On Tue, May 08, 2018 at 01:28:59PM -0400, Juancarlo Añez wrote:
> So the way I envision it is that *in the absence of a nonlocal or global
> > declaration in the containing scope*, := inside a comprehension or genexpr
> > causes the compiler to assign to a local in the containing scope, which is
> > elevated to a cell (if it isn't already). If there is an explicit nonlocal
> > or global declaration in the containing scope, that is honored.
> >
>
> This seems to be getting awfully complicated. Proof? Try to write the docs
> for the proposed semantics.
Okay, I'll bite. I don't know why you think its complicated: it is
precisely the same as ordinary ``=`` assignment scoping rules. It is
comprehensions that are the special case.
* * *
The binding expression ``<name> := <value>`` evaluates the right hand
side <value>, binds it to <name>, and then returns that value.
Unless explicitly declared nonlocal or global (in which case that
declaration is honoured), <name> will belong to the current scope, the
same as other assignments such ``name = value``, with one difference.
Inside comprehensions and generator expressions, variables created with
``for name in ...`` exist in a separate scope distinct from the usual
local/nonlocal/global/builtin scopes, and are inaccessible from outside
the comprehension. (They do not "leak".) That is not the case for those
created with ``:=``, which belong to the scope containing the
comprehension. To give an example:
a = 0
x = [b := 10*a for a in (1, 2, 3)]
assert x == [10, 20, 30]
assert a = 0
assert b = 30
> I don't understand why we went so astray from the original requirements,
> which could all be met by having `if` and `while` accept `as` to bind an
> expression to a variable that would be local to the structured statement.
That is not the original motivation for binding expressions. The
original requirements were specifically for comprehensions.
https://mail.python.org/pipermail/python-ideas/2018-February/048971.html
This is hardly the only time that something similar has been raised.
--
Steve
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