Thank you all, for your useful contributions. I particularly value the insight you've given me regarding the experience that underlies your views. I'll respond to your comments tomorrow.

I am able today to respond to one question. Ricky asked if

>>> key_object = K(a=1, b=2) # where K is some new key object type

>>> d1 = {key_object: 3}

>>> d2 = {}

>>> d2[a=1, b=2] = 3

>>> assert d1==d2

was what I had in mind. The answer is YES, and in some ways better expressed. (And the new key object type is introduced only if necessary for the desired user experience.)

The use of dict literals makes the role of the key_object even clearer. For clarity, when we write

>>> d[1, 2, 3] = 4

the associated key_object, for backwards compatibility, must be (1, 2, 3), which is a tuple.

Again for clarity, in today's Python the statements

>>> d[1, 2, 3] = 4

>>> d[(1, 2, 3)] = 4

are equivalent. I willingly accept this as a constraint. In my own mind, I require it.

Once again, thank you all, and I'll say more tomorrow (about 18 hours from now).

Jonathan