How to properly use the "choices" field option in Django

I think no one actually has answered to the first question:

Why did they create those variables?

Those variables aren't strictly necessary. It's true. You can perfectly do something like this:

MONTH_CHOICES = (
    ("JANUARY", "January"),
    ("FEBRUARY", "February"),
    ("MARCH", "March"),
    # ....
    ("DECEMBER", "December"),
)

month = models.CharField(max_length=9,
                  choices=MONTH_CHOICES,
                  default="JANUARY")

Why using variables is better? Error prevention and logic separation.

JAN = "JANUARY"
FEB = "FEBRUARY"
MAR = "MAR"
# (...)

MONTH_CHOICES = (
    (JAN, "January"),
    (FEB, "February"),
    (MAR, "March"),
    # ....
    (DEC, "December"),
)

Now, imagine you have a view where you create a new Model instance. Instead of doing this:

new_instance = MyModel(month='JANUARY')

You'll do this:

new_instance = MyModel(month=MyModel.JAN)

In the first option you are hardcoding the value. If there is a set of values you can input, you should limit those options when coding. Also, if you eventually need to change the code at the Model layer, now you don't need to make any change in the Views layer.

For Django3.0+, use models.TextChoices (see docs-v3.0 for enumeration types)

from django.db import models

class MyModel(models.Model):
    class Month(models.TextChoices):
        JAN = "1", "JANUARY"
        FEB = "2", "FEBRUARY"
        MAR = "3", "MAR"
        # (...)

    month = models.CharField(
        max_length=2,
        choices=Month.choices,
        default=Month.JAN
    )

Usage::

>>> obj = MyModel.objects.create(month='1')
>>> assert obj.month == obj.Month.JAN == '1'
>>> assert MyModel.Month(obj.month) is obj.Month.JAN
>>> assert MyModel.Month(obj.month).value is '1'
>>> assert MyModel.Month(obj.month).label == 'JANUARY'
>>> assert MyModel.Month(obj.month).name == 'JAN'
>>> assert MyModel.objects.filter(month=MyModel.Month.JAN).count() >= 1

>>> obj2 = MyModel(month=MyModel.Month.FEB)
>>> assert obj2.get_month_display() == obj2.Month(obj2.month).label

Let's say we known the label is 'JANUARY', how to get the name 'JAN' and the value '1'?

label = "JANUARY"
name = {i.label: i.name for i in MyModel.Month}[label]
print(repr(name))  # 'JAN'
value = {i.label: i.value for i in MyModel.Month}[label]
print(repr(value))  # '1'

Personally, I would rather use models.IntegerChoices

class MyModel(models.Model):
    class Month(models.IntegerChoices):
        JAN = 1, "JANUARY"
        FEB = 2, "FEBRUARY"
        MAR = 3, "MAR"
        # (...)

    month = models.PositiveSmallIntegerField(
        choices=Month.choices,
        default=Month.JAN
    )