How about the following patch. With it, you get an IOError.

>>> s = shelve.open('/tmp/t', 'c')
>>> s.has_key('foo')
0
>>> s.close()
>>> s.has_key('foo')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "shelve.py", line 107, in has_key
    return self.dict.has_key(key)
  File "shelve.py", line 94, in getdict
    raise IOError, 'shelf has been closed'
IOError: shelf has been closed