On Thu, Aug 06, 2020 at 02:46:01PM +0100, MRAB wrote:
> On 2020-08-06 14:16, Stestagg wrote:
> > In the "New syntax", wouldn't these examples map to:
> >
> > d[1, 2, a=3] => d.__getitem__((1, 2), a=3)
> > and
> > d[(1, 2), a=3] => d.__getitem__((1, 2), a=3)
That is certainly what I would expect. All positional arguments are
packed into a single positional parameter, and any keyword arguments
passed separately.
Any current subscript dunder is likely to look like this:
def __getitem__(self, index): # or key, whatever
i.e. it will only have a single positional argument. (Aside from self of
course.)
> Not quite. The second should be:
>
> d[(1, 2), a=3] => d.__getitem__(((1, 2),), a=3)
Aside from the keyword argument, which is a syntax error, that's not
what happens now.
py> class Demo:
... def __getitem__(self, index):
... print(index, type(index))
...
py> d = Demo()
py> d[(1, 2)] # Tuple single arg.
(1, 2) <class 'tuple'>
py> d[1, 2] # Still a tuple.
(1, 2) <class 'tuple'>
Adding a keyword arg should not change this.
[Stestagg]
> > I.e. My understanding was that the existing conversion of all
> > positional parameters in a subscript would be packed into a tuple
> > (strictly for consistency with legacy behaviour) while any keywords
> > would be passed as kwargs?
To be clear, a single argument is not packed into a tuple.
I think that what is actually going on is a matter of precedence. Commas
do double duty, separating arguments in calls and creating tuples.
Inside a function call, the argument separate takes priority:
func(1, 2, 3)
is interpreted as three arguments separated by commas. If you want a
tuple, you need to change the priority by using round brackets:
func((1, 2, 3))
But inside a subscript, there is currently no concept of multiple
arguments. There's always only one item, and commas just have their
normal meaning of creating a tuple.
I think that's how it works, but I'm not quite good enough at reading
the grammar specs to be sure.
https://docs.python.org/3/reference/grammar.html
--
Steven
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