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Installing Python 3.6 on MacOS Mojave for QGIS

I'm trying to install QGIS 3.4.1 on Mac OS Mojave. QGIS install comes as a pkg file, specifically requires python 3.6 (will not work with 3.7), Mac ships with 2.7, and homebrew installs python3 with 3.7.1.
To the Python devs out there - what's the best way of installing Python 3.6 and then QGIS here? Pyenv, venv, pipenv, virtualenv? How would QGIS always find python 3.6 - during install and later when I run it?
What I want to accomplish is run QGIS on python 3.6 and not have python 3.6 change either default Mac 2.7 python, or homebrew python3.

As stated in QGIS documentation:
The current QGIS package uses the python.org Python 3.6, at least version 3.6.5, the “macosx10.9” build - other distributions are not supported.
So you can install Python 3.6 downloaded from python.org website. Please follow these simple steps:
Prerequisites
Please verify the Xcode developer tools are installed:
xcode-select -p
should return
/Library/Developer/CommandLineTools
if not, please run this command
xcode-select --install
Python installation
Go to https://www.python.org/downloads/mac-osx/ and download the Python 3.6 macOS 64 bit installer (currently 3.6.7). This is a pkg installer you can run like many other software. If you don't want to replace the Python 3.7 you installed from brew, ensure this Python distribution is NOT added to the system $PATH. By default, it will install under /Library/Frameworks/Python.framework/Versions/3.6.
QGIS install
Now you have the correct Python installed, download QGIS package.
Install the pkg in the right order
Run QGIS and open "Preferences". Go to System > Environment.
Check "Use custom variables" and click green "plus" sign. Under "Apply", select "Prepend", set PATH as variable and paste
/Library/Frameworks/Python.framework/Versions/3.6/bin:
as Value. This will help QGIS locating the correct python interpreter. Click OK, restart QGIS and you should be done.

Use updated version of Python on Linux

I have Python 3.4 installed on my Linux computer.
sudo apt-get install python3.4
However, when I run python -V, it shows that Python 2.7.6 is being used.
How do I tell the system to use the updated version of Python?
The answer to this question for a windows computer is at How to update version of Python?, but I couldn't find an answer for Linux.

On Linux, installations of python3 installed by the package manager (e.g. apt) can be called as python3. You might need to specify the version - e.g. python3.5 if the package manager has installed more than one, or you've compiled your own installations from source.

you can specify the version in the shebang.
write #!/usr/bin/env python3.
when run via ./my_script.py it will run in python3.
otherwise run it via python3 my_script.py.
if you just want to start an interactive python shell start it with python3
you also can be more specific with the version. just replace python3 with python3.4 (if installed)

In ubuntu various python executable are places under /usr/bin/ and might look like
/usr/bin/python
/usr/bin/python3.2
/usr/bin/python3.4
etc. so when you execute a command python -v it looks for a file with that name in that location. so to choose your version specify it like python3.4 -v
you can replace the simlink /usr/lib/python with /usr/bin/python3.4 to make that "default"

Django is installing to the wrong python version

I had python 2.6 and I downloaded Django. I found that python 2.6 throws errors when trying to run django, so I downloaded python 2.7. Now, typing python in the terminal runs 2.7 but the django library isn't in the 2.7 folder. So I uninstalled django using:
sudo pip uninstall django
and that worked just fine. When i used the command:
sudo pip install django
it installed into the python 2.6 instead of python 2.7.
How can I install django into python 2.7 instead of python 2.6?
(I am running a MacBook Pro on 1.6, and I was told to not uninstall the base version of python because so many of the systems use 2.6)

You need to install pip for python2.7. If it's installed, you should be able to see it using which pip-2.7.
It's better not to touch system python. Use homebrew to install your own.

