Mailman 3 PEP 472 -- Support for indexing with keyword arguments - Python-ideas

3 May 2020

      Hello!

I'm not sure I'm addressing the right audience here, so please direct me 
to the appropriate channel if that's the case...

My name is Andras Tantos and I'm working on a Python library desscribing 
HW designs. I came across this problem of __getitem__ and co. not 
supporting kwargs. Apparently this extension was proposed and rejected 
as PEP 472.

Apart from my use-case, which is arguably a corner-case and not worth 
modifying the language for, I believe there are two important use-cases 
that are worth considering with the latest improvements in the language:

1. With the recent type-hint support, the feature could be made way more 
descriptive if this PEP got implemented.

    For example, instead of doing the following:

         def func(in: Dict[str, int])

    one could write:

         def func(in: Dict[key=str, value=int])

2. It would also make 'generic classes' much cleaner to implement, 
similar to the way type-hints look. Consider the following code:

    class_Generic(object):
    Specializations = []
    @classmethod
    def__getitem__(cls, *args):
    name = f"Generic_{len(cls.Specializations)}"
    Specialized = type(name, (cls,), {"specials": tuple(args)})
    cls.Specializations.append(Specialized)
    returnSpecialized
    def__init__(self, value= None):
    self.value = value
    def__str__(self):
    ifhasattr(self, "specials"):
    return(f"[{type(self)}- "+ ",".join(str(special) forspecial
    inself.specials) + f"] - {self.value}")
    else:
    return(f"[{type(self)}- GENERIC"+ f"] - {self.value}")
    Generic = _Generic()
    #g = Generic() - fails because of no specialization is given
    s1 = Generic[12]()
    s2 = Generic[42]("Hi!")
    print(s1)
    print(s2)

    Running this simple example results in:

    python3 -i python_test.py
    [<class '__main__.Generic_0'> - 12] - None
    [<class '__main__.Generic_1'> - 42] - Hi!

    You can see how the specialized parameters got passed as well as the
    ones to '__init__'. Obviously, in real code the idea would be to
    filter generic parameters and set up 'Specialized' with the right
    set of methods and arguments.

    Now, without kwargs support for __getitem__, it's impossible to pass
    named arguments to the specialization list, which greatly limits the
    usability of this notation.

I don't know how convincing these arguments and use-cases are for you, 
but could you advise me about how to start the 'ball rolling' to drum-up 
support for re-activating this PEP?

Thanks again,
Andras Tantos

PEP 472 -- Support for indexing with keyword arguments

Andras Tantos

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