Those two controversial 2nd & 3rd paragraphs of my ch 1

Daniel Fetchinson fetchinson at googlemail.com
Wed Jan 13 17:30:52 CET 2010


>> Also, I would replace
>>
>> "in practice it’s very hard to create programs"
>>
>> with
>>
>> "in practice it’s very hard to create complex programs"
>>
>> because for small programs it's very possible to write code that will
>> work with both python 2 and 3. The question is of course what program
>> is simple/small and what program is large/complex, but without
>> qualifications your sentence is misleading, I think.
>
> We had the example of Cheetah over in the other thread. Would you say that
> Cheetah doesn't qualify as "complex" program?

One code base of cheetah works under python 2 and 3? I doubt it, but I
could be wrong. What I can easily imagine is that somebody ported
cheetah to python 3. In this case there are two code bases, one for
python 2 and another for python 3. So it's not the same program that
runs under python 2 and 3.

What the sentence in Alf's book is about is having the same code base
working for both python 2 and 3.

> There are also other
> examples, like Django.

Again, django has been ported to python 3, that's fine, everybody
acknowledges that, but it's not the case that one code base works with
both python versions.

> Often enough it's just a couple of cleanups and tiny
> try-except blocks in the program header that enables running the program in
> both Py2 and Py3.

Yes, this is true. But I'd say it's fair to say that with complex
programs this is usually not the case, but I definitely not want to
enter into a discussion into whether any given program is complex or
simple. It's a matter of judgement and gut feeling it's pointless to
argue about this too much.

> If it's more, it can usually be done using 2to3.

Again, 2to3 helps with porting, but does not help with having a single
code base that will run unmodified on python 2 and 3, which is what
Alf was writing about.

> So I
> would change the above statement into something more like "for some
> programs, especially large existing code bases, it can be hard to get the
> code to work in both Python 2 and Python 3".

I actually agree with this sentence :)

> Nevertheless, it has been  done, more than once.

Example? Just to be clear I'm looking for an example where one given
code runs on python 2 and 3 unmodified. I think django and cheetah
doesn't count because they simply take their python 2 code, run it
through 2to3 which gives them a python 3 code (I could be wrong
though). Two codes for the two python versions.

> Personally, I don't see much value in deliberately trying to keep people
> from porting their code to Py3 by producing underqualified statements like
> the above.

Nobody is deliberately trying to keep people from porting! I think you
misunderstand what is being said, these two statements are very
different: (1) single code base working on both python versions (2)
creating a second code from a code so that the second code works with
python 3 and the first one with python 2. Statement (2) is about
porting, statement (1) is about something else.

Having said all that I actually seriously doubt (probably in agreement
with you) that Alf is able to write good and helpful material on the
relationship between python 2 and 3, porting, migrating, etc, based on
his emails :)

Cheers,
Daniel




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