[Python-ideas] PEP 572: Statement-Local Name Bindings

[Paul Moore]

On 28 February 2018 at 15:18, Chris Angelico <rosuav at gmail.com> wrote:

>>     a = (2 as a)
>>
>> there is a temporary variable 'a', which gets assigned 2, and the SLNB is
>> evaluated as 2, which should then get assigned back to the local variable
>> 'a'.  In other words, the final print from `f()` above should be 2, not 1.
>> (Slightly different names would help avoid confusion when referencing
>> different locations of the PEP.)
>
> Except that assignment is evaluated RHS before LHS as part of a single
> statement. When Python goes to look up the name "a" to store it (as
> the final step of the assignment), the SLNB is still active (it's
> still the same statement - note that this is NOT expression-local), so
> it uses the temporary.
>
> Honestly, though, it's like writing "a = a++" in C, and then being
> confused by the result. Why are you using the same name in two
> assignments? Normal code shouldn't do this. :)

Eww. I can understand the logic here, but this sort of weird gotcha is
precisely why people dislike C/C++ and prefer Python. I don't consider
it a selling point that this proposal allows Python coders to make the
sort of mistakes C coders have suffered from for years.

Can you make sure that the PEP includes a section that covers weird
behaviours like this as problems with the proposal? I'm happy if you
just list them, or even say "while this is a potential issue, the
author doesn't think it's a major problem". I just don't think it
should be forgotten.

Paul