while expression feature proposal

[Ian Kelly]

On Fri, Oct 26, 2012 at 4:03 PM, Cameron Simpson <cs at zip.com.au> wrote:
> It will work anywhere an expression is allowed, and superficially
> doesn't break stuff that exists if "as" has the lowest precedence.

Please, no.  There is no need for it outside of while expressions, and
anywhere else it's just going to be bad practice.  Even if it's
considered an expression, let's only allow it in while expressions.

> Any doco would need to make it clear that no order of operation is
> implied, so that this:
>
>   x = 1
>   y = (2 as x) + x
>
> does not have a defined answer; might be 2, might be 3. Just like any
> other function call with side effects.

Actually, the docs are clear that expressions are evaluated left to
right, so the expected result of the above would be 4.

> It would probably mean folding the except/with "as" uses back into
> expressions and out of the control-structural part of the grammar. I can't
> see that that would actually break any existing code though - anyone else?

Yes it would, because the meaning is a bit different in both of those
cases.  For except, the result of the expression (an exception class
or tuple of classes) is not stored in the target; the exception
*instance* is.  Similarly for with, the result of the expression is
not stored; the result of calling its __enter__ method is, which is
often but not always the same thing.