No macros in Python

[holger krekel]

Michael Hudson wrote:
> jepler at unpythonic.net writes:
> 
> > I'd rather just write 'with lock: suite'.
> 
> Right.  Here's a proposal:
> 
> with <exp>:
>   <suite>
> 
> is equivalent to
> 
> _x = <exp>
> _x.__enter__()
> try:
>   <suite>
> finally:
>   _x.__exit__()

I like the idea.  but i wonder if these semantics couldn't
be wrapped into a generic python protocol (the ':' indentation operator)
instead of adding a new keyword.  The basic idea is that you can omit
'with' and do

    <exp>:
        <suite>

which would still hold the exact same equivalence as above. 
If the parser and the python grammar allows this construct
unambigously then the ':' + indented block would signal
the 'enter/exit' protocol.  

with or without 'with', i wonder if allowing the <exp> part 
to manipulate the execution namespaces (globals, locals) of 
the <suite> wouldn't provide nice features, e.g.:

    class myclass:
        def some_method_which_uses_lots_of_attributes(self):
            with my_exec_in(self):
                # do lots of computations with attributes 
                #
                # use name, email, value directly instead of 
                #
                # self.name, self.email, self.value
                return result

        def some_const_method(self, *args):
            with const_attr(self):
                # as above but you can't modify self's name-bindings
            
for this to work the 'with' construct (or the ':' indentation operator)
would be equivalent to:

    _x = <exp>
    _x_entered = _x.__enter__()
    try:
        if _x_entered:
            _g,_l = _x_entered
            exec code_object(<suite>) in _g,_l
        else:
            <suite>
    finally:
        _x.__exit__()

i am not sure how these exec/code_object statements
are implementable, though. 

just my 2 keyword-cents,

    holger