Updated the 'Installing Astropy in develop mode' section ... - #1212

I've just noticed the " Error — The Travis CI build could not complete due to an error" message, but I'm not sure why ... I ran "python setup.py build_sphinx" to check that the docs built correctly and they did :( How can I fix this?

@adrn tells me that the Travis build failure looks unrelated to this pull request ...

Thanks for your work on this - I'm restarting the Travis build.

Aside from the two comments above, this looks good to me - thanks!

I think @eteq and @iguananaut will also want to review this.

@ehogan - just for info, once you make new commits to your branch, you can just push the changes to the same remote branch and the commits will appear here.

@astrofrog - ah, ok, I wasn't sure how that worked, thanks for the info! You mentioned two comments but I can only see one (giving the full command) ... am I missing something?

@ehogan - ah you are right, there was only one comment - ignore me!

IIRC, the reason we didn't encourage usage of develop in the past is because it doesn't work with Python 3 (since 2to3 needs to be run). It appears this is still the case (but thankfully it doesn't overwrite the files). Do we want to encourage this if it doesn't work everywhere? At the very least, we need a prominent comment that this doesn't work with Python 3.

@mdboom - I think the instructions should be there with a caveat that it doesn't work on Python 3, and we can always say it's not the main recommended method.

I still want to strongly encourage it--it greatly cuts down on iteration time. I would almost refuse to do development without it. I don't think most of our contributors (@astrofrog aside) are working regularly in Python 3, though I agree there should be a warning that it won't work there (although I might be able to fix that, it's probably not worth the effort).

In #1202 there seems to be a grown consensus toward creating a unified codebase sooner rather than later, at which point this would become a non-issue.

I would say that it is the main recommended method for developers. Outside of Python 3 it makes development vastly easier.

I guess this is a good motivation to try and phase out the use of 2to3 ;)

Agreed--I think that it's great you're doing most of your development in Python 3 and I wouldn't want to make Python 3 users second-class citizens.

A little OT, but do you think it would be feasible to add some hook into setup_packages.py to disable running 2to3 for individual sub-packages that are python 2 and 3 source compatible? io.ascii is pretty much there already, and I'm hoping that time and table wouldn't be too much trouble.

@taldcroft - good idea - let's discuss that in #1219.

I can add a warning to say it doesn't work in Python 3 ... is there something that does work in Python 3 that I could mention instead?

@ehogan - I think in that case one just has to use the build_ext which is already mentioned.

@iguananaut - since this is pretty much ready apart from the comment about Python 3, how about merging this and then rebasing #1126 to add the comment about develop?

Yeah, I'll do that.