API call:  get a variable via a ``GET`` request.

        This will return None if the variable doesn't exist::

          >>> from google.cloud import runtimeconfig
          >>> client = runtimeconfig.Client()
          >>> config = client.config('my-config')
          >>> print(config.get_variable('variable-name'))
          <Variable: my-config, variable-name>
          >>> print(config.get_variable('does-not-exist'))
          None

        :type variable_name: str
        :param variable_name: The name of the variable to retrieve.

        :type client: :class:`~google.cloud.runtimeconfig.client.Client`
        :param client:
            (Optional) The client to use.  If not passed, falls back to the
            ``client`` stored on the current config.

        :rtype: :class:`google.cloud.runtimeconfig.variable.Variable` or None
        :returns: The variable object if it exists, otherwise None.

def get_variable(self, variable_name, client=None):
        """API call:  get a variable via a ``GET`` request.

        This will return None if the variable doesn't exist::

          >>> from google.cloud import runtimeconfig
          >>> client = runtimeconfig.Client()
          >>> config = client.config('my-config')
          >>> print(config.get_variable('variable-name'))
          <Variable: my-config, variable-name>
          >>> print(config.get_variable('does-not-exist'))
          None

        :type variable_name: str
        :param variable_name: The name of the variable to retrieve.

        :type client: :class:`~google.cloud.runtimeconfig.client.Client`
        :param client:
            (Optional) The client to use.  If not passed, falls back to the
            ``client`` stored on the current config.

        :rtype: :class:`google.cloud.runtimeconfig.variable.Variable` or None
        :returns: The variable object if it exists, otherwise None.
        """
        client = self._require_client(client)
        variable = Variable(config=self, name=variable_name)
        try:
            variable.reload(client=client)
            return variable
        except NotFound:
            return None