The problem is that you are running pip from your default Python installation (2.6), read this: How to run multiple python version on Windows, maybe answers give you how to solve in your OS X.
You can view the version of your default Python installation by executing python -V, there is a way to specify which version to use when you execute python, in Linux you can create an alias (alias python=python2.7) in your $HOME/.bash_profile, OS X must have something similar, then install pip using your preferred version.
BTW, It's recommended to use virtualenv

How can I uninstall python 2.6.5 and install the newest version(2.7.3) in ubuntu 10.04

I want to uninstall python 2.6.5, and install python 2.7.3
but there is so many other software relay on it.
so how can I up upgrade python in ubuntu10.04?

I used this link to do the same(i.e the installation part), after doing this the default python version will change to 2.7.3.
Another useful link How do I install python 2.7.2 on 10.04?.

Why do you want to uninstall the old one? You can install python 2.7 and just create a virtualenv that points to it: Use different Python version with virtualenv
Aside from that, it's rather risky to upgrade it, if you don't want to upgrade the whole system.

switch versions of python

Story:
One of the app that i have works on python 2.4 and other on 2.6. I tried to do a sym link of python2.4 to python and things started to break loose on ubuntu jaunty.
Now i am downloading every dependency of 2.4 and installing it using python2.4 setup.py install. The dependencies seem to be endless.
Question1: How will i tell any framework that go and use version so and so pf python like day django to use 2.6 and say mjango to use 2.4? Something like we say use database databasename kinda syntax.
Question2: Is there more elegant way to switch between version as my hack of symlinking was a virtual disaster?
Question3: Can I download a deb for say hardy and make jaunty believe its for her?

Use Virtualenv.
There is more information here: Working with virtualenv.
Using virtualenv you can create a new virtual python environment with whatever version of Python you want for each project or application. You can then activate the appropriate environment when you need it.
To expand on my answer:
You can install multiple versions of Python on your computer (I have 2.4, 2.5, 2.6 and 3.1 on my machine - I install each from source). I use a Mac, and keep my system Python as whatever OS X sets as the default.
I use easy_install to install packages. On ubuntu you can get easy_install like this:
sudo apt-get install python-setuptools
To install virtualenv then do:
easy_install virtualenv
I tend to create a new virtualenv for each project I'm working on and don't give it access to the global site-packages. This keeps all the packages tight together and allows me to have the specific versions of everything I need.
virtualenv -p python2.6 --no-site-packages ~/env/NEW_DJANGO_PROJECT
And then whenever I am doing anything related to this project I activate it:
source ~/env/NEW_DJANGO_PROJECT/bin/activate
If I run python now it uses this new python. If I use easy_install it installs things into my new virtual environment.
So, virtualenv should be able to solve all of your problems.

Pythonbrew is a magical tool. Which can also be called as Python version manager similar to that of RVM-Ruby version manager but Pythonbrew is inspired by Perlbrew.
Pythonbrew is a program to automate the building and installation of Python in the users $HOME.
Dependencies – curl
Before Installing the Pythonbrew, Install “curl” in the machine, to install curl use the below command in the terminal, give the the password for the user when prompted.
$sudo apt-get install curl
After Installing the curl, Now Install Pythonbrew, copy and paste the following commands in the terminal and type the password for the user when prompted.
Recomended method of installation - Easy Install
$ sudo easy_install pythonbrew
To complete the installation, type the following command
$pythonbrew_install
Alternate method of installation:
Use curl command to download the latest version of pythonbrew from github.
curl -kLO http://github.com/utahta/pythonbrew/raw/master/pythonbrew-install
After downloading, change “pythonbrew-install” to “executable”
chmod +x pythonbrew-install
Then, run the pythonbrew-install in the terminal
./pythonbrew-install
Now the Pythonbrew has been installed in the “Home Directory” i.e., /home/user/.pythonbrew
Next, copy and paste the following line to the end of ~/.bashrc
*NOTE: change “user” to your user name in the system
source /home/user/.pythonbrew/etc/bashrc
Thats it! Close the terminal.
Steps to Install different versions of Python:
Open a new terminal, type the following command or copy and paste it.
$pythonbrew install 2.6.6
This will install Python 2.6.6 and to install Python 2.7 or Python 3.2, change the version number in the previous command.
$pythonbrew install 2.7
or
$pythonbrew install 3.2
Update: If you get error while Installing then Install using the below command.
$pythonbrew install --force 2.7
or
$pythonbrew install --force 3.2
How to manage different versions of Python installed in system
For instance, if Python 2.6.6, Python 2.7 and Python 3.2 is installed in your system, switching between the versions can be done as follows:
By default, Python 2.6.6 will be active and in order to switch to Python 2.7 use the below command
$pythonbrew switch 2.7
The default Python is changed to Python 2.7.
Now, to switch to Python 3.2 change the version number in the previous command.
$pythonbrew switch 3.2
Use the below command to check or list the installed Python versions
$pythonbrew list
Use the below command to check or list the available Python Versions to install
$pythonbrew list -k
To uninstall any of the installed Python version (for example to uninstall Python 2.7), use the below command.
$pythonbrew uninstall 2.7
Use the below command to update the Pythonbrew
$pythonbrew update
Use the below command to disable the Pythonbrew and to activate the default version
$pythonbrew off
Enjoy the experience of installing multiple versions of Python in single Linux / ubuntu machine!

