django-rest-framework/rest_framework/authtoken/serializers.py

from django.contrib.auth import authenticate
from rest_framework import serializers

class AuthTokenSerializer(serializers.Serializer):
    username = serializers.CharField()
    password = serializers.CharField()

    def validate(self, attrs):
        username = attrs.get('username')
        password = attrs.get('password')

        if username and password:
            user = authenticate(username=username, password=password)

            if user:
                if not user.is_active:
                    raise serializers.ValidationError('User account is disabled.')
                attrs['user'] = user
                return attrs
            else:
                raise serializers.ValidationError('Unable to login with provided credentials.')
        else:
            raise serializers.ValidationError('Must include "username" and "password"')