[Python-Dev] PEP 572: Why not := as standard assignment operator?

[Chris Angelico]

On Fri, Apr 27, 2018 at 1:38 AM, Gustavo Carneiro <gjcarneiro at gmail.com> wrote:
>
>
> On 26 April 2018 at 16:18, Chris Angelico <rosuav at gmail.com> wrote:
>>
>> On Thu, Apr 26, 2018 at 11:13 PM, Martin Teichmann
>> <lkb.teichmann at gmail.com> wrote:
>> > Hi list,
>> >
>> > when reading PEP 572 I actually liked it a lot - I think it's actually
>> > a cool idea. I think it's actually that cool an idea that it should be
>> > made the default way of doing an assignment, over time phasing out the
>> > good ole =.
>> >
>> > This would have several benefits:
>> >
>> > - people wouldn't have to worry about two different options
>> > - different things would have a different look: assignment is :=,
>> > keyword args is =, while comparison is ==. Especially beginners would
>> > benefit from this clarity.
>> >
>> > in this case, for sure, we should make it possible to chain :=s, for
>> > example by making it bind right-to-left, so that  a := b := 3 would be
>> > a := (b := 3)
>> >
>> > I'm sorry if somebody brought that up already, but the discussion has
>> > grown so huge that I couldn't read through it entirely.
>>
>> It has indeed grown huge. And in the interests of not growing it even
>> huger, I'm not going to rehash the arguments against making := into
>> the one and only operator, save to say one thing: there's no way that
>> "x = 1" can be removed from the language any time soon, and by "soon"
>> I mean even by the Yes Prime Minister definition, where "any day now",
>> in strategic terms, meant "within the next half century".
>
>
> In the interest of that, do you think := can be made illegal, by the
> grammar, if used outside an expression?
>
> a = 1  # legal
> a := 1  # Syntax error
> if a := 1:  # legal
>

No. Any expression may be used as a statement, so this isn't "outside
an expression".

ChrisA