[Python-ideas] For-loop variable scope: simultaneous possession and ingestion of cake
ntoronto at cs.byu.edu
Sat Oct 4 01:48:12 CEST 2008
Andrew Clover wrote:
> Greg Ewing wrote:
> > 1. If the loop variable is referenced by such a nested
> > function, a new local scope is effectively created
> > to hold each successive value of the loop variable.
> Why only the loop variable? What about:
> >>> for i in range(3):
> ... j= str(i)
> ... funs.append(lambda: j)
> >>> funs()
Spanking good point. To hack this "properly" all cell variables closed
over within the loop would have to go into the per-iteration scope.
> It seems an odd sort of scope that lets rebindings inside it fall
> through outwards.
True, but a lot of Python programs would depend on this - even new ones
because of social inertia.
>> Also, most other languages which have lexical scoping
>> and first-class functions don't seem to suffer from
>> problems like this.
> That's not always because of anything to do with introducing scopes
> though. Some of those languages can bind the variable value early, so if
> you were to write the equivalent of:
> >>> i= 3
> >>> f= lambda: i
> >>> i= 4
> f() would give you 3. I would love to see an early-value-binding
> language with similar syntax to Python, but I can't see how to get there
> from here.
Python could close over the values rather than a cell. That's *almost
always* what people really care about anyway! It'd break almost no code.
Of course, to satisfy Paul Graham, there'd have to be a way to go back
to cells. A "cell" keyword? Other languages make this explicit - they're
often called "boxes". I wonder if "nonlocal" could go if there were a
It wouldn't mean the variable carries pass-by-reference semantics,
though. That would be bad.
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