The following commands will remove your make altinstall-ed python:

rm -f /usr/local/bin/python2.7
rm -f /usr/local/bin/pip2.7
rm -f /usr/local/bin/pydoc
rm -rf /usr/local/include/python2.7
rm -f /usr/local/lib/libpython2.7.a
rm -rf /usr/local/lib/python2.7

You might also have to do

rm -f /usr/local/share/man/python2.7.1
rm -rf /usr/local/lib/pkgconfig
rm -f /usr/local/bin/idle
rm -f /usr/local/bin/easy_install-2.7

Although make altinstall has served me well if the "system python" has a different major.minor number from the one you install, it doesn't work that well if only the micro number (the third position) differs. That number is excluded from the installed binary, and you end up with two versions pythonX.Y. This was always a problem but once distributions started shipping with system utilities based on 2.7.X this problem has been more severe as 2.7 is supposed to be the last of the Python2 series.

IMO the best approach to solve this problem is to prevent it from becoming one: configure python to install in a directory not used by any other python. On my system they go under /opt/python/X.Y.Z.

To use any of the Pythons installed there you use [virualenv][1] to make a new environment:

virtualenv --python=/opt/python/2.7.9/bin/python2.7 venv
source venv/bin/activate

or use [virtualenvwrapper][2]. I have some aliases for the latest versions in the series I work with.

If you are using tox for testing against multiple versions (you should) the following alias will help it find the various version:

alias tox='PATH=/opt/python/2.7.9/bin:/opt/python/2.6.9/bin:/opt/python/3.4.3/bin:/opt/python/3.3.6/bin:/opt/python/3.5-dev/bin:$PATH tox'

(these are currently the latest versions, I use a slightly different setup by maintaining links from /opt/python/2.7 to the latest /opt/python/2.7.9, and for the other minor numbers as well, within the process for downloading, building and installing a new python version)

These installs are never used directly. They are always used as the basis for virtualenv environments only, hence I don't care that they are not in my normal PATH.

in case you still need it.

first of all, it's never advisable to uninstall python. check for instance this link.
having said this, Ubuntu 16.04 does not depend (wasn't build) on Python 3.9 (check this out, for instance). thus, and in principle you shouldn't be having problems fully uninstalling it... as it was my case.

i had no troubles following these steps; which for you/me would be something like:

sudo apt-get clean
sudo apt-get autoremove --purge
sudo apt-get remove python3.9
sudo apt-get autoremove

How I do:

Copy# Remove python2
sudo apt purge -y python2.7-minimal

# You already have Python3 but 
# don't care about the version 
sudo ln -s /usr/bin/python3 /usr/bin/python

# Same for pip
sudo apt install -y python3-pip
sudo ln -s /usr/bin/pip3 /usr/bin/pip

# Confirm the new version of Python: 3
python --version

Warning: This will break Ubuntu 18.04 and 18.10. These instructions apply to the specific situation described in the question, on Ubuntu 16.04.

Warning: I have tested this solution, but not very thoroughly. Make a backup, and proceed with caution.

1. Remove the repo:

   sudo add-apt-repository --remove ppa:fkrull/deadsnakes
   

2. Refresh apt cache:

   sudo apt-get update
   

3. Remove the package:

   sudo apt-get remove --purge python3.6
   

My python was corrupted due to some module. So I planned to re-installed or remove the python completely from my Ubuntu 16.04 machine. But sudo apt-get install --reinstall python2.7 command was also failing and was throwing same error. So I finally Did few hacks and cracks. Here are the steps -

Copy - sudo rm -rf /usr/bin/python2.x as well as python3.x
 - sudo rm -rf /usr/lib/python2.x as well as python3.x
 - sudo rm -rf /usr/local/lib/python2.x as well as python 3.x  

Copy - sudo apt-get update

In Between if you get this error The package needs to be reinstalled ubuntu Then Run following command

Copysudo vi /var/lib/dpkg/status 

And delete all the lines from above file for the package which was expecting re-install and run sudo apt-get update again.

Now download a python tgz file from https://www.python.org/downloads/ and unzip it and CD into it

Copy./configure
make test
sudo make install

Python should be installed now. Check by running python

Don't uninstall python3 in ubuntu as it is depended on many python packages if you delete that terminal, firefox browser,idle , will also delete and at last it will show you authentication error