[Python-Dev] Single- vs. Multi-pass iterability

[Tim Peters]

Note that it's easy to make objects cooperate with gc.  We've historically
only done so when the need was clear, because the gc header takes about a
dozen extra bytes per gc-tracked object.  There aren't enough files or
xreadlines objects in existence to care about the extra memory burden here,
though; we simply thought that objects of these types could never be in
cycles.  OTOH, if that means lazy code like

    for fname in os.listdir('.'):
        for line in file(fname):
            n += 1

would accumulate an ever-growing number of open file objects until gc
happened to run and break cycles, I expect a lot of CPython programs would
"suddenly break" (they rely on refcount semantics now closing the anonymous
file object the instant it becomes unreachable).