what is the "/" mean in __init__(self, /, *args, **kwargs) ?

[Terry Reedy]

On 8/13/2014 10:20 PM, Ethan Furman wrote:
> On 08/13/2014 07:01 PM, luofeiyu wrote:
>>>>> help(int.__init__)
>> Help on wrapper_descriptor:
>>
>> __init__(self, /, *args, **kwargs)
>>      Initialize self.  See help(type(self)) for accurate signature.
>>
>> what is the "/" mean in __init__(self, /, *args, **kwargs) ?
>
> The '/' means that all arguments before it must be positional only.

In particular, int.__init__(self = <subclass instance>) will not work 
because 'self' is positional only. (I don't think int.__init__ actually 
does anything, but a subclass of int might call it.)

> This looks like an artifact of the new Argument Clinic for C code.
>
> For example, if this also worked at the Python level, you could say:
>
>     def some_func(this, that, /, spam, eggs, *, foo, bar):
>        pass
>
> Meaning that the first two parameters could not be specified by name,
> the next two could be either name or position, and the last two by name
> only.
>
> Oh, and the * is valid Python now (the / is not -- it's solely a
> documentation feature at this point).

I hope / will be valid Python in 3.5.

-- 
Terry Jan Reedy