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Installing python 3.4.x on OSX El Capitan

I love "brew" package manager, but sometimes being too progressive breaks things.  I have several python apps that I maintain that get deployed to AWS using Elastic Beanstalk.  AWS eb can deploy with python 2.7 or 3.4.  Any recent 'brew install python3" will get 3.5.1. #annoying
The next annoying thing I find is that OSX now uses /usr/local/bin/python3 as the default location for its binary.  Of course AWS uses /usr/bin/python3 so it breaks. #annoying

Also, pythin 3.5.x handles some libraries differently (particularly flask and blueprints)

AND if you install any libs with virtual environments (venv), site packages will be installed under a python 3.5.1 dir, not 3.4, so when you run your app with 3.4, it won't see your libs and will bomb.

What is the fix?

First, remove Python 3.5.1 using brew:
brew uninstall python3
(test by typing "which python3" at a shell prompt, and python3 should be gone)
Now install python 3.4
You were hoping for a brew formula to do this, weren't you? too bad.  Brew no longer has a python 3.4.x package.  You have to go to the python website and get the release.  As of the time of writing this blog, the link for python 3.4 is https://www.python.org/downloads/release/python-344/
 Download the 64-bit installer and boom, it works.  Still installs the binary to /usr/local/bin/python3 though, different from AWS :( 
Link python3 to /usr/bin/python3
To do this, you need to reboot OSX and turn off "rootless" protection.  Don't worry, after we are done, you can turn this security feature back on.
The proper way to disable the "rootless" mode (aka System Integrity Protection, "SIP") is to boot temporarily into Recover Mode (hold Command+R during boot) and use the csrutil disable command (or csrutil enable to reenable security) from the Terminal. The Terminal is reachable via the menu of the Installer that launches in Recovery Mode.
Once rebooted and rootless is off, simply link your python to the correct place.
ln -s /usr/bin/python3 
Now to fix up your virtual environment, you can remove and manually create symbolic links in your venv/bin dir.  python3 would be pointing to /usr/local/bin/python3, so remove and do a ln -s /usr/local/bin/python3 /usr/bin/python3

After you are done linking things, re-build any of your virtual environments using pip etc., and then you can turn back on "rootless" security.

Now your code will be much more portable as-is, at least to be dropped onto an AWS instance.

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