python, legacy, flexx  -  2016-02-12

Write Python 3, while supporting Python 2.7

In this post I discuss an approach for writing code in Python 3, and still support Python 2.7. I've recently used this approach in one of my own projects. Most projects should get away with only minor modifications and an automatic translation step during package-build. However, there are some pitfalls (bytes/str) that might need special attention.

I've always been a proponent of Python 3, and I have been bashing Python 2 (a.k.a. legacy Python) at many opportunities. To be fair, Python 3 was announced just a few months after I started using Python, so I did not have much existing code that was holding me back. In fact I started writing Python 3 wherever the dependencies allowed it, and indeed the Pyzo IDE was written in Python 3 (almost) from the start. For most other projects, though, I have written code that runs on both Python 3 and Python 2.

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Supporting both Python 3 and Python 2

Initially, most projects started supporting Python 3 by making use of the 2to3 tool, which translates the code (to make it compatible with Python 3) during the installation. In other words, developers were still writing in Python 2, but put some effort to support "early adopters" of Python 3.

An approach that is increasingly popular is to write code that runs on both Python 2 and 3 without the need for a translation step. Tools to help with this are (amongst others) and python-future.

I have been following this approach for several years. It's not even that hard if you know what to watch out for. However, I am increasingly annoyed by this approach, and I feel a great sense of satisfaction when I write code for Python 3 only, when I can just write super() and don't have to worry about isinstance(x, basestring), etc.

Therefore, I think we should move to an approach of writing Python 3, and support legacy Python (i.e. Python 2.7) where needed by making use of an automated translation step at build time. It does not seem like there are currently many projects that follow this approach, but my guess is that we will be seeing this more, because Python 2 is going to be depreciated by 2020.

Why support Python 2

Python 3 is obviously the future, and as a community we should try to get rid of Python 2. By supporting legacy Python, we are holding back progres, because we can not always make use of certain features (e.g. super() or the matrix multiplication operator).

Making new (and useful) packages only available in Python 3 can be an effective incentive for users to transition to Python 3, and thereby speeding up the transition process of the scientific Python community.

At the same time, however, not supporting legacy Python can hold back the adoption of a project, and there may be social reasons, e.g. an important cliet still relies on Python 2. I don't think I would care working on supporting legacy Python in unpaid time though...

My experience

I have been writing the Flexx project in Python 3 from the start. The project contains a few quite interesting pieces, such as PyScript, a Python to JavaScript transpiler. We have been adopting this transpiler in the Bokeh project to allow users to write client-side callbacks in Python. The initial approach (that I've been advocating) was to only support this functionality on Python 3, and look at Python 2 in case there was demand. As Bryan Van de Ven anticipated, one of the first user questions were about when this functionality would be available in Python 2.

We agreed to make Flexx available in Python 2.7, but I did not want to sprinkle the code with references to basestring or snippets from, so I set out to look at the approach to translate the code at build time.

Some googling did not reveal many projects following this approach (yet), but I did find a library called lib3to2, which more or less promised to do what I needed. I was not quite happy with some of the translations though (especially the handling of bytes and str), and I found it quite slow. While the lib3to2 library supports Python 2.6 and even 2.5, I am only interested in 2.7, which means some things could be done much simpler.

The translate_to_legacy module

I decided to write my own translater module, that uses a tokenizer instead of a full AST parser. This means higher speed and much simpler code, for a less detailed description of the code, but it proved enough to do all the translations that I wanted to do.

I decided to only target Python 2.7, which means that much less translations are necessary. For instance, you can just keep b'xx', and there is a bytes class as an alias for str.

A brief overview of the applied translations:

With these translations, and only minor tweaks to the code, things were looking good.

The loose ends

There are however, a few situations that need special attention. Most notably places that use isinstance(x, bytes). I used this in one spot to determine whether a filename or actual data was provided. But on legacy Python a str is the same as bytes, so it would work incorrectly. In this case I wrote a little function to check whether the given input looked like a filename or not (e.g. no zero bytes and no newlines). It's not 100% bulletproof, but good enough. (On Python3 it obviously still just checks whether the input is bytes.)

Another situation that needs special attention is the creation of metaclasses, e.g. type('MyName', bases, {}). On Python 2, the name cannot be unicode. Together with the 'unicode_literals', this causes problems. It's easily resolved, but it takes a few extra lines of code.

Likewise, setting environment variables must be done with str objects, and not unicode in Python 2.

Using this approach in your projects

To adopt this approach in another project you need to:

You might need to make a few small modification to make the translations work correctly, or resolve tricky situations like isinstance(x, bytes).

Note that the code is still a single-source distribution, so if your code is pure Python, you can e.g. still build universal wheels, or noarch conda packages.


If you want to write in Python 3, but still support legacy Python, using a translation step at build time seems like a viable solution. If you limit legacy support to Python 2.7, the number of required translations is certainly feasible, though you may have to make small adjustments to your code, e.g. to make any import translations work appropriately. Further, be aware where bytes are used explicitly.

Other than that, you can just write in Python 3! Well, mostly anyway, some features like function annotations, async and matrix multiplication cannot be translated. But no worries, by 2020 we can forget about Python 2 once and for all!