on Python 3.x

Monday 13 July 2009

Last week I went through the exercise of making compatible with Python 3.1. I learned a few things along the way.

At first, I wanted to create code that would work on both 2.x and 3.x, but while that is possible to an extent, there are syntactic differences that make it impossible. Then I toyed with the idea of a preprocessor-like tool that would let me have 2.x lines and 3.x lines together in one file, but it seemed like more trouble than it was worth.

In the end I went back to the standard 2to3 tool. I had thought that this tool was meant to be a starting point for creating a 3.x codebase, and I started using it that way. But a recommendation I read somewhere suggested using it not once at the start of the project, but as a build step to create your 3.x kit from your 2.x sources. This is what I ended up doing.

2to3 is impressive: it runs over your source files, changing code to work under 3.x. It doesn’t always do the best thing, but I never saw it do the wrong thing.

My process is to copy my whole source tree into a “three” directory, then run 2to3, then run the unit tests. After fixing a problem, repeat the process. This seems like an odd way to run with an interpreted language, but works really well, and lets me keep one code base. It doesn’t run as-is on 2.x and 3.x, but it’s one set of files that produces code that runs on both.

The bulk of the changes I had to make were in the tests rather than in the code itself.’s tests consists of many small snippets of code, often in strings, so 2to3 wasn’t able to fix it all up for me. In these snippets, I had often used print statements where any statement would do, so I ended up converting a lot of these to assignments. Where I really did want printing I added parentheses to make them compatible between 2.x and 3.x.

Here are some other differences I had to accommodate:

  • There’s no setuptools on 3.x. The one feature I really used from it was its auto-generated coverage script, so I wrote a simple script and conditionalized Unfortunately, it means I can’t use “coverage” as a command name under 3.x, but have to run it as “python /blah/Python3.1/Scripts/coverage etc”.
  • 3.x no longer has os.popen4, so I wrote a helper function to run commands, with different implementations for 2.x and 3.x.
  • filter() is no longer available, but easy to replace.
  • exec is no longer a statement, so those tests had to be conditionalized by version.
  • Variables in comprehensions are local to the comprehension in 3.x, whereas they are available outside the comprehension in 2.x. This ended up making a difference because of the way coverage tracing doesn’t start until the next call, and in 3.x, the new scope for comprehensions means it is traced as a call.
  • Bytes vs. strings: this took a few go-rounds to get right, and wasn’t helped by the fact that the 3.x docs say write() takes a string. It doesn’t: what it expects depends on how the file was opened. In binary mode, write() expects a bytes argument, in text mode, it expects a string. This makes perfect sense, and is part of the new logical goodness in 3.x, but I learned about it the hard way. I dealt with it by moving around some encode() and decode() calls, and still might have it a little wrong, but it works, so I don’t think so.
  • Comparison special functions: __cmp__() is gone in 3.x. This is too bad, since now I have to implement the comparison as __lt__() and __eq__(). But once I do that, the 2.x code doesn’t work properly, since it wants all six functions defined. So where I used to have __cmp__(), now I have __lt__(), __le__(), __eq__(), __ne__(), __gt__(), and __ge__(). Is there a simpler way to define custom comparisons that work on both 2.x and 3.x?
  • The module containing the built-in functions has changed. In 2.x, it’s __builtin__, in 3.x, it’s builtins.
  • An obscure difference: at one point in my tests I was appending to the PYTHONPATH environment variable, and doing it repeatedly, adding the same entries over and over again. In Python 2.x, this worked fine. In 3.x, once the variable got longer than some limit, it was ignored, and my tests failed. I hadn’t meant to append repeatedly like that, so I fixed the tests not to, but I don’t know why 3.x minded when 2.x didn’t.

After all of these changes, now I have code that passes all its unit tests on 3.x. I still haven’t tackled packaging kits for 3.x, but that’s next.



To simplify defining custom comparison, you could create a Mixin that requires you to define only two of his methods, like __eq__() and __gt__(). I wouldn't be completely happy having to use it, though.


A little late to this post, but:

> Is there a simpler way to define custom comparisons that work on both
> 2.x and 3.x?

Of course *class* decorators are only in Python 3, so you could:
- use the other (heavier) recipe linked to there (which supports Python 2 and 3, I think)
- roll your own quick mixin
- or just bake in the extra method boilerplate, e.g.


Cool. Your blooged popped up in a kit search for Python.


They did realize this problem and add functools.total_ordering, but that came a bit late (2.7).

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