[Python-Dev] Changing semantics of issubclass when accepting atuple (original) (raw)

Tim Peters tim.one at comcast.net
Sun Dec 14 20:33:50 EST 2003

[Brett]

But this is a change in semantics. Tim said I should clear it here first so that is what I am doing. Anyone have issues if I do this? And if so, any objections of backporting to 2.3?

[Guido]

Let's not mess with 2.3 semantics.

That's the rub: the segfaults occur in 2.3 too, of course, so should also be fixed there. Looks like isinstance() has the same vulnerability, and that the code comments don't match the docs.

""" class C(object): pass

classtup = C for i in xrange(50000): classtup = (classtup,)

if 0: # segfaults on both branches; maybe on your box 50000 is too small print issubclass(C, classtup) else: print isinstance(C(), classtup) """

The implementations even have this shared prophetic comment:

    /* Not a general sequence -- that opens up the road to
       recursion and stack overflow. */

The error messages plainly say that the example segfaulting program isn't a supported use:

        "isinstance() arg 2 must be a class, type,"
        " or tuple of classes and types"))

                "issubclass() arg 2 must be a class"
                " or tuple of classes"))

The documentation could well have meant that, too:

print isinstance.doc isinstance(object, class-or-type-or-tuple) -> bool

Return whether an object is an instance of a class or of a subclass thereof. With a type as second argument, return whether that is the object's type. The form using a tuple, isinstance(x, (A, B, ...)), is a shortcut for isinstance(x, A) or isinstance(x, B) or ... (etc.).

print issubclass.doc issubclass(C, B) -> bool

Return whether class C is a subclass (i.e., a derived class) of class B. When using a tuple as the second argument issubclass(X, (A, B, ...)), is a shortcut for issubclass(X, A) or issubclass(X, B) or ... (etc.).

The idea that arbitrarily nested tuples is allowed comes from that the docstrings imply it (more accidentally than on purpose, it seems to me), and that the implementations don't actually check for what its error messages claim must be the case.

Finally, the docs in the Library Reference Manual are schizophrenic about the issue at hand. The issubclass() docs appear to disallow the possibility of nested tuples, while the isinstance() docs explictly allow them.

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