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How to downgrade ubuntu to python 3.6?

I'm trying to install an older version of tensorflow and it needs python3.6 to support the whl file of installation.
I'm now running Ubuntu 20.04 with python 3.8.5, I've already done this :
sudo add-apt-repository ppa:deadsnakes/ppa
followed by :
sudo apt-get update
sudo apt-get install python3.6
By doing that it installs python 3.6 but when I see the version of python installed it's still 3.8.5. Should I do something to remove python3.8.5 ? maybe apt-get purge ?
P.S: I'm installing the wheel file through pip3 should I downgrade it too ?

You shouldn't hack the system installation of Python I'd say.
Instead you should use something that let's you manage multiple versions of it, something like pyenv.
It is a well-known and widely accepted utility and according to its readme:
pyenv lets you easily switch between multiple versions of Python. It's simple, unobtrusive, and follows the UNIX tradition of single-purpose tools that do one thing well.
You can find the installation instructions here: https://github.com/pyenv/pyenv#basic-github-checkout.
Once you installed it you can install your preferred Python distribution which will live besides your system one and all the others you want later.
E.g. if you want to install 3.6.7 and then use it globally you can do:
pyenv install 3.6.7
pyenv global 3.6.7
Then test it like:
python -V
And that's only to scratch the surface, you can do many more things with it, check out the documentation for more.

after the commands you've run, you should have a python3.6 binary installed on the path
that said, I'd recommend using virtualenvs instead of system installations
for deadsnakes, you can install the venv module by (sudo) apt install python3.6-venv (debian decided to split venv into a separate module, so the deadsnakes packaging follows that)
from there you can create and activate a virtualenv:
python3.6 -m venv venv # create the environment
. venv/bin/activate # activate the environment
pip install ... # install things to your isolated environment
inside this virtualenv the python command will refer to your own isolated python installation
disclaimer: I'm the maintainer of deadsnakes

How to install python-distutils for old python versions

I am running Ubuntu 20.04 with python 3.6, 3.7, and 3.8 installed.
I am trying to install some packages using pip on 3.6 and 3.7 versions using 'python3.7 -m pip install package' but, I am getting this error:
ModuleNotFoundError: No module named 'distutils.util
I already have python3-distutils and python3-distutils-extra installed but pip only works for python 3.8.
How can I make pip work for installing packages on python 3.6 and 3.7?

What about things like deadsnakes or pyenv, would those help? Otherwise you might need to add the apt repositories for older Ubuntu versions (say 18.04), but I am not sure what the side effects might be. This is a question that I would rather ask on Ask Ubuntu or Super User.

(from bounty description)
I want a definitive solution that does NOT involve someone telling me to install:
python-distutils
python3-distutils
python3-distutils-extra
I will be handing out FREE DOWNVOTES to anyone who mentions installing these.
Well, I have some bad news for you. There is no "official" way to get distutils on the Debian-provided python* packages without installing these packages, not unless you go outside of Debian's default package repositories.
This is a consequence of Debian's decision to break up various parts of Python's standard library into separate packages, on the basis of their policy to split things into runtime packages and development packages. They consider distutils to fall in the "development" part, and thus, don't distribute it as part of the standard python package.
Basically, your options are:
install a python*-distutils package.
This is what your OS maintainers recommend doing in your situation.
You've not explained why you'll "be handing out free downvotes" for anyone recommending this, but this is genuinely the "most correct" solution.
install Python, from somewhere that does not break up the standard library.
This would mean using a package source other than the official debian repositories.
The easiest source would be the deadsnakes ppa.
compile your own Python!
This isn't actually that difficult, and tools like pyenv make it easy to compile and manage multiple python versions.
hack around the problem!
You could... download the sources from CPython's repo, and place the Lib/distutils folder somewhere on the import path for pythonX.Y? This is a 100% hack though and I strongly recommend not doing this. If something bad happens, because you did this, I'm not responsible.

