I wrote recently about why I didn't use the attrs module recently; the short version is that it would have forced my co-workers to learn about it in order to work on my code. Talking about this brings up a potentially awkward issue, namely Python 3. Just like the attrs module, working with Python 3 code involves learning some new things and dealing with some additional concerns. In light of this, is using Python 3 in code for work something that's justified?

This issue is relevant to me because I actually have Python 3 code these days. For one program, I had a concrete and useful reason to use Python 3 and doing so has probably had real benefits for our handling of incoming email. But for other code I've simply written it in Python 3 because I'm still kind of enthused about it and everyone (still) does say it's the right thing to do. And there's no chance that we'll be able to forget about Python 2, since almost all of our existing Python code uses Python 2 and isn't going to change.

However, my tentative view is that using Python 3 is a very different situation than the attrs module. To put it one way, it's quite possible to work with Python 3 without noticing. At a superficial level and for straightforward code, about the only difference between Python 3 and Python 2 is print("foo") versus 'print "foo". Although I've said nasty things about Python 3's automatic string conversions in the past, they do have the useful property that things basically just work in a properly formed UTF-8 environment, and most of the time that's what we have for sysadmin tools.

(Yes, this isn't robust against nasty input, and some tools are exposed to that. But many of our tools only process configuration files that we've created ourselves, which means that any problems are our own fault.)

Given that you can do a great deal of work on an existing piece of Python code without caring whether it's Python 2 or Python 3, the cost of using Python 3 instead of Python 2 is much lower than, for example, the cost of using the attrs module. Code that uses attrs is basically magic if you don't know attrs; code in Python 3 is just a tiny bit odd looking and it may blow up somewhat mysteriously if you do one of two innocent-seeming things.

(The two things are adding a print statement and using tabs in the indentation of a new or changed line. In theory the latter might not happen; in practice, most Python 3 code will be indented with spaces.)

In situations where using Python 3 allows some clear benefit, such as using a better version of an existing module, I think using Python 3 is pretty easily defensible; the cost is very likely to be low and there is a real gain. In situations where I've just used Python 3 because I thought it was neat and it's the future, well, at least the costs are very low (and I can argue that this code is ready for a hypothetical future where Python 2 isn't supported any more and we want to migrate away from it).

Sidebar: Sometimes the same code works in both Pythons

I wrote my latest Python code as a Python 3 program from the start. Somewhat to my surprise, it runs unmodified under Python 2.7.12 even though I made no attempt to make it do so. Some of this is simply luck, because it turns out that I was only ever invoking print() with a single argument. In Python 2, print("fred") is seen as 'print ("fred")', which is just 'print "fred"', which works fine. Had I tried to print() multiple arguments, things would have exploded.

(I have only single-argument print()s because I habitually format my output with % if I'm printing out multiple things. There are times when I'll deviate from this, but it's not common.)