[Python-ideas] "Immutable Builder" Pattern and Operator

[Guido van Rossum]

On Mon, Jan 23, 2017 at 8:07 AM, Soni L. <fakedme+py at gmail.com> wrote:

>
>
> Since it desugars into `x = x.y`, you can literally use anything for `y`.
>
> x .= __call__().whatever().unwrap() * 3
>
> is equivalent to
>
> x = x.__call__().whatever().unwrap() * 3
>
> and
>
> x .= 1
>
> is equivalent to
>
> x = x.1
>
> which is equivalent to
>
> SyntaxError: invalid syntax
>

And that's exactly the problem. Users would be greatly confused because
what's to the right of `.=` is *not* an expression, it's something more
restricted (in particular it must start with a plain identifier). This
makes the `.=` operator a very different beast from `=`, `+=` and friends.

I assume you think that's fine, but given your cavalier attitude about `x
.= 1` my feeling is that you don't have a lot of experience designing and
implementing language features. That is okay, you are learning it here. But
perhaps you should take the hint from the large number of people here who
have gently tried to explain to you that while this is a good idea, it's
not a great idea, and there's no sufficiently important use case to make up
for the confusion (indicated above) that it will inevitably cause.

-- 
--Guido van Rossum (python.org/~guido)
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