I've been looking for an answer to this question for almost as long as you, and finally found a solution.
Be warned, I may have screwed up my python3.8 installation as a side-effect, and it's possible the package will be uninstalled again and break things as soon as you run apt-update.
Also, it may be possible to reduce the number of commands, but here's what I ended up doing:
Run apt list to see a list of available packages:
$ apt list -a python3-distutils
Listing... Done
python3-distutils/focal-updates,focal-updates,focal-security,focal-security,now 3.8.5-1~20.04.1 all [installed]
python3-distutils/focal,focal 3.8.2-1ubuntu1 all
python3-distutils/bionic-updates,bionic-updates 3.6.9-1~18.04 all
python3-distutils/bionic,bionic 3.6.5-3 all
Then download the "bionic" deb package,
because it actually contains multiple versions of distutils,
for Pythons 3.6, 3.7 & 3.8:
$ apt download python3-distutils/bionic
Get:1 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 python3-distutils all 3.6.9-1~18.04 [144 kB]
Fetched 144 kB in 0s (661 kB/s)
Finally, install the deb package:
$ sudo dpkg --install python3-distutils_3.6.9-1~18.04_all.deb
dpkg: warning: downgrading python3-distutils from 3.8.5-1~20.04.1 to 3.6.9-1~18.04
(Reading database ... 193375 files and directories currently installed.)
Preparing to unpack python3-distutils_3.6.9-1~18.04_all.deb ...
Unpacking python3-distutils (3.6.9-1~18.04) over (3.8.5-1~20.04.1) ...
Setting up python3-distutils (3.6.9-1~18.04) ...
At this point, I was finally able to use pip with python3.6.
Note-1: The printouts of apt list -a above depend on whether you run an Ubuntu or a Debian distribution, and which apt-repos you have activated in /etc/apt/sources.list*.
Note-2: As seen in the case of python3-distutils/bionic, above, the name of the package does not always coincide with its contents.
To view them, you have to download it and inspect it with dpkg -C <pafkage-file>, or from remote with apt-file list python3-distutils.
Note-3: In case you cannot find the exact distutils version for a specific Python release, you may install an earlier version and symlink to it.
For example in Debian-unstable("sid") and "bullseye" as of January 2020, the python3-distutils package contained in their apt-repos has files just for Python3.9, and the previous apt-repos from "buster" contain files for Python3.7 only.
So if you wish to install distutils for Python3.8, you have to download the "buster" package and execute:
$ sudo dpkg --unpack python3-distutils_3.7.3-1_all.deb
$ sudo rm /usr/lib/python3.8/distutils/distutils/
$ sudo ln -w /usr/lib/python3.{7,8}/distutils/distutils/

The instructions here worked for me: https://www.linuxcapable.com/how-to-install-python-3-7-on-ubuntu-20-04-lts/
Namely this one sudo apt install python3.7-distutils
Was trying to install checkov and getting the ModuleNotFound error.
After the above, it installed just fine following these instructions

Here is what has worked for me on Ubuntu 22.04 for Python 3.6.
pip won't install as distutils is not available.
install python3.6 via apt
check the exact version with python3.6 --version
find a matching tag here: https://github.com/python/cpython.git, i.e. 3.6.9
adapt and call git clone --depth=1 -b v3.6.9 https://github.com/python/cpython.git
export PYTHONPATH=`pwd`/cpython/Lib
curl https://bootstrap.pypa.io/pip/3.6/get-pip.py | python3.6 - --user
python3.6 -m pip list
If the old version is not available, cd into the cpython source directory and use e.g. ./configure --enable-optimizations --prefix=$HOME/.local and make -j4 and make altinstall (set some PREFIX to install to when calling configure, so it won't clobber the system, but note that the install path has to be in your search paths)

How do I replace a Python installed from source with a packaged version?

As Python 3.7 was being prepared I installed Python 3.7.0b3 from source. Now 3.7 is out and I want to use the version packaged for Ubuntu. So I've done
apt-get install python3.7
but
$ which python3.7
/usr/local/bin/python3.7
$ /usr/local/bin/python3.7 --version
Python 3.7.0b3
How does one uninstall a Python installed from source (on Ubuntu)?
Or how can I replace it with the apt repo packaged version?

