Django Model Field Initialization Mechanism

When you define a custom __init__ method in a Django model field subclass, the parent class's keyword arguments are still processed because of how Python inheritance works with the super() call.

In a properly implemented custom field, you would typically call the parent class's __init__ method using super(), which passes along the arguments:

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def __init__(self, *args, **kwargs):
    # Custom initialization code
    
    # Process your custom parameters
    my_custom_param = kwargs.pop('my_custom_param', default_value)
    
    # Pass remaining kwargs to parent class
    super().__init__(*args, **kwargs)

How Django Handles Field Parameters

Django's Field class has a comprehensive __init__ method that accepts many parameters (as shown in the documentation):

• verbose_name
• name
• primary_key
• max_length
• unique
• blank
• null
• And many others

When you create a custom field and don't explicitly call super().__init__(), Django's field parameters won't be properly processed. This is why the documentation warns against changing the calling signature when overriding __init__.

Best Practices

The Django documentation specifically cautions against customizing models by overriding __init__ and suggests alternative approaches:

1. Use a classmethod on the model class:

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class Book(models.Model):
    title = models.CharField(max_length=100)

    @classmethod
    def create(cls, title):
        book = cls(title=title)
        # do something with the book
        return book

2. Add a method on a custom manager (usually preferred):

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class BookManager(models.Manager):
    def create_book(self, title):
        book = self.create(title=title)
        # do something with the book
        return book

class Book(models.Model):
    title = models.CharField(max_length=100)
    objects = BookManager()

Field Options Behavior

It's worth noting that Django's model fields are designed to be permissive with options. As the documentation states:

Many of Django's model fields accept options that they don't do anything with. For example, you can pass both editable and auto_now to a DateField and it will ignore the editable parameter (auto_now being set implies editable=False). No error is raised in this case.

This behavior simplifies field classes since they don't need to check for unnecessary options. They pass all options to the parent class and then use only what they need.