Migrating to python3Sat 25 June 2016
I was recently asked, by my boss, about the possible benefits of migrating our code base at work from python 2.6 to python 3. Instead of sending an email back to my boss with my answer, I decided that it worth a blog post. Why? because the internet is full with comparisons between python 2 and python 3, but it is hard to tell what are the real gains from doing the migration. Here, I will try to explain, in a very subjective manner, the benefits I see from such move.
So let's start with the obvious, python 3 is clearly a better language than python 2.6: print is a function instead of a statement, now with sane arguments; dict and set comprehensions reduce unnecessary, error prone, verbosity; real division by default, because that's what you want 99% of the time anyway; iterators by default (range replaced xrange, dict.items() replaced dict.iteritems(), etc.); explicit relative imports... Actually, the list goes on and on. However, I don't think that the features above are good reasons to migrate a large project. Modern python code, written with python 3 in mind, will benefit from them, that's for sure. But we do not want to rewrite our code base, we want the transition to be as smooth as possible.
So what is valuable? Here is my list:
We must leave python 2.6
Our server is based on twisted, which already dropped support for python 2.6. Our other main dependency is SQLAlchemy. I don't think that they are going to drop support for python 2.6 anytime soon but this day will surely come. In general, staying with python 2.6 will soon mean that for each dependency upgrade we will have to upgrade everything at once, which is kind of a nightmare.
So, if we must leave python 2.6 behind and upgrade, I really don't think that migrating to python 3.5, for example, will be way too complicated than to 2.7. Moreover, moving to python 2.7 now will require another migration, with similar effort, in the next 4 years.
What features do worth the upgrade
In python 3, when an exception is raised from within an except block, it is concatenated to the former exception, and the traceback of both is available to whoever catch them. That way, we can always be sure that our top most logging system will log the entire traceback, even if the latest exception "mask" the original one.
This feature could solve us several debugging hours. Even more, I'm sure that we gave up on lots of bugs we couldn't investigate properly, as the original exception was lost due to a bug in the except block.
If something is broken, I prefer it to raise exception as soon as possible. In python 2, the following won't fail!
>>> # python2 >>> 'hello' < 1
WTF?!? Why should anyone allow comparison between a string and an int!?! Moreover, what is the result, True or False? In python 3 this issue was solved for good:
>>> # python3 >>> 'hello' < 1 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unorderable types: str() < int()
Again, we saw such bugs before in our system, and they are hard to spot. Having the language solve this corner for us will be great.
List comprehensions that doesn't leak
Again, another source for nasty bugs.
>>> # python2 >>> i = 42 >>> squares = [i ** 2 for i in xrange(10)] >>> print(i) 9
>>> # python3 >>> i = 42 >>> squares = [i ** 2 for i in xrange(10)] >>> print(i) 42
And much more
Apart from new features there are the plethora of packages that we can't use with python 2.6. External dependencies gradually drop support, while the standard library continuously improves with new and shiny tools, such as concurrent.futures and asyncio.