When guessing the type of a file, tarfile.py tries to open it in .tar, .tar.gz and .tar.bz2 modes successfully. The problem is that in conjunction with a fileobj argument, failed attempts can advance the current position of the fileobj, and cause the following attempts to fail as well. This didn't show up so far because, by chance, the order in which the OPEN_METH dictionary is enumerated is somehow the "correct" one. The problem can be seen by changing this order and re-running test_tarfile.py; for example, reverse the order (tarfile.py line 1013): for comptype in reversed(cls.OPEN_METH):
Logged In: YES user_id=591932 I took a look at tarfile.py and though there is this comment about OPEN_METH # This concept allows one to subclass TarFile without losing the comfort of # the super-constructor. A sub-constructor is registered and made available # by adding it to the mapping in OPEN_METH. because adding items to OPEN_METH could change the order and break the Tarfile.open() classmethod I doubt adding to OPEN_METH is done in practice. +1 for renaming OPEN_METH to _OPEN_METH and making it a list of two-tuples. +0 on changing the two tups from ('typename', 'funcname') to ('typename', function_ref) and dropping the getattr functionality. The three default openers are not listed in the public interface so if anyone is subclassing Tarfile and overriding them (doubtful) they are doing so at their own peril.
Logged In: YES user_id=4771 If possible, I would prefer that the constructors were fixed to become order-independent. Not sure how easy this is, though.
Logged In: YES user_id=591932 The invidual openers could tell() the fileobj and revert with a seek() on failure. The module requires those methods be implemented already to work so that is safe. I'll cook up a patch. But basically open() isn't as extensible as it seems. Stream reading and writing isn't supported for injected compression types and appending only works on plain tarfiles. I'm guessing that isn't a problem in practice or someone would have mentioned it by now ;)
