[Python-ideas] PEP 572: about the operator precedence of :=

[Terry Reedy]

On 5/10/2018 9:53 AM, Guido van Rossum wrote:
> On Thu, May 10, 2018 at 3:32 AM, Terry Reedy 
> <tjreedy at udel.edu 
> <mailto:tjreedy at 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 `<=').

a := f(2,4) # corrected ;-)

> 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,

until someone tries a := [0]*10000000

> 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`.

Although the reason the same as for any expression, I can believe that 
people will see it as different.  A bare assignment expression *looks* 
like a statement in a way that other expressions do not.
> 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.

-- 
Terry Jan Reedy