Please open a separate issue for your enhancement request in your second message (assuming there isn't already one open).  I'm not sure what your third message is about, but it also sounds off topic for your original bug report.

For the FileCookieJar issue, I agree that the documentation is unclear.  However, it is perfectly reasonable to have a documented base class that is an Abstract Base Class, and there is no reason for us to invent our own unique cookie file format just to make FileCookieJar work by itself.  ABCs didn't exist when FileCookieJar was implemented, so it should be turned in to one for 3.4 (it might break working code, so we shouldn't do that for bug fix releases).  That way you would get a useful error when you try to instantiate it, rather than when you try to use it.  And the docs should document it as being a base-class-only for all active releases.