New assignmens ...

Jon Ribbens jon+usenet at
Sat Oct 23 15:06:03 EDT 2021

On 2021-10-23, Chris Angelico <rosuav at> wrote:
> On Sun, Oct 24, 2021 at 4:39 AM Jon Ribbens via Python-list
><python-list at> wrote:
>> On 2021-10-23, Chris Angelico <rosuav at> wrote:
>> > In what situations do you need to mutate an attribute and also test
>> > it, and how much hassle is it to simply break it out into two lines?
>> It's not hard to imagine something like:
>>     def get_expensive(self):
>>         return self.expensive or self.expensive := self.calculate_expensive()
> I usually write this sort of thing the other way:
> def get_expensive(self, key):
>     if key not in self.cache:
>         self.cache[key] = ...
>     return self.cache[key]
> and then if you don't like the duplication, the cleanest way is to put
> the expensive calculation into the __missing__ method of a dict
> subclass.

Sure, but if "there's already another way of doing it" was a winning
argument then assignment expressions wouldn't have been accepted into
the language at all.

>> > The onus is on you to show that it needs to be more flexible.
>> Is it though? It seems to me that the onus is on you to show that
>> this special case is special enough to be given its own unique
>> existence. It's a bit surprising that the PEP doesn't discuss this
>> decision at all.
> The PEP was accepted. Thus it is now up to someone proposing a change
> to show that the change is worthwhile.
> Python has frequently started with a more restricted rule set, with
> the option to make it less restricted in the future. Case in point:
> Decorator syntax used to be limited to a small set of options, aimed
> at the known use-cases at the time. Then very recently, that was
> opened up to basically any expression.
> Read over that document for an excellent example of how to take a
> tight proposal and recommend that it be made more flexible. Assignment
> expressions are currently in the restricted form, allowing only simple
> names, and it's up to you to propose and demonstrate the value of the
> increased flexibility.

I think we're allowed to discuss things in this group without them
having to turn into formal proposals. Personally I've never written
a Python assignment expression, and I think it'll be a few years
before Python 3.8 is old enough for them to be conveniently usable.

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