[Python-ideas] One more time... lambda function <--- from *** signature def.

[Nick Coghlan]

On 1 Mar 2014 05:43, "Ron Adam" <ron3200 at gmail.com> wrote:
>
>
>
> On 02/28/2014 11:54 AM, Chris Angelico wrote:
>>
>> On Sat, Mar 1, 2014 at 4:17 AM, Ron Adam<ron3200 at gmail.com>  wrote:
>>>
>>> >This returns a lambda-like function.
>>> >
>>> >       def star_lambda(***expr): return expr
>>> >
>>> >
>>> >And is used this way...
>>> >
>>> >       result = star_lambda(a * b + c)  # captures expression.
>>> >
>>> >       actual_result = ***result        # *** resolves "result" here!
>>> >
>>
>> Interesting, but I don't like the way the interpretation of a function
>> call depends on the target function. With both * and ** notations,
>> there's absolutely no difference: the function is called with these
>> positional and those keyword arguments, whether they came from actual
>> args or from * or ** unpack/repacks;and there's no difference between
>
> > a function that collects args with *args,**kwargs and one that
> > collects them with individual names (or a C-level function that might
> > do something altogether different).
>
>
> It's not clear what differences you mean here... can you show some
examples?

Remember that at compile time, Python has *no idea* what the actual
signature of the target function is. Thus, all Python function calls use
the following sequence (ignoring optimisations of special cases):

1. At the call site, the arguments are collected into a tuple of positional
arguments and a dict of keyword arguments.
2. The interpreter hands that tuple and dict over to the target callable
3. The *target callable* then maps the supplied arguments to the defined
parameters including filling in any default values.

Any function related proposals need to account for the fact that from the
compiler's point of view *every* function signature looks like "(*args,
**kwds)" (although it may have optimised paths for the no-args case and the
positional-args-only case), and that the target callable may not even be
written in Python.

Cheers,
Nick.
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