Setter for **self.__source** attribute.

        :param value: Attribute value.
        :type value: unicode

def source(self, value):
        """
        Setter for **self.__source** attribute.

        :param value: Attribute value.
        :type value: unicode
        """

        if value is not None:
            assert type(value) is unicode, "'{0}' attribute: '{1}' type is not 'unicode'!".format(
                "source", value)
            assert os.path.exists(value), "'{0}' attribute: '{1}' file doesn't exists!".format("source", value)
        self.__source = value