From: Ville Vainio <ville@...>
Date: 2004-04-06T15:34:20+09:00
Subject: Re: deciding between ruby and python

>>>>> "Gavin" == Gavin Sinclair <gsinclair@soyabean.com.au> writes:

    >> Python. And if the needs of your anonymous function exceed what lambda
    >> provides, you can use what I call big L notation:

    >> def L(matchobj):
    >> return matchobj.group(0).upper()

    >> res = re.sub('u|a', L, 'huijaahuijaa')

    Gavin> That's a poor man's substitute.  Seriously.  It means you have to name
    Gavin> everything you're doing, and pollute the function namespace *for no
    Gavin> good reason*.  Yes, you can "do it", but it's ugly in comparison.

You don't really pollute it much, because you can always use the same
name. The old name (and as such, the old definition too) is clobbered
by the new name. If you always use big L for this purpose, 

Also, you can do the deffing inside functions too. So it's not even
visible at module level.

    Gavin> Can you provide an implementation in the same style in Python?  It's
    Gavin> not that I don't believe Python can count characters, lines and words
    Gavin> in a string.  But I don't believe it can carry outside state into its
    Gavin> "substitute for blocks".

Yes. Python supports lexical closures. The only gotcha is that you
can't 'assign' the names of free variables to point to new objects,
i.e. it has to be the same object, though it can be mutated. So you
can use e.g. length 1 array to contain the other object.

The following code:

------------------

def get_char_counter():
    chars = [0]

    def filepusher(f):
        chars[0] += len(open(f).read())
        return chars[0]

    return filepusher

acc = get_char_counter()

print acc("/bootsect.dos")
print acc("/bootsect.dos")

-------------------

Prints out:

-------------------
201
402
-------------------

    Gavin> # Connection: the database connection is automatically released at
    Gavin> # the end of the block, including if there is an exception therein.

try - finally blocks are used for this in Python. 

    Gavin> # Transaction support: any exception in this block will cause a
    Gavin> # "ROLLBACK".  If all is fine, a "COMMIT" is performed at the end.

try:
   stuff()
except:
   rollback()
else:
   commit()

But I admit that the ruby approach seems cleaner here. It's probably
easier to use and the logic doesn't clutter the code.

    Gavin> I'd like to see the same program written in Python.  I don't know the
    Gavin> Python database interface, but I expect it would need to have explicit
    Gavin> exception handling.  There would also be no automatic releasing of
    Gavin> resources.

Back when I did db programming, I didn't think it was such a big
deal. It seems bigger on the paper, though. 

    Gavin> To prove me wrong, please present your best effort at translating the
    Gavin> database programming example above into Python.  Of course, you may
    Gavin> disagree with my frame of reference instead, but I think that's pretty
    Gavin> good.

Yes, I do indeed disagree with your frame of reference. I just don't
think this issue such big a deal. you might want to go googling for
more elaborate discussion - code blocks have indeed been suggested for
Python, they just didn't happen. It seems to pop up every now and
then, so perhaps they will be in Python some day if they are deemed
valuable enough. In the meantime, I don't care much - Try-finally is
good enough for me :-). I would go googling myself, but I don't really
have the time. There's work to do, and I'm 'between ADSL operators' at
home for this month.

-- 
Ville Vainio   http://tinyurl.com/2prnb