The following comment is from the thread about adding kwd arg support to
the square bracket operator (eg, `Vec = Dict[i=float, j=float]`).
On Tue, Aug 4, 2020, 2:57 AM Greg Ewing <greg.ew...@canterbury.ac.nz> wrote:
On 4/08/20 1:16 pm, Steven D'Aprano wrote:
> Why would we want to even consider a new approach to handling keyword
> arguments which applies only to three dunder methods, `__getitem__`,
> `__setitem__` and `__delitem__`, instead of handling keyword arguments
> in the same way that every other method handles them?
These methods are already kind of screwy in that they don't
handle *positional* arguments in the usual way -- packing them
into a tuple instead of passing them as individual arguments.
I think this is messing up everyone's intuition on how indexing
should be extended to incorporate keyword args, or even whether
this should be done at all.
--
Greg
So here is the main question of this thread:
Is there really not a backwards compatible, creative way a transition to
positional args from a tuple in the item dunders could not be accomplished?
It's not my intention to champion any specific idea here, just creating a
specific space for ideas to be suggested and mulled over.
There could be several reasons for changing the item dunder signatures. The
immediate reason is making the intuition around how to add kwd arg support
to square brackets more obvious and sane.
A second reason is it might be more intuitive for users who have to learn
and remember that multiple arguments to [ ] get packed into a tuple, but
this doesn't happen anywhere else.
Another reason: it could make writing code for specialized libraries that
tend to abuse (for the good of us all!) item dunders, like pandas, much
easier. Right now such libraries have to rely on their own efforts to break
up a key:
def __getitem__(self, key):
try:
k1, k2 = key
except TypeError:
raise TypeError("two tuple key required")
But for regular function calls (as opposed to item getting) we get to write
our signature however we want and rely on the language to handle all of
this for us:
def f(k1, k2):
# no worries about parsing out the arguments
-----------
One idea: change the "real" names of the dunders. Give `type` default
versions of the new dunders that direct the call to the old dunder names.
The new get and del dunders would have behavior and signatures like (I am
including **__kwargs since that could be an option in the future) :
def __getx__(self, /, *__key, **__kwargs):
return self.__getitem__(__key, **__kwargs)
def __delx__(self, /,, *__key, **__kwargs):
del self.__delitem__(__key, **__kwargs)
However the set dunder signature would be a problem, because to mirror the
current behavior we end up writing what is now a syntax error:
def __setx__(self, /, *__key, __value, **__kwargs):
self.__setitem__(__key, __value, **__kwargs)
The intended meaning above would be that the last positional argument gets
assigned to __value. Maybe someone could suggest a way to fix this.
Item getting, setting, deleting would call these new dunders instead of the
old ones. I haven't thought through how to handle inheritance-- I'm sort of
hoping someone smarter than me could come up with a way to solve that...:
class My:
def __getx__(self, my_arg, *args, my_kwarg, **kwargs):
# the way I have written things this super call will cause a
recursion error
v = super().__getitem__(*args, **kwargs)
return combine(my_arg, my_kwarg, v)
By the way I'm not bike shedding on what these new dunder names should be,
though we could probably do worse then getx, setx, and delx (nice and
short!).
-----------
Ok now this is the part where I get to wait for everyone smarter than me to
show the errors of my ways/how naive I am. :)
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