How Meta classes works in python?

Question

Origin of question I’m recently working with django and became used to of Meta class in models, Serializers, and Forms.

My Understanding so far I learned that meta classes are used for creating classes.

When one class is defined, Python will go inside the class and collect all attributes and methods and store as dictionary, after that it searches for __metaclass__ attribute. If defined, it will use that class to create the defined class else it will use default object.
Object is default class which is inherited to all classes, and this object class must have __metaclass__ which is type by default.

type class have __new__ and __init__ methods which is used to create classes.

My question
What is the flow of creating a class when we declare Meta class inside definition of class

For example

class Transformer(models.Model):
    name = models.CharField(max_length=150, unique=True)

    class Meta:
        ordering = ('name',)

Where and When this Meta class is used?

Edit 1:
Cleared one thing that metaclasses and django Meta are different.
So Meta is just nested class of Transformer Model Class.

Question: Still my quesition is how this Meta class is used by Model Class?

Asked By: Parth

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Source

Answer 1

Model Meta is basically the inner class of your model class. Model Meta is basically used to change the behavior of your model fields like changing order options,verbose_name_plural, and a lot of other options. It’s completely optional to add a Meta class to your model.

example:

class Category (models.Model):
    name = models.CharField(max_length=255)
    slug = models.SlugField(max_length=255, unique=True)

    def __str__(self):
    return self.name

    class Meta:
         verbose_name_plural= 'Categories'

Answered By: happycoder

Answer 2

As put in the comments: Python metaclasses are different from django metaclasses: Django just, for historical reasons, use the same terminology for the inner class where one annotates extra parameters about a class, where the primary members of the outer class are meant to correspond to fields in a model or form.

A Python metaclass, on the other hand, are what you are describing in your example, though you have checked some Python 2 documentation. In current Python, the metaclass is determined by passing the keyword argument "metaclas=" in the declaration of a new class, where the base classes go:

class MyClass(Base1, Base2, metaclass=MyMeta):
   ...

As far as I know it, the Django behavior had origin in which early versions of Django actually used a custom (Python) metaclass to annotate some of the parameters now used in the nested Meta – and in doing so, it took a shortcut of defining the metaclass inline inside the class body: instead of assigning the __metaclass__ name to an externally defined metaclass, as the usual for normal use, it would just define the class inplace: from the point of view of the language runtime, it would find the name __metaclass__ bound to a valid metaclass and use that to build the class.

Later versions, even in Python 2, modified this approach – the inner class was no longer the actual "metaclass" of the Model or Form (as the previous approach was clearly overkill).

Answered By: jsbueno

How Meta classes works in python?

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