For review: PEP 343: Anonymous Block Redux and Generator Enhancements

[Nicolas Fleury]

Andrew Dalke wrote:
> The implementation would need to track all the with/as forms
> in a block so they can be __exit__()ed as appropriate.  In this
> case ghi.__exit() is called after jkl.__exit__() and
> before defg.__exit__
> 
> The PEP gives an easy-to-understand mapping from the proposed
> change to how it could be implemented by hand in the existing
> Python.  Can you do the same?

I think it is simple and that the implementation is as much 
straight-forward.  Think about it, it just means that:

 > with abc
 >
 > with defg:
 >   with ghi
 >   with jkl:
 >     1/0

is equivalent to:

 > with abc:
 >
 >   with defg:
 >     with ghi:
 >       with jkl:
 >         1/0

That's it.  Nothing more complex.  It's only about syntax.

> I have not idea if the problem you propose (multiple with/as
> blocks) will even exist so I can't comment on which solution
> looks good.  It may not be a problem in real code, so not needing
> any solution.

Good point.  As a C++ programmer, I use RAII a lot.  However, there's 
situations in C++ that don't apply, like allocating dynamic memory in a 
scope.  However, I still expect some Python programmers to use it enough 
to ask for that syntax to be added, in the same way operators like += 
have been added.

But there's another point that has nothing to do with how many "with" 
statements you have in a function.  In C++, very very rarely I've seen 
something like:

void foo() {
     {  // (define a scope for the Lock object)
        Lock locking(myMutex);
        ...
     }
     ...
     {
        Lock locking(myMutex);
        ...
     }
}

So I come to another conclusion: the indentation syntax will most of the 
time result in a waste of space.  Typically a programmer would want its 
with-block to end at the end of the current block.

So basically, there's two 10-90% points, one in favor of my proposal, 
one against:

- Most of the time, you don't have a lot of with-statements in a single 
function, so not so much indentation.
- Most of the time, a with-statement ends at the end of current block, 
so indentation-syntax most of the time result in a waste of space.

The way to see my proposal is not "to be used when you have multiple 
with-blocks" but instead "never use the ':' syntax, unless necessary". 
The day some code need it, it's very easy to add a ':' and indent some 
code with our favorite editor.

Regards,
Nicolas