Comment by exDM69 | Hacker News Reader

Comment by exDM69 | Hacker News Reader

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Simply put: it's a better language. The whole "discussion" is whether it makes sense to migrate since many parts of the ecosystem (many important libraries and frameworks) have not made the transition. And many distros ship Python 2 by default, Python 3 is optional. Python 3 only is not feasible.

To me, the killer feature is better lazy evaluation (generators). In particular, important builtins like map, filter, zip, enumerate, etc are generators, instead of returning lists. This makes it feasible to write things like

    (process(line) for line in map(str.upper, open('giantfile.txt')) if line.lstrip()[0] != '#')

Some of the above can also be done with itertools package in Python 2, but not everything.

Python 3.4 changelog is here, it contains e.g. asynchronous io facilities (asyncio module): https://www.python.org/downloads/release/python-342/

edit: added enumerate() in the example above, for line in open(filename) returns a generator in Python 2.x too.

edit2: enumerate is lazy in python2, I replaced it with map(str.upper)

The example you gave works perfectly in Python 2.7 (would also be a generator, and you're not using map filter or else); but I agree: those should've been generators from day 1, especially zip and enumerate since they make more elegant code but often come with a performance overhead in Python 2.7

Python 2 enumerate returns an 'enumerate' object that is more or less a light weight wrapper of the sequence that was passed in.

Generators provide a convenient syntax to implement that sort of object.

D'oh. I put in a map(), that returns a list.

Turning them into generators would have broken a lot of existing code, though, so it's reasonable to leave them until a major version change. Rather, these are the kind of small, obviously-useful changes that should have come immediately in 3.0, giving people some encouragement to switch.

Most of the breaking changes (including making map, zip, filter, etc lazy) were done in Python 3.0.

Ah, you're absolutely right. for line in open(filename) returns a generator. I added map(str.upper, xxx) there to make it more it make sense.

But the point should be obvious, without generators that would potentially consume a lot of memory.

        (process(line) for line in open('giantfile.txt') if line.lstrip()[0] != '#')

Is that line really using any new features in Python 3? The lazy evaluation there is in the file object and the generator expression, both of which have long been present in python2.

No, it wasn't. I added map(str.upper, xxx), now it does.

Then again a lazy map is just an import away in P2. It removes pitfalls from Python but the improvement is… very limited (as opposed to e.g. `yield from` which is a big convenience, or for much more specialised uses the ellipsis literal being universal)

  (process(line.upper()) for line in open('giantfile.txt') if line.lstrip()[0] != '#')

That's besides the point. It's trivial to write that as a for loop or in a million different ways to avoid the issue. It's a contrived example written to demonstrate a difference.

Here's another one you can't change that easily:

    tests_pass = all(process(input) == output for (input, output) in zip(open('inputs.txt'), open('outputs.txt'))

You can change that easily, with izip from itertools.

The fact that a bunch of builtins and the values/items methods of dictionaries have become iterators is not very siginificant IMHO. Python 2 code could already be written to use iterators or generator expressions, so in the parts where it was crucial it was already done. In this regard Python 3 has not added new functionality but only changed defaults.

The unicode change is the big one.

In this case (checking that process(input) == output), itertools.izip_longest is probably that right solution, unless there's an out-of-band way to know that inputs.txt is the same length as outputs.txt.

You felt the need to correct yourself earlier, so I think your "besides the point" should be directed to yourself. I was pointing out that your correction wasn't persuasive.

We went to Python 3 for the multiprocessing module. At the time it was 3.3, but now 3.4 has all that async magic. I wish I still worked on that project.

You'll get an IndexError exception on that if there are any blank lines in the file.

Changing that to line.lstrip().startswith('#') would be an alternate approach.

You're right but that is irrelevant, it's a somewhat contrived example anyway. It's not like I spent a lot of time trying it out.

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