Since you opened a bounty, I can't vote to close as a duplicate, but this question would seem to provide a possible solution. Quoting from the accepted answer:
You can use checkinstall to remove Python. The idea is:
Install checkinstall
Use
checkinstall to make a deb of your
Python installation
Use dpkg -r to
remove the deb.
checkinstall basically wraps a make install command and creates a Debian .deb package based on what was installed. Then, you can just uninstall that package to reverse make install completely. To be perfectly safe you may want to uninstall the packaged Python 3.7 first and reinstall it afterwards to avoid any conflicts (I wouldn't expect any, though, since the packaged version lives in /usr while your source version lives in /usr/local).
If you don't have your source files around anymore, you can always download them again (https://www.python.org/downloads/release/python-370b3/) and rebuild Python. Specifically, the checkinstall commands would look something like this:
sudo apt install checkinstall
wget https://www.python.org/ftp/python/3.7.0/Python-3.7.0b3.tgz
tar xf Python-3.7.0b3.tgz
cd Python-3.7.0b3
./configure && make
sudo checkinstall -D --fstrans=no make install
sudo dpkg -r Python-3.7.0b3.deb
(-D creates a Debian package, --fstrans=no disables use of a temporary directory for installation).

Since I was also moving to python 3.7, I came across this question and decided to answer it, as well as finish my installation.
This are the two sources, which I used for installing python 3.7 on ubuntu 16.04:
https://askubuntu.com/questions/865554/how-do-i-install-python-3-6-using-apt-get
https://superuser.com/questions/241865/updating-python-on-ubuntu-system
Apparently from the first source the deadsnakes PPA contain Python 3.7 - Link: https://github.com/deadsnakes/python3.7/tree/ubuntu/xenial/Python
So following from my first source, I used the following commands to install Python 3.7:
sudo add-apt-repository ppa:deadsnakes/ppa
sudo apt-get update
sudo apt-get install python3.7
It installed Python 3.7 successfully but my Python 3.5.2 remains default. You can invoke/use Python 3.7 by running the commands:
python3.7 script.py
It works, so I decided to set Python 3.7 as default by removing python 3.5, but I came across my second source, and just decided to stick with using python3.7, when running my script in the terminal.
Apparently it says:
Ubuntu policies extensively for writing end-user software. So basically, a large part of the system is written in Python. To switch to Python 3.7, there needs to be done a piece of work consisting of updating and re-testing all the scripts.
So to say you can't just switch to Python 3.7 and delete the older version.
Also from a comment from my first source it states deleting the older version might break the system. I haven't been around to deleting my older version in case it might break the system, but since you are asking for how to download Python 3.7, I think my first source and the first part of my answer should to the work.
I hope it helps :)

It seems that your Python built from source is under /usr/local, and your PATH variable has /usr/local/bin before /usr/bin, since running python3.7 gets you the one under /usr/local rather than the packaged one which would be /usr/bin/python3.7.
Look at your PATH to verify this.
echo $PATH
(When you run a program in bash, that particular running bash instance will remember the location and not rescan the directories in the PATH for that program again, so it will not notice a new file that has appeared somewhere earlier in the PATH. You can prevent this by running hash -r to reset the cache or by just exiting the shell and launching it again.)
I presume your goal is for python3.7 (or any of the other commands provided by Python) to run the versions from your packaged install in /usr.
Unfortunately the python build process does not provide an uninstall method -- the only automated way to remove just the files installed by a source Python install requires using other tools ahead of time (such as checkinstall).
So you have some choices :
Change your PATH so that /usr/local/bin is after /usr/bin. To do this, edit your ~/.profile file or whatever other script you have configuring your PATH and logout/login. This will also affect any other commands you run that are available in both /usr/local/bin and /usr/bin.
Remove /usr/local and reinstall anything else you want there. If a Python install is the only thing in your /usr/local, or if you can easily reinstall anything else you had there, this might be the way to go.
Painstakingly figure out what files under /usr/local/bin were part of Python and remove them. You might be able use the corresponding files in /usr/bin from your installed python3 packages as a starting point to figure out the similar names for /usr/local/bin.
One-liner to get the list of files in /usr/bin from installed python3* packages:
$ for pkg in $(dpkg -l 'python3*' | grep '^ii' | cut -f 3 -d' '); do dpkg -L $pkg | grep '^/usr/bin/'; done | sort
This should produce a list of files like:
/usr/bin/2to3-3.x
/usr/bin/chardet3
...
(I've tested this one-liner on Debian, I'm not sure if any changes are required for Ubuntu)

How do we separately use, maintain & install libraries for python 2.7 and python 3.5 on the same Ubuntu OS?

