEAR_IN_SCHOOL_CHOICES = [
        (FRESHMAN, 'Freshman'),
        (SOPHOMORE, 'Sophomore'),
        (JUNIOR, 'Junior'),
        (SENIOR, 'Senior'),
        (GRADUATE, 'Graduate'),
    ]
    year_in_school = models.CharField(
        max_length=2,
        choices=YEAR_IN_SCHOOL_CHOICES,
        default=FRESHMAN,
    )

Source: Model field reference

Enums

enum.Enum uses SCREAMING_SNAKE_CASE for enum values. For example:

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

Source: enum.Enum documentation