[Python-ideas] Access to function objects

海韵 lyricconch at gmail.com
Mon Aug 8 07:27:06 CEST 2011

when function body access function name, it requires "name lookup",
that is a "runtime" behavior.
i would like that python offer some "compile time" behavior just like
this proposal - things(here, it's function) declare by the "as clause"
is always "runtime independent" and only visible on it's own
suite(which means you can not use the "as declared" NAME outside its
indent block).

2011/8/8 Nick Coghlan <ncoghlan at gmail.com>:
> On Sun, Aug 7, 2011 at 11:07 PM, Guido van Rossum <guido at python.org> wrote:
>> On Sun, Aug 7, 2011 at 8:46 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>> With a PEP 3135 closure style solution, the cell reference would be
>>> filled in at function definition time, so that part shouldn't be an
>>> issue.
>> Yes, I was thinking of something like that (though honestly I'd
>> forgotten some of the details :-).
> I'd forgotten many of the details as well, but was tracking down some
> super() strangeness recently (to answer a question Michael Foord
> asked, IIRC) and had to look it up.
>> IMO there is no doubt that if __function__ were to exist it should
>> reference the innermost function, i.e. the thing that was created by
>> the 'def' statement before any decorators were applied.
> Yeah, I'd mostly realised that by the time I finished writing by last
> message, but figured I'd record the train of thought that got me
> there.
>>> Reference by name lazily accesses the outermost one, but doesn't care
>>> how the decorators are applied (i.e. as part of the def statement or
>>> via post decoration).
>> What do you mean here by lazily?
> Just the fact that the reference isn't resolved until the function
> executes rather than being resolved when it gets defined.
>>> A __class__ style cell reference to the result
>>> of the 'def' statement would behave differently in the post decoration
>>> case.
>> Oh you were thinking of making it reference the result after
>> decoration? Maybe I know too much about the implementation, but I
>> would find that highly confusing. Do you even have a use case for
>> that? If so, I think it should be a separate name, e.g.
>> __decorated_function__.
> The only reason I was thinking that way is that currently, if you do
> something like [1]:
> @lru_cache()
> def fib(n):
>    if n < 2:
>        return n
>    return fib(n-1) + fib(n-2)
> then, at call time, 'fib' will resolve to the caching wrapper rather
> than to the undecorated function. Using a reference to the undecorated
> function instead (as would have to happen for a sane implementation of
> __func__) would be actively harmful since the recursive calls would
> bypass the cache unless the lru_cache decorator took steps to change
> the way the reference evolved:
> @lru_cache()
> def fib(n):
>    if n < 2:
>        return n
>    return __func__(n-1) + __func__(n-2) # Not the same, unless
> lru_cache adjusts the reference
> This semantic mismatch has actually shifted my opinion from +0 to -1
> on the idea. Relying on normal name lookup can be occasionally
> inconvenient, but it is at least clear what we're referring to. The
> existence of wrapper functions means that "this function" isn't as
> clear and unambiguous a phrase as it first seems.
> (I think the reason we get away with it in the PEP 3135 case is that
> 'class wrappers' typically aren't handled via class decorators but via
> metaclasses, which do a better job of playing nicely with the implicit
> closure created to handle super() and __class__)
>>> While referencing the innermost function would likely be wrong in any
>>> case involving function attributes, having the function in a valid
>>> state during decoration will likely mandate filling in the cell
>>> reference before invoking any decorators. Perhaps the best solution
>>> would be to syntactically reference the innermost function, but
>>> provide a clean way in functools to shift the cell reference to a
>>> different function (with functools.wraps doing that automatically).
>> Hm, making it dynamic sounds wrong. I think it makes more sense to
>> just share the attribute dict (which is easily done through assignment
>> to the wrapping function's __dict__).
> Huh, I hadn't even thought of that as a potential alternative to the
> update() based approach currently used in functools.wraps (I had to
> jump into the interactive interpreter to confirm that functions really
> do let you swap out their instance dict).
> It's interesting that, once again, the status quo deals with this
> according to ordinary name resolution rules: any wrapping of the
> function will be ignored, *unless* we store the wrapper back into the
> original location so the name resolution in the function body will see
> it.
> Since the idea of implicitly sharing state between currently
> independent wrapper functions scares me, this strikes me as another
> reason to switch to '-1'.
>>> This does seem like an area ripe for subtle decoration related bugs
>>> though, especially by contrast with lazy name based lookup.
>> TBH, personally I am in most cases unhappy with the aggressive copying
>> of docstring and other metadata from the wrapped function to the
>> wrapper function, and wish the idiom had never been invented.
> IIRC, I was the one who actually committed the stdlib blessing of the
> idiom in the form of 'functools.wraps'. It was definitely a hack to
> deal with the increasing prevalence of wrapper functions as decorators
> became more popular - naive introspection was giving too many wrong
> answers and tweaking the recommended wrapping process so that
> 'f.__doc__' would work again seemed like a better option than defining
> a complex introspection protocol to handle wrapped functions.
> I still think it was a reasonable way forward (and better than leaving
> things as they were), but it's definitely an approach with quite a few
> flaws.
>>> While this may sound a little hypocritical coming from the author of
>>> PEPs 366 and 395, I'm wary of adding new implicit module globals for
>>> problems with relatively simple and robust alternatives. In this case,
>>> it's fairly easy to get access to the current module using the idiom
>>> Guido quoted:
>>>    import sys
>>>    _this = sys.modules[__name__]
>>> (or using dict-style access on globals())
>> Yeah, well, in most cases I find having to reference sys.modules a
>> distraction and an unwarranted jump into the implementation. It may
>> not even work: there are some recipes that replace
>> sys.modules[__name__] with some wrapper object. If __this_module__
>> existed it would of course refer to the "real" module object involved.
> Some invocations of runpy.run_module also cause the 'sys.modules'
> based idioms to fail, so there may be a case to be made for this one.
> I suspect some folks would use it to avoid global declarations as well
> (i.e. by just writing '__module__.x = y').
> It might cause the cyclic GC some grief, though,so the implementation
> consequences would need to be investigated if someone wanted to pursue
> it.
> Cheers,
> Nick.
> [1] http://docs.python.org/dev/library/functools.html#functools.lru_cache
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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