[jdormit]

> It's not hard to upgrade your code

People keep writing blog posts about how easy it is to migrate your code, and I just can't imagine that these people have any experience maintaining a Python2 codebase outside of toy projects or libraries.

At work I maintain a Python2 codebase that's been under development since 2016. It's huge, relies on now-deprecated Py2-only libraries that we can't realistically switch away from, full of Py2-specific syntax, and reliant on Py2's handling of Unicode strings. We scoped out what it would be like to upgrade it to Py3, and determined that we would basically need to rewrite large parts of it from scratch. This isn't an acceptable trade-off for the business in terms of development time and priorities.

Instead we are slowly breaking out the monolith's functionality into Py3 services, which is much more tightly-scoped (and more palatable for management). But realistically speaking we will be maintaining Py2 code for the foreseeable future, and I can't imagine we are the only company in this situation.

Upgrading a production codebase from Py2 to Py3 isn't easy, or even possible in some cases. And frankly, all these blog posts suggesting that migration is trivial are insulting.

[joshuamorton]

95% of the migration effort is trivial (I say this as someone who has migrated 100k+ loc myself as part of a much larger migration effort).

There's things that are tricky, don't get me wrong: unicode handling can be tricky, though six.ensure_str/bytes usually allows you to defer that problem. Futurize and 2to3 do so much of the work that in my experience the majority of files can be migrated by automation, unicode warts included.

The hard stuff is when you have extension modules or weird metaprogramming stuff (there's also one particular case I had to deal with around code generation). That takes effort to migrate, but it's the long tail.

If you need to rewrite your code to enable the py3 transition, the issue isn't python, it's your coding choices, and you should rewrite your code anyway.

[jdormit]

> If you need to rewrite your code to enable the py3 transition, the issue isn't python, it's your coding choices, and you should rewrite your code anyway.

I... don't even know what to make of this advice. Have you ever worked for an engineering organization with more than 5 members? I can't just go to my boss and say, "I need the next six months to rewrite our whole backend". I'd get laughed out of the room.

Without a clear, business-first reason for an engineering project of that magnitude, it's not ever going to be a priority or a realistic option.

[aprdm]

100k is a toy project compared to how many LoC a company that has been around the block for a decade or more has.

[t-writescode]

If the code base started its development in 2016, why did they choose python 2 over python 3?

[jdormit]

I wasn't at the company at that time, so I don't really know. Sure would have made my life easier if they had...

EDIT: Just checked, looks like the first commit was in 2015. But Py3 was still an option back then :(

[wenc]

> People keep writing blog posts about how easy it is to migrate your code

As someone who has had to migrate from Py2 to Py3, it wasn't easy. Unicode/bytes/strings was the hardest aspect of it -- because Python is dynamic, there were breakages that unit tests couldn't exhaustively test for so things broke in production and we had to scramble to fix.

Other aspects were mostly automatically handled by the 2to3 tool.

[_bxg1]

It seems like the main problem is the network effect from all the libraries. It'd probably be much more feasible to upgrade your code alone than to upgrade your code and also find/build replacements for all of your dependencies.

If so, maybe a more concerted effort needs to be made around getting those upstream projects to upgrade. If some of them are refusing/dragging their feet, maybe the core python team could even get involved to help smooth the transition for everybody.