super in Python 3 and variadic arguments

[Ned Batchelder]

On 10/10/13 3:22 AM, Marco Buttu wrote:
> On 10/09/2013 06:47 PM, Ned Batchelder wrote:
>
>>> >>> class B(A):
>>> ...     def bfoo(*args):
>>> ...         super().afoo(*args[1:])
>>> ...
>>> >>> B().bfoo(1, 2, 3)
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in <module>
>>>   File "<stdin>", line 3, in bfoo
>>> RuntimeError: super(): no arguments
>>>
>>> How come?
>>
>> The no-args super() call inspects the calling environment to determine
>> the class and self.  "self" is the first local name stored in
>> frame.f_code.co_localsplus, but *args doesn't put "args" into that entry
>> of the code object
>
> But is it a bug or the behavior we want? The first (implicit) argument 
> is stored as expected as the first one in the args tuple, and the args 
> tuple is inserted as expected in frame.f_locals:
>
> >>> import inspect
> >>> class B(A):
> ...     def bfoo(*args):
> ...         frame = inspect.currentframe()
> ...         for obj, value in frame.f_locals.items():
> ...             print(obj, value, sep=' --> ')
> ...         # super().afoo(*args[1:])
> ...
> >>> B().bfoo(1, 2, 3)
> args --> (<__main__.B object at 0x7f28c960a590>, 1, 2, 3)
> frame --> <frame object at 0x7f28cad4b240>
>
> So, why does not super use it?
>

I haven't seen the discussion that decided the behavior of super(), but 
I'd guess that if you reported this as a bug, it would be closed as 
wontfix, because: 1) the use case you describe isn't something people 
actually write, 2) it would add to the complexity of super() to support 
it, and 3) there's a simple way to write your code that does work:

     class B(A):
         def bfoo(self, *args):
             super().afoo(*args)

(though it's a bit odd to call afoo from bfoo.)

Python has never claimed the kind of purity that makes everything work 
in a totally simple consistent way.  super() with no args is a kind of 
hack to begin with.  It involves a special case in the compiler (so that 
using the name "super" as a function call will act as if you had 
accessed the name "__class__" so that super can find it later), and 
inspecting the stack frame during execution.

It's an interesting case of the Zen of Python.  It violates one 
("explicit is better than implicit"), but only because of another one 
("practicality beats purity").  super(MyClass, self) in Python 2 is the 
kind of brain-bender that so many people get wrong at first, that it's 
helped plenty of people to do the arguments implicitly, even if there 
are oddball edge cases that it doesn't seem to handle properly.

--Ned.