I find http://github.com/utahta/pythonbrew much easier to install and use than any other solution.
Just install it and you'll have these options:
pythonbrew install 2.7.2
pythonbrew use 2.7.2 # use 2.7.2 for a current terminal session
pythonbrew switch 2.7.2 # use 2.7.2 by default system wide
pythonbrew uninstall 2.7.2
Note: if you're using a Linux-based operating system with preinstalled Python, switching (system wide) to another version can make things go wrong, so be careful.

A more grassroot approach than Virtualenv is the side-by-side installation of two Python versions.
If there is an existing installation, and you want a second installation into the same root path (e.g. /usr/local), use this target when making install:
make altinstall
When your second installation is Python 2.6, this will leave you with a /usr/local/bin/python2.6 alongside the old /usr/local/bin/python.
A simple way to switch between these two versions is using a shell alias (alias python=/usr/local/bin/python2.6) on the shell where you invoke the interpreter. But this won't work across sub-shells and she-bang invocations.

pyenv is yet another Python manager. The README.md at that link has a good set of instructions, but they basically are:
$ cd
$ git clone git://github.com/yyuu/pyenv.git .pyenv
Then set up your $PATH.
$ echo 'export PYENV_ROOT="$HOME/.pyenv"' >> ~/.bash_profile
$ echo 'export PATH="$PYENV_ROOT/bin:$PATH"' >> ~/.bash_profile
$ echo 'eval "$(pyenv init -)"' >> ~/.bash_profile
Install the desired versions of Python:
$ pyenv install 2.7.8
After installing you need to run this:
$ pyenv rehash
Then switch to the version of Python you want to run, for the shell:
$ pyenv shell 2.7.8

"Question1: How will i tell any framework that go and use version so and so pf python like day django to use 2.6 and say mjango to use 2.4?"
You simply run them with the specific python version they need. Run mjango with /usr/bin/python2.4 and django with /usr/bin/python2.6. As easy as that.
"Question2: Is there more elegant way to switch between version as my hack of symlinking was a virtual disaster?"
Yes, see above. Have two separate installs of Python, and run explicitly with the different versions.
"Question3: Can I download a deb for say hardy and make jaunty believe its for her?"
That generally works. If it doesn't, it's because it has dependencies that exist in Hardy, and does not exist in Jaunty, and then you can't.
And here is a Question 4 you didn't ask, but should have. ;)
"Is there an easier way to download all those Python modules?"
Yes, there is. Install setuptools, and use easy_install. It will not help you with library dependecies for those Python modules that have C code and need to be compiled. But it will help with all others. easy_install will download and install all the Python dependencies of the module in question in one go. That makes it a lot quicker to install Python modules.

Move to the project directory :
Create an environment :
virtualenv -p python2.7 --no-site-packages ~/env/twoseven
Then activate your source :
source ~/env/twoseven/bin/activate