Re: [Python-Dev] code blocks using 'for' loops and generators

[Brian Sabbey]

On Sat, 12 Mar 2005, Steven Bethard wrote:
Brian Sabbey <[EMAIL PROTECTED]> wrote:
I agree that this is a confusing way to use generators.  But it is the
expected way to use "code blocks" as found in other languages.  It would
take some getting used to that 'for' can be used this way, but I think it
would be worth it.
I guess I need some more convincing...  I don't find your approach[*], e.g.
def pickled_file(name):
   f = open(name, 'r')
   data yield pickle.load(f)
   f.close()
   f = open(name, 'w')
   pickle.dump(data, f)
   f.close()
for data in pickled_file('greetings.pickle'):
   data.append('hello')
   data.append('howdy')
   continue data
any clearer than, say:
class PickledFile(object):
    def __init__(self, name):
        self.name = name
        f = open(self.name, 'r')
        self.data = pickle.load(f)
        f.close()
    def close(self):
        f = open(self.name, 'w')
        pickle.dump(self.data, f)
        f.close()
p = PickledFile('greetings.pickle')
p.data.extend(['hello', 'howdy'])
p.close()
Note that I'm not using the iteration construct (for-in) because I'm
not really doing an iteration here.  Pehaps I could be taught to read
for-in otherwise, but without an obvious benefit for doing so, I'm
really not inclined to.
In the real world, I need to lock and unlock the pickled files.  So if I 
forget to call p.close(), then it would be a problem.  I would need a 
try/finally block.  If I could use the above 'for' loop approach, I am 
able to encapsulate the try/finally code.

This is the same problem that is addressed in PEP 310.  Copied from the 
PEP, here is the basic idea:

   The syntax of a 'with' statement is as follows::
        'with' [ var '=' ] expr ':'
            suite
    This statement is defined as being equivalent to the following
    sequence of statements:
        var = expr
        if hasattr(var, "__enter__"):
            var.__enter__()
        try:
            suite
        finally:
            var.__exit__()
I prefer re-using the 'for' loop for this purpose because it allows the 
problem to be solved in a general way by re-using a structure with which 
most users are already familiar, and uses generators, which are easier to 
use in this case than defining a class with __exit__, __enter__, etc.

-Brian
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