[Python-ideas] Control Flow - Never Executed Loop Body

[Chris Angelico]

On Tue, Mar 22, 2016 at 11:07 PM, Sven R. Kunze <srkunze at mail.de> wrote:
> However, I assume that when I do "object()" I got something unique among all
> other objects. So,
>
> item = sentinel = object()
> for item in collection:
>     main_suite(item)
> if item is sentinel:
>     empty_suite()
>
> Is still quite correct, right?

Sure it is. Perfectly correct.

>>> def nope():
...     item = sentinel = object()
...     for item in locals().values():
...         print("We have:", item)
...     if item is sentinel:
...         print("We have no items.")
...
>>> nope()
We have: <object object at 0x7f17b1c6b0a0>
We have: <object object at 0x7f17b1c6b0a0>
We have no items.
>>>

An iterator can return *any* *object*. That's why Python uses an
exception (StopIteration) to signal the absence of an object.

More reliable is to use that absence, either by exception or by
probing a dictionary's keys:

def better():
    # Make sure item is unbound
    try: del item
    except NameError: pass
    for item in locals().values():
        print("We have:", item)
    if 'item' not in locals():
        print("We have no items.")

But now we're getting into the realm of ugly code to deal with edge
cases. Like with "yield from", language support can be justified when
there's a simple and obvious *but imperfect* way to do something
("yield from x" is not the same as "for item in x: yield item").

ChrisA