[HPH the BDFL]

I suggest you file those as products of an overactive imagination. :-)

At least not only mine. ;)

Have you even tried to define precise semantics for any of those, like the expansion of "with E as V: B" in PEP 343?

Easily. if expr as name: BLOCK would be equivalent to name = expr if name: BLOCK del name Same for while. Georg

Guido van Rossum wrote:

On 3/7/06, Georg Brandl wrote:

...

...

Have you even tried to define precise semantics for any of those, like the expansion of "with E as V: B" in PEP 343?

Easily.

if expr as name: BLOCK

would be equivalent to

name = expr if name: BLOCK del name

You need to be a little more formal. What happens when there are elif or else clauses? Why the del at all? (with doesn't delete VAR).

You also need to present a better motivation. What would be the point of having a separate name for the value True or False when you already know its value based on which branch you execute? And what exactly is the big savings? If you look at the motivation for with E as V, V is *not* assigned the value of E (but rather E.__context__().__enter__()) and the with statement as a whole has a very good reason for its existence. Your proposal here lacks that.

Thinking over it, this is too much a difference between the with-"as" and my "as", so I'm abandoning this idea. My "as" would just have been a shortcut to avoid writing longish expressions that have to be checked for true-ness and then tinkered with.

...

Same for while.

You aren't really trying, are you?

No ;) I would have written a PEP anyway. Cheers, Georg

On 3/7/06, Andrew Koenig wrote:

As it turns out, Python has similar ways of decomposing data structures:

(x, y) = foo

or

def bar((x, y)): # etc.

and I have sometimes wished I could write

z as (x, y) = foo

or

def bar(z as (x, y)): # etc.

However, it's not real high on my list of priorities, and I suspect that many Pythonists consider these usages to be a frill anyway.

For the assignment case, you can do this:

...

...

foo = (1,2) (x,y) = z = foo x 1 y 2 z (1, 2)

Function arguments are not covered by this trick, but def bar(z): (x,y) = z probably isn't too much overhead... (Or did I miss your point?) Paul.

Andrew Koenig wrote:

...

Function arguments are not covered by this trick, but

def bar(z): (x,y) = z

probably isn't too much overhead...

It's not the machine overhead, it's the intellectual overhead. I know there are some who will disagree with me, but I would find it easier to read

def foo(origin as (x1, y1), corner as (x2, y2)):

than to read

def foo(origin, corner): (x1, y1) = origin (x2, y2) = corner

It's not a big deal, but it is not completely negligible either.

On the third hand, what about

...

...

def foo((x1, y1), (x2, y2)): ... print x1, y1, x2, y2 ... origin = (10, 1) corner = (20, 2) foo(origin, corner) 10 1 20 2

Follow-ups set to comp.lang.python :-) regards Steve -- Steve Holden +44 150 684 7255 +1 800 494 3119 Holden Web LLC/Ltd www.holdenweb.com Love me, love my blog holdenweb.blogspot.com

Alex Martelli wrote:

I think the best use cases for 'assignment inside an if or while' condition, as far as they go, require `capturing' a SUB-expression of the condition, rather than the whole condition. E.g., in C,

while ( (x=next_x()) < threshold ) ...

being able to capture (by `as') only the whole (true or false) condition meets a minority of the use cases, so I'm unenthusiastic about it.

There are use cases (because every value can be tested for true-ness), such as the likes of while file.readline() as line: BLOCK (which is not needed for builtin file objects, I know) or something like m = re.match(...) if m.group(1) as filename: do something with filename Georg

Alex Martelli wrote:

On Mar 7, 2006, at 6:15 AM, Georg Brandl wrote:

...

with expr as f: do something with f

I expect the "with" example here is a red herring, not intended to have anything to do with the new "with" statement that's just been added.

I think the best use cases for 'assignment inside an if or while' condition, as far as they go, require `capturing' a SUB-expression of the condition, rather than the whole condition. E.g., in C,

while ( (x=next_x()) < threshold ) ...

IIUC, that would be while (next_x() as x) < threshold: ... i.e. 'x as y' would be an expression. -- Greg Ewing, Computer Science Dept, +--------------------------------------+ University of Canterbury, | Carpe post meridiam! | Christchurch, New Zealand | (I'm not a morning person.) | greg.ewing@canterbury.ac.nz +--------------------------------------+