[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20

[Chris Angelico]

On Thu, Apr 26, 2018 at 3:34 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Thu, Apr 26, 2018 at 05:22:58PM +1200, Greg Ewing wrote:
>> Łukasz Langa wrote:
>> >What was its own assignment before
>> >now is part of the logic test. This saves on vertical whitespace but makes
>> >parsing and understanding logic tests harder.
>>
>> Another way to say this is that expressions are no longer
>> restricted to being trees, but can be general DAGs, which
>> require more mental effort to understand.
>
> Is that right? I presume you mean that there can be cycles in
> expressions involving binding-expressions. If not, what do you mean?
>
> Can you give an example of a Python expression, involving PEP 572
> binding-expressions, that is not a tree but a more general DAG or that
> contains cycles?

A DAG is a directed *acyclic* graph, so it still can't contain cycles.
But I have no idea what kind of expression isn't a tree as a
consequence of having an assignment in it.

ChrisA