1. For the cases like read/write handles conflicting, ValueError doesn't seem quite right to me - I take ValueError to mean that you have passed an invalid value to a function. RuntimeError feels like the closest fit for 'the current state does not allow this operation'. It is rather broad, though.

If I was writing it anew, I would use ValueError for something like an invalid mode parameter: zf.open('foo', mode='q') . That particular case was already in the code, though, so I think backwards compatibility should take precedence.

2 & 3. Agreed, I'll prepare a patch