Custom fields django

fields.py

from django.db import models

class ZipField(models.Field):

    def __init__(self, length=8, *args, **kwargs):
        self.length = length
        super().__init__(*args, **kwargs)

    def db_type(self, connection):
        return f"char({self.length})"

models.py

from django.db import models
from .fields import ZipField

class Address(models.Model):

    zip_default = ZipField()
    zip_cinco = ZipField(length=5)
    zip_doze = ZipField(length=12)