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

[Chris Angelico]

On Thu, Mar 1, 2018 at 2:25 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
> On 02/28/2018 02:43 AM, Chris Angelico wrote:
>>
>> On Wed, Feb 28, 2018 at 8:04 PM, Robert Vanden Eynde wrote:
>
>
>>> 3) "C problem that an equals sign in an expression can now create a name
>>> binding, rather than performing a comparison." The "=" does variable
>>> assignement already, and there is no grammar problem of "=" vs "=="
>>> because
>>> the "with" keyword is used in the expression, therefore "with a == ..."
>>> is a
>>> SyntaxError whereas "where a = ..." is alright (See grammar in thektulu
>>> implemention of "where").
>>
>>
>> Yes, but in Python, "=" does variable assignment *as a statement*. In
>> C, you can do this:
>>
>> while (ch = getch())
>>      do_something_with(ch)
>>
>> That's an assignment in an arbitrary condition, and that's a bug
>> magnet. You cannot do that in Python. You cannot simply miss out one
>> equals sign and have legal code that does what you don't want. With my
>> proposed syntax, you'll be able to do this:
>>
>> while (getch() as ch):
>>      ...
>>
>> There's no way that you could accidentally write this when you really
>> wanted to compare against the character.
>
>
> Given the current (posted) proposal, wouldn't 'ch' evaporate before the ':'
> and be unavailable in the 'while' body?
>

Not my proposal. Others have suggested various forms of *expression*
local name bindings, but the definition in the PEP is *statement*
local. So it is indeed available.

ChrisA