'Get one value matching the key, raising a KeyError if multiple
values were found.'

def getone(self, key):

    v = self.getall(key)
    if (not v):
        raise KeyError(('Key    not    found:    %r' % key))
    if (len(v) > 1):
        raise KeyError(('Multiple    values    match    %r:    %r' % (key, v)))
    return v[0]
