Working with Python 2.6.5, I find I cannot put multiple CABs in the same installer.  This is due to this statement in msilib.CAB.commit():

        add_data(db, "Media",
                [(1, self.index, None, "#"+self.name, None, None)])

The key, 1, must be different for each record in the 'Media' table.  The symptom is an exception something like this:

_msi.MSIError: Could not insert [(1, 3, None, '#foo', None, None)] into Media

You can have at most one CAB object per database, yes. However, you can have certainly multiple cab files in the installer; just call add_data yourself.

If you are using the API provided by msilib, there should be no need to ever have more than one CAB instance.

I'm certainly using the API provided by msilib, but perhaps I'm using it badly.  Which API did you have in mind?  I'm using msilib directly, not through bdist_msi.

This seems like an artificial limitation to put on the Python library; the standard MSI spec supports 65K different cabs in a single installer.

I still don't see the need to create multiple CABs. Just use the Directory class to add files, and that will automatically record them in the singleton CAB.

Martin, are you rejecting this bug report as worksforme or is it a valid feature request?

As a feature request, it's fine - but I have no intention to work on this feature.