[Python-Dev] PEP 572: Assignment Expressions

[Rob Cliffe]

On 17/04/2018 15:01, Chris Angelico wrote:
> On Tue, Apr 17, 2018 at 10:17 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>
>>> Style guide recommendations
>>> ===========================
>>>
>>> As this adds another way to spell some of the same effects as can already be
>>> done, it is worth noting a few broad recommendations. These could be included
>>> in PEP 8 and/or other style guides.
>>>
>>> 1. If either assignment statements or assignment expressions can be
>>>     used, prefer statements; they are a clear declaration of intent.
>>>
>>> 2. If using assignment expressions would lead to ambiguity about
>>>     execution order, restructure it to use statements instead.
>>>
>>> 3. Chaining multiple assignment expressions should generally be avoided.
>>>     More than one assignment per expression can detract from readability.
>> Given the many different uses for ":" identified on python-ideas, I'm
>> inclined to suggest making these proposed style guidelines more
>> prescriptive (at least initially) by either:
>>
>> 1. Listing out specific approved unambiguous use cases (i.e. if
>> statement conditions, while loop conditions, list comprehensions,
>> generation expressions)
>> 2. Making the 3rd admonition more general by advising against using
>> ":" for more than one purpose in the same expression (i.e. don't
>> combine assignment expressions with slicing syntax, lambda
>> expressions, function headers, variable annotations, dict or set
>> displays, dict or set comprehensions)
> I'm actually dubious about the third point as it stands.
I'm more than dubious - I disagree with Nick on this point.  It is 
already possible to have multiple uses of ":" in an expression; surely 
we wouldn't advise that such existing code should be changed, in cases 
where it is arises naturally and is genuinely useful.
Best wishes
Rob Cliffe