[Python-Dev] Re: anonymous blocks

ron adam rrr at ronadam.com
Tue Apr 26 20:18:17 CEST 2005

Hi, this is my first post here and I've been following this very 
interesting discussion as is has developed. 

A really short intro about me,  I was trained as a computer tech in the 
early 80's... ie. learned transistors, gates, logic etc...  And so my 
focus tends to be from that of a troubleshooter.  I'm medically retired 
now (not a subject for here) and am looking for something meaningful and 
rewarding that I can contribute to with my free time.

I will not post often at first as I am still getting up to speed with 
CVS and how Pythons core works.  Hopefully I'm not lagging this 
discussion too far or adding unneeded noise to it.  :-)

>> So maybe the 'with' keyword should be dropped (again!) in
>> favour of
>>   with_opened(pathname) as f:
>>     ...
> But that doesn't look so great for the case where there's no variable
> to be assigned to -- I wasn't totally clear about it, but I meant the
> syntax to be
>    with [VAR =] EXPR: BLOCK
> where VAR would have the same syntax as the left hand side of an
> assignment (or the variable in a for-statement).

I keep wanting to read it as:

   with OBJECT [from EXPR]: BLOCK

>> 2) I'm not sure about the '='. It makes it look rather deceptively
>> like an ordinary assignment, and I'm sure many people are going
>> to wonder what the difference is between
>>   with f = opened(pathname):
>>     do_stuff_to(f)
>> and simply
>>   f = opened(pathname)
>>   do_stuff_to(f)
>> or even just unconsciously read the first as the second without
>> noticing that anything special is going on. Especially if they're
>> coming from a language like Pascal which has a much less magical
>> form of with-statement.

Below is what gives me the clearest picture so far.  To me there is 
nothing 'anonymous' going on here.  Which is good I think. :-)

After playing around with Guido's example a bit, it looks to me the role 
of a 'with' block is to define the life of a resource object.  so "with 
OBJECT: BLOCK" seems to me to be the simplest and most natural way to 
express this.

def with_file(filename, mode):
   """ Create a file resource """
   f = open(filename, mode)
       yield f        # use yield here
       # Do at exit of 'with <resource>: <block>'

# Get a resource/generator object and use it.
f_resource = with_file('resource.py', 'r')
with f_resource:
   f = f_resource.next()   # get values from yields
   for line in f:
       print line,

# Generator resource with yield loop.

def with_file(filename):
   """ Create a file line resource """
   f = open(filename, 'r')      try:
       for line in f:
           yield line
          # print lines in this file.
f_resource = with_file('resource.py')
with f_resource:
   while 1:
       line = f_resource.next()
       if line == "":
       print line,

The life of an object used with a 'with' block is shorter than that of 
the function it is called from, but if the function is short, the life 
could be the same as the function. Then the 'with' block could be 
optional if the resource objects __exit__ method is called when the 
function exits, but that may require some way to tag a resource as being 
different from other class's and generators to keep from evaluating 
__exit__ methods of other objects.
As far as looping behaviors go, I prefer the loop to be explicitly 
defined in the resource  or the body of the 'with', because it looks to 
be more flexible.

# "The right question is a good start to finding the correct answer."

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