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I don't have python but I have python3

I recently formatted my laptop and installed the Ubuntu operating system. I did not explicitly install Python. To check if it is pre-installed, I ran python --version in the terminal and got this:
$ python --version
Command 'python' not found, but can be installed with:
sudo apt install python3
sudo apt install python
sudo apt install python-minimal
You also have python3 installed, you can run 'python3' instead.
Then I checked python3 --version and got this: Python 3.6.9.
How is this possible? Should I install python as well? Or can I continue to use python3? Would this cause any problems?

Recent Ubuntu versions do not install python 2 by default, as python 2 is now "dead". However, the command python is not (yet) linked to python 3, to avoid some confusion; traditionally, python was for python 2 while python3 was for python 3.
In case you want to change this behavior and use python to run python 3, try installing a package named 'python-is-python3'. Then you can run python --version and see that it points to python 3.
In theory it might cause confusion for some legacy programs, but I haven't seen any issue so far for about a year.
In 20.04 LTS, the python included in the base system is Python 3.8. Python 2.7 has been moved to universe and is not included by default in any new installs.
Remaining packages in Ubuntu which require Python 2.7 have been updated to use /usr/bin/python2 as their interpreter, and /usr/bin/python is not present by default on any new installs. On systems upgraded from previous releases, /usr/bin/python will continue to point to python2 for compatibility. Users who require /usr/bin/python for compatibility on newly-installed systems are encouraged to install the python-is-python3 package, for a /usr/bin/python pointing to python3 instead.
— https://wiki.ubuntu.com/FocalFossa/ReleaseNotes

Just python on Linux traditionally refers to Python 2. python3 is the correct command if you want to use Python 3 (which you should, as Python 2 is EOL).

TL;DR: Just use python3
Python on Ubuntu refers to Python v2. If you were to run
python --version
you would see an output in the format of
Python 2.x.y
Python 2 has been sunset, which means it no longer will be updated and the recommendation is to move to Python3.
To run a program with Python 3, you just need to use python3 command instead.
python3 ./app.py

Use python 3.8 instead of 3.6 on elementary OS (or Ubuntu)

I recently started using elementary OS. It's based on Ubuntu.
During installation it installs python 3.6.
I installed python 3.8 manually by installing the following packages: python3.8, python3.8-dev, python3.8-minimal, python3.8-venv.
I also updated the link to the python binary with:
sudo ln -sf /usr/bin/python3.8 /usr/bin/python3
After this a couple of things stopped working.
For example when I tried to execute a non-existing command it didn't print the error message that it cannot found the command, but it displayed a python stack trace. That one I solved with:
cd /usr/lib/python3/dist-packages
sudo ln -s apt_pkg.cpython-36m-x86_64-linux-gnu.so apt_pkg.so
sudo ln -s apt_inst.cpython-36m-x86_64-linux-gnu.so apt_inst.so
As you can see the python error was because it couldn't find the compiled binaries for the apt module.
So this issue was solved, but there are a couple of similar ones, and none of them can be solved this way, as the module binary is not compatible with python 3.8.
Is it possible to remove python 3.6 completely and override it with 3.8 so that the module binaries also get updated? Or can python 3.8 and 3.6 coexist? I would be fine with the /usr/bin/python3 link pointing to python 3.6 and I would manually execute /usr/bin/python3.8 or create a different alias or link for it. However when I print out the sys.path with /usr/bin/python3.8 I get this:
/usr/lib/python38.zip
/usr/lib/python3.8
/usr/lib/python3.8/lib-dynload
/home/{username}/.local/lib/python3.8/site-packages
/usr/local/lib/python3.8/dist-packages
/usr/lib/python3/dist-packages
The trouble is with the last one. That's where the modules are with the 3.6 compatible so files. Can I somehow force python 3.8 to completely ignore the last module search path (without always stating sys.path.remove in my scripts) ?

I recommend you undo your symlink overwriting your systemwide version of Python 3. As you have found, replacing the python3 executable might create some problems, as it is used under the hood.
In general, you should leave your system python[3] installation alone for this reason and it is common to use virtual environments. This can be done as follows (assuming you are in some project directory):
python3.8 -m venv venv
source venv/bin/activate
The first command will create a virtual environment ('venv') in the directory venv. The second command will 'activate': now python (and in this case, python3 and python3.8) all refer to your original python3.8 in the context of this shell. You will have to repeat this if you launch a new shell.
This will also allow you to install packages using pip without cluttering your system installation. The use of virtual environments and pip is an incredibly common workflow in the Python development world.
In terms of shell and 'global' Python management, you can also use pyenv to manage your Python versions and what is available in the shell. pyenv is quite nice if you want to run a particular version of python, say 3.8.0 but not 3.8.1.

