[Python-ideas] Syntax for passing lambdas to functions

[Andrew Barnert]

On Feb 27, 2014, at 13:24, Chris Angelico <rosuav at gmail.com> wrote:

> On Fri, Feb 28, 2014 at 8:14 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>>>    class A: ...
>>>    a = A()
>>>    l = [0]
>>> 
>>>    for a.x in [1, 2, 3]: ...
>>>    for l[0] in [1, 2, 3]: ...
>> 
>> I wonder whether that is deliberate feature, or an accident of the way
>> the syntax works. It does seem to be pretty useless though -- why are
>> you using an attribute or list as a loop variable?
> 
> I would say it's no accident that the for loop can accept any lvalue.
> That's how we can do this, which I'm sure you'll agree is *extremely*
> useful:
> 
> for x,y in [(1,2), (3,4), (5,6)]: ...
> 
> Assigning to the tuple works beautifully there (especially with
> enumerate or zip). Being able to assign to other complex targets is a
> bonus that you'll probably never come across (I can imagine someone
> might have a use for "for self.blah in ...", but can't concoct any
> right now)

Something I've seen in real code (not _good_ code, but actually deployed):

    for self.index in range(len(self.values)):
        if self.values[self.index] == spam:
            break
    else:
        self.index = None

Apparently someone didn't want to catch the exception from list.index. (Or, more likely, they were trying to write C code in Python.)