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

[Steven D'Aprano]

On Wed, Mar 23, 2016 at 12:21:34AM +1100, Chris Angelico wrote:

> > It's perfectly safe if somebody passes you locals() from *another*
> > scope, because it cannot have access to *your* local sentinel. So you
> > only need worry about one case: when *you* pass *your own* locals as the
> > iterable. Hence, *just don't do it*.
> 
> And make sure the function you call doesn't use sys._getframe to get
> them for you.

Are you saying that I might have code like this:

def test():
    x = sentinel = object()
    iterable = some_function()
    for x in iterable:
        # you know the rest


and some_function() might use _getframe to steal sentinel and return it 
back to me as iterable = [sentinel]?

I'll take my chances with that, thanks very much. I am as concerned by 
that as I am that some_function might use ctypes to hack the value of 
integers so that 1 + 1 returns 4. Yes, it could happen. No, I don't care 
to take any special precautions to avoid it happening.

I think a far more likely possibility is that somebody or something has 
monkey-patched object() to always return the same instance, regardless 
of who calls it from where, and now my sentinel is the same as everyone 
else's sentinel. Am I bothered by this? Not in the least.

I mean, seriously, we write code all time relying on the behaviour of 
builtin functions, knowing full well that any piece of code anywhere 
might monkey-patch them to do something weird. Should we insist on 
special syntax that returns the len() of sequences because of an 
edge-case "what if len has been monkey-patched?". No. We just say "well 
don't monkey-patch len."

In case it's not obvious by now, I don't think there's any need for 
syntax to handle the empty iterable case. -1 on the proposal.



-- 
Steve