Opening a dbm database which doesn't exist without a "c" or "n" flag
results in this exception:

>>> import dbm
>>> dbm.open("abc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/MP.shadow/hagenf/local/src/py3k/Lib/dbm/__init__.py", line
79, in open
    raise error("need 'c' or 'n' flag to open new db")
TypeError: 'tuple' object is not callable

"error" is a tuple of dbm's own exception class and IOError, but this
doesn't seem to make sense in the present code and Python 3.0. The
attached patch fixes the problem and adds a test for the correct
exception being raised.

dbm.error is documented as a tuple, and I'd prefer not to change this:
  http://docs.python.org/dev/3.0/library/dbm.html#dbm.error
Since it says that its first member is another dbm.error exception,
we could simply raise error[0](message)

Attached another patch, with the same test case.

Amaury's patch looks good.

Did you know that...
with python 2.x, "raise (x,y,z)" is equivalent to "raise x"!
I could not find this in the documentation.

Committed the patch to py3k as r66622.

I didn't know until I had googled this:

http://mail.python.org/pipermail/python-3000/2007-March/005916.html