[Python-Dev] assignment expressions: an alternative proposal

[Chris Angelico]

On Wed, Apr 25, 2018 at 7:29 AM, Anthony Flury via Python-Dev
<python-dev at python.org> wrote:
> On 24/04/18 17:11, Yury Selivanov wrote:
>>
>> On Tue, Apr 24, 2018 at 12:03 PM, Ethan Furman <ethan at stoneleaf.us> wrote:
>> [..]
>>>
>>> But I do write this:
>>>
>>>    def wrapper(func, some_value):
>>>      value_I_want = process(some_value)
>>>      def wrapped(*args, **kwds):
>>>        if value_I_want == 42:
>>>           ...
>>
>> But this pattern is more rare than comparing local variables. That's
>> the point I'm trying to use.  Besides, to make it an assignment
>> expression under my proposal you would need to use parens. Which makes
>> it even less likely that you confuse '=' and '=='.
>
>
> Just because you wrap a set of character in parens doesn't mean that you
> wont potentially mistype what you should type inside the parens. The failure
> mode of in C :
>
>     if (a = 3)
>         do_something_with_a(a);
>
> Is Incredibly common even with very experienced developers - so much so that
> most linters flag it as a likely error, and I think gcc has an option to
> flag it as a warning - even though it is valid and very occasionally it is
> useful.

Technically what you have there trips *two* dodgy conditions, and
could produce either warning or both:

1) Potential assignment where you meant comparison
2) Condition is always true.

The combination makes it extremely likely that this wasn't intended.
It's more dangerous, though, when the RHS is a function call...

> Also many developers who come to Python from languages such as C will still
> place parens around conditionals - this means that a typo which will cause a
> Syntax Error in current versions, but would cause a potentially subtle bug
> under your implementation (unless you maintain the rule that you can't
> rebind currently bound names - which renders the whole idea useless in loops
> (as already discussed at length).

Yuri, look how many of the python-dev readers have completely
misinterpreted this "can't rebind" rule. I think that's a fairly clear
indication that the rule is not going to be well understood by anyone
who isn't extremely familiar with the way the language is parsed. I
hesitate to say outright that it is a *bad rule*, but that's the lines
I'm thinking along.

ChrisA