[Python-Dev] PEP 572: Assignment Expressions

[Chris Angelico]

On Sat, Apr 21, 2018 at 5:11 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Sat, Apr 21, 2018 at 12:30:36PM +1000, Chris Angelico wrote:
>> Introducing expression assignments will make these oddities even more
>> obvious. You'd be able to demonstrate things like this at function
>> scope, not just with a class.
>
> In what way?
>
> And are you absolutely sure they will be oddities? To give an analogy, I
> don't think this is an oddity:
>
> py> def func(a=1, b=a+1):
> ...     pass
> ...
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> NameError: name 'a' is not defined
>
> If anyone expected that the default value for b could make use of the
> default value for a, the answer is: no, Python function declarations
> don't work that way. Maybe they could, if we wanted them to, but we
> don't, so they don't.
>
> So can you explain specifically what odd function-scope behaviour you
> are referring to? Give an example please?

doubled_items = [x for x in (items := get_items()) if x * 2 in items]

This will leak 'items' into the surrounding scope (but not 'x').

[x for x in x if x] # This works
[x for y in x if x := y] # UnboundLocalError

(x for x in 5) # TypeError
(x for _ in [1] for x in 5) # Works

I'm sure you can come up with more examples. The outermost iterable is
special and magical.

ChrisA