Recently there have been some discussions about Python 3 adoption. Adoption rates aren't great. There are a number of theories on why Python 3 is not adopted more and what should be done about it. I'll expand a bit on the analysis by Ian Bicking and add my own. I'll offer a few suggestions on what might help.

The Stick Some people blame Python 2.7. The core developers were way too nice to us Python developers by maintaining Python 2.x. If they had dropped support for Python 2.x years ago, Python 3 adoption would've been much faster. I think this argument is based on counterfactuals that are very hard to back up. I wish people would stop making these assertions, as it's impossible to prove either way. Even if this argument could be shown to be true, I for one am very glad that the core developers didn't throw the existing community into the deep end. Somehow punishing those holdouts on Python 2 so they'll move I'll call the "stick" approach. As Ian points out, this supposes some kind of moral authority for Python 3, or at least the progress of the Python language as a whole. People who are using Python 2 and who don't port code are holding the language back. We should have some sort of allegiance to Python, and why aren't we getting off our behinds and do the work? It's not that hard! You're holding back The Python! This kind of moral argument may work on those who are active members of the Python community, but it won't work those who are using Python as a tool for a job. I'll get back to them, as they're important.

The Default Some people blame the Linux distributions. People new to Python apparently don't do the research, but instead type "python" only to discover Python 2 and then stick to it. Once Linux distros upgrade their Python, people will start using Python 3. I can't believe that this is a major force holding everything back. Most Python beginners will check out python.org. Python 3 is quite widely available - it's in my Fedora installation and I'm two releases behind. Won't a beginner at least check out python.org? On Windows, installing Python 3 is no problem. A more realistic problem is that of arcane system administration practices and obsolete Linux distributions making it harder to get Python 3 installed. I still don't think this is in any way a dominant factor.

The Carrot Other people blame Python 3. The message is that Python 3 was immature early on, which is why people didn't adopt it, but that has changed. It's mature now, and people should adopt it now. If only Python 3 is made attractive enough, people will adopt it. I think there is some truth to this. But the problem is that we've had about five years of people claiming that Python 3 is the superior version of Python, and it's still not commonly used. There are other forces at work that I want to highlight here.

Dependencies A common theory is that people aren't moving to Python 3 because their dependencies haven't moved to Python 3 yet. What's a dependency? If you write code that depends on, say, the Reg library, and the Reg library only exists for Python 2, then you have to wait for the developer of the Reg library (me) to port to Python 3 first before you can port my own code. Or you have to dive in and port it yourself, hopefully somehow coordinating that with the original author. This is the situation that holds you back from Python 3. I think this theory has a lot of merit. The problem is widely understood, which is why we see a lot of people urging us to port. What seems to be widely neglected is the problem of the reverse dependencies.

Reverse Dependencies I am sure that dependencies are an important factor, but what seems to be ignored in these discussions is the issue of reverse dependencies. Reverse dependencies have a gravity all their own. What's a reverse dependency? If you are developing a cool new library, and you know that this library needs to be integrated into an existing Python 2 codebase, you need to make sure that this library works in Python 2.

Python 2, Python 3 or Polyglot? There's a common response to this situation. You are writing a Python library, and you need it to work in a Python 2 context. Writing the library in Python 3 is now out of the question. You can't enjoy the cleaner feel of Python 3. You can't use any of the cool new Python 3 features. If you feel some moral obligation to The Python, or at least want to be cool and modern, you might consider writing polyglot code: code that works in Python 2 and Python 3. You gain for this some coolness points, potential users who use Python 3, and a harder to maintain codebase. You gain none of the benefits of Python 3. You can also just write Python 2 code. Now what is the better programming language to work with? Python 3, of course! But you can't use it. If you have to make the choice between Python 2 and Polyglot, Python 2 is the better programming language. So if you don't care about The Python, in this context you are better off writing Python 2.

The Gravity Okay, forget about all those legacy codebases! If we write all our new code at least in Polyglot, we can start more new projects in Python 3. I think that this does a disservice to those old projects. They were happy to upgrade to new Python 2.x versions, as that was doable with little effort. They might've picked Python before Python 3 came along and created this upgrade bump, or at least before Python 3 was mature (whatever point in time we pick where Python 3 is considered mature). But that's a moral argument. We have no allegiance to other people's codebases. But those existing Python 2 codebases are the ones with business value to people. Those are the codebases with the bosses and customers who want new functionality, not a port to a new version of Python that involves an effort with no near-future benefits and the potential to introduce bugs. Those are the codebases with the money. Those codebases have gravity. That's the gravity of Python 2.

Circular Dependencies So now we have dependencies holding us back, and reverse dependencies holding us back. Those create a circular dependency structure. We know that those are the worst kind to deal with from a maintenance perspective! Our code has dependencies. More dependencies are available for Python 2 than Python 3. Those dependencies that are polyglot are more likely to be battle tested in a Python 2 context than Python 3. Motivation not to move to Python 3. The gravity caused by reverse dependencies written in Python 2 then pulls at people so that the situation remains that way. You will write your new library in Python 2 or Polyglot, because you have to integrate with Python 2 codebases. That is going to make more Python 2 dependencies available, not less.

Passing the porting buck down the Python 2 gravity well The gravity field of Python 2 only pulls lightly on the core developers, who are high above us in the sky working on The Python. So after an upgrade path (2to3) was figured out, the porting buck was passed to the library developers. The library developers are lower down in the gravity field, so 2to3 didn't work out so well for them. So they figured out how to write polyglot, and to the credit of the core developers, they made modifications in both Python 2 and Python 3 to make it easier to write polyglot. That works okay enough for many, so more and more polyglot libraries are appearing. Now the porting buck is passing down the gravity field again, to those mired in existing Python 2 codebases. The force of bosses and customers and money is heavy. Porting is harder than for the library authors in all that weight, not helped by the fact that most applications have much less test coverage than libraries. What is going to happen? Are they going to port their codebases? Are they going to be able to do so in that gravity?

Escape Velocity? How do we get out of this situation? One option is for Python 3 to reach escape velocity. Python 3 the language becomes attractive enough, along with enough polyglot libraries being ported, that new projects will be written in Python 3. As we leave the gravity well old Python 2 codebases become just a pale blue dot in the cosmos. A Python 2 community is left behind. I don't think that Python 3 reaching escape velocity is exactly friendly to those dealing with Python 2 every day for no fault of their own, a "bye bye we don't care about you anymore". Another argument against escape velocity is that this is not healthy for our community; it would tear it apart. We're feeling some of the strains today. Another argument against escape velocity is that this might cut off significant resources from the Python community. The high gravity Python 2 area that we're trying to leave behind has all the money. Is this wise? So should we wish for escape velocity in the first place? Is it desirable?

Escape Velocity cannot be reached? It's also possible that the Python 2 gravity field is too strong, circular dependencies will keep holding us back, and that escape velocity cannot be reached. This means we would be in this Python 2 versus Python 3 situation forever. That would suck. Above I listed some arguments against going for escape velocity in the first place. What if we take those seriously?