password_serializer.py

class PasswordSerializer(serializers.ModelSerializer):
    """
    Changes user password
    """
    old_password = serializers.CharField(required=True)
    new_password = serializers.CharField(required=True)
    _required_fields = ['old_password', 'new_password']

    def validate_old_password(self, value):
        """
        Validates that the old_password field is correct.
        """
        if not self.instance.check_password(value):
            raise serializers.ValidationError(
                "Your old password was entered incorrectly. Please enter it again.")
        return value

    def validate_new_password(self, value):
        """
        Validates that the old_password field is correct.
        """
        self.instance.set_password(value)
        return value

    class Meta():
        model = Client
        fields = ['old_password', 'new_password', ]