I need to work with Both Python 2.7.12 and python 3.5.2 simultaneously on my Ubuntu 16.04.1 LTS. Python 3 came pre-installed so I've no idea where it sits, in terms of path to the directory, while python 2 sits in /usr/local/lib/python2.7/.
I found lots of questions on SO and on askubuntu about how to install but nothing about how to use them separately, installing different libraries, and what should I avoid or be careful of, if I maintain this dual python thing for the long term? For example, I usually run pip install to install a library and I can check that its installed in my python2 directory but how do I install the same package for my python3 without conflicts? Something like: python3 pip install <package> ?? Where is the default python3 installed? And how do I call python3 for paths where python is not part of the command for example: pip freeze, sudo-apt get, etc.?
PS: I've not officially worked with Virtualenv but I've been informed that is usually good for isolating projects within a python language version, rather than isolating two different language versions from each other.
Please let me know.
Thanks

This is absolutely no problem, as Python does that for you. You don't need a virtualenv at all.
If you use Ubuntu packages, make sure you use the python3- versions for Python 3, and the normal python- versions for Python 2.
For example, python3-numpy and python-numpy.
If you use pip to install extra packages, you an either use the pip script with the version number appended: pip2.7 or pip3.5, or, my preferred method, call pip as a module for the respective Python executable:
python2.7 -m pip install <whatever>
and
python3.5 -m pip install <whatever>
Other than that, there should not be any issue: Python stores the packages in completely separate directories, and each Python executable only uses its respective directive.
Do not fiddle around with PYTHONPATH, unless you really know what you're doing. This has the danger of setting your PYTHONPATH to a directory with Python 2.7 modules and then using Python 3.5 to run things.
If you start from scratch, you may need to install pip first.
For the system Python(s), use the relevant package:
sudo apt install python-pip
sudo apt install python3-pip
For your locally installed Python(s), use the built-in bootstrapper module:
pythonx.y -m ensurepip
Note on the OS-installed Python executables:
Python 3.5 lives at /usr/bin/python3.5, Python 2.7 (the OS one) at /usr/bin/python2.7.
You could even use the OS 2.7 one next to your locally installed /usr/local/bin/python2.7 (and confuse yourself when a package can't be found because you used the wrong one).
Or install Python 3.6 next to Python 3.5 (provided you've used make altinstall, so python3 doesn't get overwritten).
This is also why you don't really want to run pip (or even pip2.7) as is: pip2.7 may get you the system one, instead of the one in /usr/local/bin/pip2.7, depending on your PATH.
(The same goes for the python2.7 executable, so if you need to specify the full path /usr/local/bin/python2.7 to run that one (or have an alias), the same holds for pip2.7. If, on the other hand, /usr/local/bin is first on your PATH, you should in principle never run into the same pip and python executables.)

What is the suggested way to install multiple Python interpreters?

With Ubuntu, only certain Python versions are available from the repositories. They're easy to get, but with the rest, it's not so obvious. While I can certainly build them from source, I'm thinking there must be tools to automate the process but I can't find them.

Windows and OS X users can simply use executable installers available on the Python website. This is not true with the other OS options. If your Linux distribution's repositories do not contain all the versions you want, building from source is the recommended way, at least as far as the website is concerned.
Automating this process is possible. pythonbrew is a now-deprecated installation manager that recommends shell script-based pyenv instead. With this, installing a new version is as simple as
pyenv install <version number, e.g. 2.7.3>
Another alternative is the more Python-based (read: cross platform) pythonz, a fork of the original pythonbrew. It is just as easy:
pythonz install <version number, e.g. 2.7.3>

One way of installing multiple versions of Python in Ubuntu is to use Felix Krull's deadsnakes ppa, which includes all the major releases from 2.3 on (not point releases) if they are not already in the Ubuntu repositories. It only supports currently supported Ubuntu versions. There is no guarantee of updates, but it does make getting different versions easy.
To install the necessary repositories:
$ sudo add-apt-repository ppa:fkrull/deadsnakes
$ sudo apt-get update
If you want to install 2.7, it's as easy as:
$ sudo apt-get install python2.7
Note this only works for Ubuntu, not e.g. Debian.

$ sudo add-apt-repository ppa:deadsnakes/ppa
$ sudo apt-get update
To install Python2.7, just enter:
$ sudo apt-get install python2.7