In this post, I explore how the new API might interact with dict objects. (I think PRP is a typo for PEP.)

 Here is an example of the present behaviour

    >>> d = dict()

    >>> d[x=1, y=2] = 3
    SyntaxError: invalid syntax

with the syntax error occurring at the first '=' symbol.

So what should be the new behaviour of:

To me, it is clear that we won't get a syntax error. This is because the proposal implies that

    >>> d[x=1, y=2] = 3

executes without error, for some values of 'd'.

If these two lines

execute without error, then I would the expect

    >>> len(d)

    1

    >>> d[x=1, y=2]

    3

I would further expect

    >>> k = list(d.keys())[0]

to give an object 'k', with the property

    >>> d[k]

    3

This object 'k' corresponds to the source code fragment

    x=1, y=2

in

    d[x=1, y=2]

and I would expect 'k' to work as a key in any instance of dict.

Here is something that might help the reader. It is that current Python gives

and we are looking for an analogous object for

    >>> d[x=1, y=2] = 3

To summarize and slightly extend the above. If

executes without error, then there is an object 'k' such that

    >>> d[x=1, y=2] = 5

    >>> d[k] = 5

are equivalent. (This object 'k' is the key in the collection of (key, value) pairs that are the entries in the dict.)

Aside: The key object, if it exists, has a type. Whether this type is a new type or an existing type is an implementation question. This message is focussed on the user experience.

To conclude: If d[x=1, y=2] is allowed for 'd' a dict then consistency with the current dict behaviour requires the existence of a key object that corresponds to the fragment 'x=1, y=2'.

Finally, I have tried to be both clear and concise. I hope my contribution helps.

-- 

Jonathan