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

[Tim Peters]

[Ping]
> Name any protocol for which the question "does this mutate?" has
> no answer.

Heh -- you must not use Zope much <0.6 wink>.  I'm hard pressed to think of
a protocol where that does have a reliable answer.  Here:

    x1 = y.z
    x2 = y.z

Are x1 and x2 the same object after that?  At least equal?  Did either line
mutate y?  You simply can't know without knowing how y's type implements
__getattr__, and with the introduction of computed attributes (properties)
it's just going to get muddier.

> (I ask you to accept that __call__ is a special case.)

It's not to me -- if a protocol invokes user-defined Python code, there's
nothing you can say about mutability "in general", and people do both use
and abuse that.

>> What's so special about "for" that it should pretend to deliver
>> purely functional behavior in a highly non-functional language?

> Who said anything about functional behaviour?  I'm not requiring that
> looping *never* mutate.  I just want to be able to tell *whether* it
> will.

I don't blame you, and sometimes I'd like to know whether y.z (or "y += z",
etc) mutates y too.  It cuts deeper than loops, so a loop-focused gimmick
seems inadequate to me (provided "something needs to be done about it" at
all -- I'm not sure, but doubt it).