On Thu, May 10, 2018 at 3:32 AM, Terry Reedy <tjreedy@udel.edu> wrote:

On 5/9/2018 11:33 PM, Guido van Rossum wrote:

I now think that the best way out is to rule `:=` in the top level expression of an expression statement completely

I would like to be able to interactively enter

>>> a: = f(2,4)

to have 'a' echoed as well as bound.

I hope that's a typo (the can be no space between `:` and `=`, since `:=` is a single token, just like `<=').

We *could* make this work while still ruling out the ambiguous cases (which involve top-level commas on either side of the assignment expression).

OTOH I worry that this particular feature would cause `:=` to become part of many a teacher's bag of tricks to show off, exposing users to it way too early for any curriculum, and it might then elicit complaints that

>>> def f():

... a := 5
...

>>> f()
>>>

doesn't print `5`.

So all in all I'm not sure I think this is important enough to support, and the rule "Use `:=` in expressions, not as a top level assignment" seems easier to explain and understand.

--Guido van Rossum (python.org/~guido)