http://docs.python.org/3/library/functions.html#issubclass

"classinfo may be a tuple of class objects."

I suggest make it clear like isinstance, use something like this:

classinfo may be a tuple of class objects and such tuples.

While this is documented for isinstance, I'm not sure it should be advertised too much, as it seems to me an implementation detail and doesn't seem too useful in practice.

This is a side-effect of the fact that
  isinstance(x, (A, B, ...))
is equivalent to
  isinstance(x, A) or isinstance(x, B) or ...
and therefore
  isinstance(x, (A, (B, C)))
is equivalent to
  isinstance(x, A) or isinstance(x, (B, C))
which in turn is equivalent to
  isinstance(x, A) or (isinstance(x, B) or isinstance(x, C))

While this behavior seems intentional [0], it doesn't seem to be tested [1].  FTR this is supported by PyPy too.

[0]: http://hg.python.org/cpython/file/default/Objects/abstract.c#l2494
[1]: http://hg.python.org/cpython/file/default/Lib/test/test_builtin.py#l704

Whether the recursive nature of the check should be documented or not, it seems inconsistent that it's documented for isinstance but not for issubclass.

@irit, you may be interested in issue44135, there's an open PR there and it seems to fix what this issue describes.

@Ken - not quite. This issue is about isinstance containing "(or recursively, other such tuples)" which is not there for issubclass.