Install Python of specific version system-wide with pyenv

I am researching possibility to upgrade to Python 3.6 in our project.
Right now we are using Python 3.5.2 from ppa:fkrull/deadsnakes on Ubuntu 14.04. The PPA doesn't have Python 3.6 yet and it's not clear when it will be available.
I don't want to install yet another PPA.
And I am trying to find a more general approach.
I found people suggesting to use pyenv which compiles Python from source, which sounds interesting, because I can upgrade Python any time without waiting until repo maintainer adds it. Also I can easily install other Python flavors like PyPy.
I am not ready to use pyenv as virtual environment yes, so I am wondering if it's possible to use it to compile and install Python globally so that I can just use it.

The documentation is a little confusing because there is no python-build binary added in PATH after installation.
python-build is a pyenv plugin (installed by default). Documentation and more info is here: https://github.com/pyenv/pyenv/tree/master/plugins/python-build.
How to install system-wide Python for all users: 1) Login as root and 2) install required Python version to /usr/local/python-X.Y.Z.
sudo ~/.pyenv/plugins/python-build/bin/python-build 3.6.1 /usr/local/python-3.6.1/
Now you can use this Python version as a normal user, for example you can create virtualenv for your project:
/usr/local/python-3.6.1/bin/python -m venv /var/www/my-app/.env/

https://github.com/yyuu/pyenv/wiki/Common-build-problems#installing-a-system-wide-python
Installing a system-wide Python
If you want to install a Python interpreter that's available to all
users and system scripts (no pyenv), use /usr/local/ as the install
path. For example:
sudo python-build 3.3.2 /usr/local/

I've contributed a package for python3.6 in deadsnakes for trusty / xenial :)
https://launchpad.net/~fkrull/+archive/ubuntu/deadsnakes/+packages?field.name_filter=python3.6&field.status_filter=published&field.series_filter=

By combining the hints from the other answers and reading through the documentation, I found a nice way to do exactly what you want that should work well in a CI system or in a Docker container or on a developer machine if they haven't already installed python3.x via Apt or Yum or Homebrew.
Assuming you have all the dependencies required to build your desired version of Python 3.x (anything above 3.4 requires some extra packages the pyenv-installer doesn't always warn you about), you can run the commands below to get a new system wide Python that should be executable by all users, which makes it easy to pass to virtualenv creations with python3.6 -m venv yourvenv.
curl https://pyenv.run | bash # or
wget -O - https://pyenv.run | bash
export PATH="$HOME/.pyenv/bin:$PATH"
$(pyenv which python-build) 3.6.10 /usr/local/
which python3.6
python3.6 --version
# If you get an error running the above commands, it probably means
# /usr/local/bin isn't in your PATH yet
# on Debian/Ubuntu and maybe others the /etc/environment or
# /etc/login.defs file puts it in the path when a user logs in
echo $PATH
export PATH="/usr/local/bin:$PATH"
python3.6 --version

Installing Python 2.x and python 3.x on the same computer

There are python 2.7 and python 3.2 on my computer. The default version is 2.7 because using python -V gives 2.7 as the version.
But when I use apt-get install numpy, scipy,pip why it install them into the python3.2 folder. After that I used pip to install the module into the 3.2 folder.
I also installed Theano this way but in the end it showed a message saying that there is no module named Theano installed although it is in the python 3.2 folder.

You haven't told on which OS you're running this, but it look likes a debian base linux, maybe ubuntu?
If so, I'd try with:
sudo apt-get install python3-numpy or
sudo apt-get install python2-numpy.
This would also work with python-pip2 and python-pip3.
After this, you could effectively use "pip2" or "pip3" to install your packages without having to go through the OS "prebuild" modules (but the os version of the packages are usually my prefered way to install them, if the exists in the repo)

Depending on what you're doing with python, it's often a good idea to run in a virtual environment, this lets you have several different versions of python with several different sets of installed packages on the same system. . .
See http://docs.python-guide.org/en/latest/dev/virtualenvs/ for the details.

You can also use Anaconda for maintaining two versions of Python. Anaconda consists of various libraries so you don't have to install them and after switching it to the different version of Python you can easily install them :
Download Anaconda for both Python versions
Open .bashrc
Add the path to new Anaconda you have installed for, e.g.:
export PATH="/home/paras/anaconda3/bin:$PATH"
Now there will be 2 export paths: one for Python 2 and one for Python 3. Comment the one which you don't want.

First install different versions of python or whichever python version you would like to use
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.6 10
sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 20
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.3 30
sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.4 40
Now create virtualenv like this and give the path of python version you want to use inside the virtualenv.
virtualenv -p /usr/bin/python3.6 <foldername>
virtualenv -p /usr/bin/python3.4 <foldername>

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