Document how to install kernel for python 2 and 3

[jhamrick]

This is a pretty frequently asked question -- how to install the python 2 kernel and python 3 kernel simultaneously? The answer is usually "in a virtualenv" but it would be nice to have this documented somewhere with more specific instructions that we can point people to.

[takluyver]

Virtualenvs shouldn't be necessary - I install both with --user.

I think the important thing to convey is the significance of ipython kernelspec install.

[jhamrick]

Won't the ipython command get overwritten, though? So if you install it with python 2 first and then python 3, you'll end up with ipython == ipython3, but if you do it the other way around, you'll get ipython == ipython2? I think that can be unexpected and confusing, which is why I usually install one in a virtualenv.

[takluyver]

That's not really related to the kernel, though - we don't use the ipython command to launch the kernel.

The command will get overwritten, but I don't think that's a big problem - you'll have ipython2 and ipython3 available in any case.

[jhamrick]

Yeah, I wasn't totally sure whether to report this here or in the main ipython repo. You don't use the ipython command to launch the kernel, but you do use it to install the kernelspec, so I think it's still relevant, at the very least to tell people specifically to use ipython2 or ipython3 instead of ipython.

I've personally been confused about the ipython command being overwritten, and have had other people be confused by it as well. I think it's kind of cumbersome to have to warn people "make sure you install whichever version of ipython you actually want second, because otherwise it will get overwritten". I would imagine that in most cases the following scenario applies:

1. Person already has ipython installed under e.g. python 3
2. Person wants to install the python 2 kernel as well
3. We tell them, "install ipython with python 2 and then run ipython kernelspec install"
4. They get the ipython2 kernel and are happy
5. At some point in the future they want to use ipython from the command line and expect it to be python 3 (since that's what they had before), and are confused when it is not.
6. They need to reinstall ipython with python 3 so that it is the default ipython and are unhappy that things are complicated.

[jhamrick]

Also, to clarify, I'm not saying we should try to address the problem of the command getting overwritten, I'm just advocating that they install it in a virtualenv (or at least mention they might want to do that) so they don't encounter the confusion above.

[minrk]

Yeah, to that effect, I would probably even recommend people do /path/to/python -m IPython kernelspec install-self so that it's really clear which Python is being used. Alternately, it could be:

ipython2 kernelspec install-self
ipython3 kernelspec install-self

[takluyver]

We have this doc now. It doesn't specifically talk about Python 2 and 3, but:

if you want to use a kernel with a different version of Python, or in a virtualenv or conda environment

Do we think that's clear enough?

[jhamrick]

I don't think it's quite clear enough, but I have some suggestions on how to make it clearer so I'll make some edits and submit a PR.

[minrk]

Closed by ipython/ipython#9155