User serializer with validate()

serializers.py

class UserSerializer(serializers.ModelSerializer):

    def validate_password(self, value):
        if value.isalnum():
            raise serializers.ValidationError('password must have atleast one special character.')
        return value

    def validate(self, data):
        if data['first_name'] == data['last_name']:
            raise serializers.ValidationError("first_name and last_name shouldn't be same.")
        return data

    class Meta:
        model = User
        fields = ('username', 'email', 'first_name', 'last_name', 'password')
        extra_kwargs = {
            'password': {'write_only': True}
        }