[Python-ideas] Tweaking closures and lexical scoping to include the function being defined
tjreedy at udel.edu
Wed Sep 28 06:12:26 CEST 2011
On 9/27/2011 9:26 PM, Ethan Furman wrote:
> Terry Reedy wrote:
>> On 9/27/2011 6:32 PM, Guido van Rossum wrote:
>>> On Tue, Sep 27, 2011 at 3:24 PM, Arnaud
>>> Delobelle<arnodel at gmail.com> wrote:
>>>> I may be unduly worrying about this, but it feels to me that it
>>>> lessens the overall tidiness of the language. I would much rather
>>>> there was a way to initialise these "own" variables outside the body
>>>> of the function.
>>> Yes, you have a point. Hiding side effects this way could be nasty,
>>> and this is actually the first point in favor of the default argument
>>> hack I've heard in a while. :-)
>> I have several times written on python-list that Python has a simple
>> rule: header expressions (for default args) are evaluated one-time
>> when the function is defined; body expressions are evaluated (perhaps)
>> each time when the function is called. If people understand this and
>> do not fight it, they are not surprised that mutating a once-defined
>> default arg mutates it, nor that defining a function inside a loop
>> magically causes define-time binding of names in the body.
>> I would hate for Python to lose this simplicity. I consider it one of
>> its positive features.
> Simplicity is good.
> I don't remember who first suggested it, and I don't remember any
> discussion on it, but what if decorator functions grew a __prepare__
> method similar to metaclasses?
At least three people, including Guido, have noted than a keyword-only
_private parameter solves most of the problems with using default args
for constants. Since the _underscore convention is already partly built
into the language (import *, help(module), __mangling, __reserved__), we
can have other tools like signature introspection also respect the
This solution *also* works for the semi-private accumulator parameter of
most tail-recursive functions.
Example: sum the values in a linked list (without iterators or explicit
def llsum(ll, *, _total = 0):
return llsum(ll, _total = _total + ll)
The private parameter _total is constant for outside calls but gets a
new value with each recursive call. It should not appear in the public
signature but is used internally. The non-parameter constants proposed
as a substitute for default args would not work for this case.
* I am QUITE aware that this can be rewritten with while, and indeed I
write the two if branches in the order I do to make it easy.
total = 0):
ll, total = ll, total + ll
But this is a different issue.
Terry Jan Reedy
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