How to define a group of constant in Django app?

Group of constants

A group of constants value will help group the constant by specific type or meaning.

For example, this blog post might have multiple status like draft, publish then we could use a group with named POST_STATUS to group all of them together.

Use cases

In a Django app, I usually use group of constants in two common use cases:

– Define choices for a field in the model

– Use this group of constants in the logic app

How to define a group of constants?

Use tuple

You might define each constant type with a name then group them by a tuple, like this:

(POST_DRAFT, POST_PUBLISHED, POST_ARCHIVED) = range(3)
POST_STATUS_CHOICES = (
   (POST_DRAFT, 'Draft'),
   (POST_PUBLISHED, 'Published')
   (POST_ARCHIVED, 'Archived')
)

You might want to use this group of constants in a model field choices:

class Post(models.Model):
    post_status = models.IntegerField(
        choices=POST_STATUS_CHOICES,
        default=POST_STATUS_CHOICES[0][0]
    )

Above code snippet also show you get the default value is POST_DRAFT with

POST_STATUS_CHOICES[0][0]

This way is work but lack of reading on code, where you might forget about the value of the first one then need to re-check on where define the group.

Use Enum

There is a better way to avoid select by the index of the value as above is using enum value.

Firstly, define a class stand for the choices by enums, and provide a method to get all values by tuple named “choices”

import enum
from enum import Enum

@enum.unique
class EnumChoices(Enum):
    @classmethod
    def choices(cls):
        return [(item.value, item.name) for item in cls]

then define a class PostStatus extend from EnumChoices

class PostStatus(EnumChoices):
    POST_DRAFT = 0
    POST_PUBLISHED = 1
finally, use in the model field choices

class Post(models.Model):
    post_status = models.IntegerField(
        choices=PostStatus.choices(),
        default=PostStatus.POST_DRAFT
    )

This solution help us avoid the index select by better meaningful name PostStatus.POST_DRAFT

Use Django choices class

Above solution is good to go, but Django makes it even better by already define enum choices class to use, like IntegerChoices, TextChoices, Choices.

Then above code will become:

from django.db.models import IntegerChoices
class PostStatus(IntegerChoices):
    POST_DRAFT = 0
    POST_PUBLISHED = 1
then in model field choices will call to get choices value instead:

class Post(models.Model):
    post_status = models.IntegerField(
        choices=PostStatus.choices,
        default=PostStatus.POST_DRAFT
    )

For summary, we should refer the Django choice class since it is the better option for clean code and readable constant value in the group.