def getone(self, key, default=_marker):
        """Get first value matching the key.

        Raises KeyError if the key is not found and no default is provided.
        """
        identity = self._title(key)
        for i, k, v in self._impl._items:
            if i == identity:
                return v
        if default is not _marker:
            return default
        raise KeyError("Key not found: %r" % key)
Get first value matching the key.

        Raises KeyError if the key is not found and no default is provided.
        
lib/multidict/_multidict_py.py