[Answered ]-Django to DjangoRestFramework – Where do I do form validation (I am no longer using forms.py – I'm using serializers.py)

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To perform validation in the serializers, we can define a function validate_username() which will validate the username and another function validate() which validates password1 and password2.

Custom Field-level validation:

We need to validate the username, we can add a function validate_username() to our serializers.

To specify custom field-level validation, we need to add .validate_<field_name> methods to our Serializer subclass. These are similar to the .clean_<field_name> methods on Django forms.

These methods take a single argument, which is the field value that
requires validation.

Your validate_<field_name>methods should return the validated value
or raise a serializers.ValidationError.

Object-level validation:

To do any other validation that requires access to multiple fields, we need to add a method called .validate() to our Serializer subclass.

This method takes a single argument, which is a dictionary of field values. It should raise a ValidationError if necessary, or just return the validated values.

Since in the clean_password2() function, you are trying to access the value of password1 also, we need to do object-level validation by defining a validate() function.

Final Code: 

from rest_framework import serializers

class UserSerializer(serializers.ModelSerializer):

    ...

    def validate_username(self, value):        
        if not re.search(r'^\w+$', value):
            raise serializers.ValidationError('Username can only contain alphanumeric characters and the underscore.') 
        if User.objects.filter(username=value):
            raise serializers.ValidationError('Username is already taken.')
        return value # must return validated value

    def validate(self, data):
        password1 = data.get('password1')
        password2 = data.get('password2')
        if password1 != password2:
            raise serializers.ValidationError('Passwords do not match.')
        return data # must return validated values
👤Rahul Gupta

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