Returns an extension instance with a given name.

        In case there are few extensions with a given name, the first one
        will be returned. If no extensions with a given name are exist,
        the `default` value will be returned.

        :param name: (str) an extension name
        :param default: (object) a fallback value
        :returns: (object) an extension instance

def get(self, name, default=None):
        """
        Returns an extension instance with a given name.

        In case there are few extensions with a given name, the first one
        will be returned. If no extensions with a given name are exist,
        the `default` value will be returned.

        :param name: (str) an extension name
        :param default: (object) a fallback value
        :returns: (object) an extension instance
        """
        try:
            value = self[name]
        except KeyError:
            value = default
        return value