[Python-ideas] Before and after the colon in funciton defs.

Devin Jeanpierre jeanpierreda at gmail.com
Fri Sep 23 03:39:39 CEST 2011

> Decorator syntax cannot work without deep magic, because the
> compiler *doesn't know* that injected names need to be given special
> treatment.

This is the last time you mention the decorator solution (aside from
further explanation of the problem). Is it being discarded for that

I was under the impression that everyone who liked that idea was fully
aware that it's deep magic.

I would assume it actually creates an entirely new function object
with new closure cell to bind the name, similar to your last solution
involving the function closing over "itself", but more general and as
part of a decorator above the function header. I'm not really seeing a
problem other than maybe distaste because it seems "hacky". It can
work, though.


On Thu, Sep 22, 2011 at 9:11 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Fri, Sep 23, 2011 at 9:51 AM, Steven D'Aprano <steve at pearwood.info> wrote:
>> With decorator syntax, the scoping rules are obvious and straightforward:
>> a = 10
>> @inject(b=a)
>> def foo():
>>    a = 20
>>    return b+a
> Please read the previous thread from June (linked earlier in this
> thread). Decorator syntax cannot work without deep magic, because the
> compiler *doesn't know* that injected names need to be given special
> treatment.
> Python's scoping relies on the compiler being able to classify names
> at compile time into 3 kinds of reference:
> - local (direct references into the local variable namespace of the
> executing frame)
> - cells (indirect references via cells stored on the function object)
> - unknown (looked up by name at runtime, first in the module globals
> and then in the builtin namespace)
> These 3 reference types are baked into the immutable code objects by
> the compiler - you *cannot* change them later without hacking the
> bytecode and recreating the function object.
> Now, we have two 'magical' names ('super' and '__cell__') that cause
> the compiler to spontaneously do interesting things with namespaces to
> make Python 3's new simplified (and incredibly convenient) super()
> invocation work. However, aside from that special case, the rules are
> very simple:
> - names bound in the current function are locals (unless marked with
> 'nonlocal' or 'global')
> - names bound as locals in an outer function and referenced from the
> current function are looked up via cells
> - anything else is treated as an unknown name
> The 'nonlocal' and 'global' keywords override the 'local by default'
> behaviour for bound names (forcing the second or third interpretations
> respectively).
> The default argument hack effectively creates a 4th namespace option
> by using the default arguments as "pre-populated locals" - the
> argument passing machinery is set up so that any parameter not
> supplied as an argument is filled in on the current frame from its
> default argument value. By adding additional parameters that are
> *never* supplied as arguments, the author of a function can create
> arbitrary locals from expressions that are evaluated when the function
> is defined rather than when it is called.
> That means there are four very different ways of looking at potential
> replacements for this technique:
> 1. Leave the technique alone, but improve the introspection tools and
> conventions associated with it
>    def f(x, _i=i):  # pydoc would, by default, display the signature as 'f(x)'
>        return x + _i
> Keyword-only arguments in Py3k already help with this approach to the
> question, especially when the 'hidden' keyword is prefixed with an
> underscore to indicate it isn't meant for public consumption. This
> approach is also highly amenable to monkey-patching, since the default
> arguments can be deliberately overridden at call time, just like any
> other parameter. It wouldn't be hard to adjust pydoc to leave out
> underscore-prefixed keyword only parameters by default, requiring an
> explicit request to include them.
> In other words, this approach just involves taking the existing
> default argument hack, tidying it up a bit, explaining it in the docs,
> and blessing it as the official way to do things and a technique that
> experienced Python programmers should know and understand.
> 2. Definition time parameters
> This approach keeps the pre-populated locals in the function header,
> but tweaks the spelling and storage so they're no longer part of the
> function signature.
> Two ideas have been put forward for this approach:
>    def f(x, **, i=i):  # extending the keyword-only syntax one step further
>        return x + i
>    def f(x) [i=i]:  # adding a dedicated set of brackets
>        return x + i
> The general consensus seems to be that these don't offer enough
> benefit over the status quo to be worth the hassle.
> 3. Definition time expressions
> With a wide variety of proposed spellings (e.g. once, static, atdef),
> this proposals aims to mark individual expressions for evaluation at
> function definition time and caching on the function object. At
> function call time, the value would be inserted in place of the
> expression.
> I explained this in my previous email, and Guido has already said '-1'
> to this approach, so I won't elaborate any further.
> 4. Function scoped variables
> This is the approach most analogous to C's static variables - named
> variables that are shared across all invocations of a function, rather
> than being local to the current invocation. In essence, each function
> becomes its own closure - just as a function can share state across
> invocations by using an outer function for storage, this technique
> would allow a function to use its *own* cell array for such storage.
> Framing the idea that way also suggests a fairly obvious spelling:
>    def f(x):
>        nonlocal i=i # Use 'f' as a closure over *itself*
>        return x + i
> With this spelling, the above would be roughly equivalent to:
>    def outer():
>        i = i
>        def f(x):
>            return x + i
>        return f
>    f = outer()
> The only visible difference would be that the cell referenced by 'i'
> would be stored directly on 'f' rather than on an outer function.
> Regards,
> Nick.
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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