How to Downgrade My Version of Python from 3.7.5 to 3.6.5 on Ubuntu

How do I downgrade my version of python from 3.7.5 to 3.6.5 on ubuntu

The following talks about upgrade from 3.6.7 to 3.7.0 but you can use the same process for downgrade. You should not change the system python unless you really know what you're doing

First Install Pyenv

Installlation Instructions are here

Look at Pyenv Options

$ pyenv 
pyenv 1.2.14
Usage: pyenv <command> [<args>]

Some useful pyenv commands are:
   commands    List all available pyenv commands
   activate    Activate virtual environment
   commands    List all available pyenv commands
   deactivate   Deactivate virtual environment
   doctor      Verify pyenv installation and deevlopment tools to build pythons.
   exec        Run an executable with the selected Python version
   global      Set or show the global Python version
   help        Display help for a command
   hooks       List hook scripts for a given pyenv command
   init        Configure the shell environment for pyenv
   install     Install a Python version using python-build
   local       Set or show the local application-specific Python version
   prefix      Display prefix for a Python version
   rehash      Rehash pyenv shims (run this after installing executables)
   root        Display the root directory where versions and shims are kept
   shell       Set or show the shell-specific Python version
   shims       List existing pyenv shims
   uninstall   Uninstall a specific Python version
   --version   Display the version of pyenv
   version     Show the current Python version and its origin
   version-file   Detect the file that sets the current pyenv version
   version-name   Show the current Python version
   version-origin   Explain how the current Python version is set
   versions    List all Python versions available to pyenv
   virtualenv   Create a Python virtualenv using the pyenv-virtualenv plugin
   virtualenv-delete   Uninstall a specific Python virtualenv
   virtualenv-init   Configure the shell environment for pyenv-virtualenv
   virtualenv-prefix   Display real_prefix for a Python virtualenv version
   virtualenvs   List all Python virtualenvs found in `$PYENV_ROOT/versions/*'.
   whence      List all Python versions that contain the given executable
   which       Display the full path to an executable

See `pyenv help <command>' for information on a specific command.
For full documentation, see: https://github.com/pyenv/pyenv#readme

Look at Python Versions

$ pyenv versions
  system
 * 3.6.7 (set by /home/taarimalta/.pyenv/version)

Install a new Python

$ pyenv install 3.7.0
Installing Python-3.7.0...
WARNING: The Python bz2 extension was not compiled. Missing the bzip2 lib?
WARNING: The Python readline extension was not compiled. Missing the GNU readline lib?
WARNING: The Python sqlite3 extension was not compiled. Missing the SQLite3 lib?
Installed Python-3.7.0 to /home/taarimalta/.pyenv/versions/3.7.0

If you run into an issue with _ctypes install libffi-dev library

Now look at the versions

$ pyenv versions
  system
* 3.6.7 (set by /home/taarimalta/.pyenv/version)
  3.7.0

Select 3.7.0 for local environment

$ pyenv local 3.7.0

See that the version changed

$ pyenv versions
  system
  3.6.7
 * 3.7.0 (set by /home/taarimalta/.python-version)

$ python
Python 3.7.0 (default, Jan  1 2020, 10:52:57) 
[GCC 9.2.1 20191008] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Switch to a different folder

cd ../project2
pyenv versions
  system
* 3.6.7 (set by /home/taarimalta/.pyenv/version)
  3.7.0

The python version may be different here depending on which python version you have set locally

Set pyenv version globally

This globally sets a python version for a user

pyenv global 3.7.0

Note that pyenv sets local version by adding a .python-version file

$ pyenv local 3.7.0
$ cat .python-version
3.7.0

Note that pyenv knows the global version by looking at the ~/.pyenv/version file

cat ~/.pyenv/version
3.8.2

conda install downgrade python version

If you want to set specific version, use it like this:

WARNING: This command will overwrite the default python version system-wise

conda install python=3.6

To create environment with a specific version, you can do:

conda create -n $PYTHON36_ENV_NAME python=3.6 anaconda  # set custom env name

The anaconda at the end allows the env to use all anaconda packages

For more information refere to Anaconda documentation

Change python version to 3.x

Interestingly, poetry is silently failing due to a missing package the tool itself relies on and continues to install a broken venv. Here's how you fix it.

sudo apt install python3-venv
poetry env remove python3
poetry install

I had to remove pytest, and then reinstall with poetry add pytest.

EDIT: I ran into this issue again when upgrading a project from python3.7 to python3.8 - for this instead of installing python3-venv, you'd want to install python3.8-venv instead

Unable to set default python version to python3 in ubuntu

EDIT:

I wrote this when I was young and naive, update-alternatives is the better way to do this. See @Pardhu's answer.

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Outdated answer:

Open your .bashrc file nano ~/.bashrc. Type alias python=python3
on to a new line at the top of the file then save the file with ctrl+o
and close the file with ctrl+x. Then, back at your command line type
source ~/.bashrc. Now your alias should be permanent.

How to downgrade python version without affecting other files

pip uninstall python 3.8.4

This command will never work. pip is a python package manager, i.e. for an installation of python, pip is responsible to install modules to that python installation. It does not manage the installation of python itself

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To your error with the tensorflow installation:

pip install tensorflow

and

ERROR: Could not install packages due to an EnvironmentError: [WinError 5] Access is denied: 'c:\\programdata\\anaconda3\\lib\\site-packages\\wrapt-1.11.2.dist-info\\INSTALLER'

This is caused by the fact that the c:\\programdata is write protected from non-admin users, so in order for your command to succeed, you should run your cmd as admin.

BUT:

It seems like you have installed anaconda and are trying to install tensorflow into the base environment using pip. Pip in turn tries to uninstall some already existing package, probably to fulfill the tensorflow requirements. But the already existing package are probably installed by conda when you first installed anaconda, so you ware getting into dangerous waters here and you are risking to break your base environment, so two suggestions:

1. Create a new environment conda create -n tf tensorflow-gpu
2. Use conda commands to install tensorflow. conda install tensorflow-gpu will give you tensorflow

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To your other question:

How to downgrade python

The solution is also to use a conda command:

conda install python=<some version>

or, even better, simply create an environment with the version you need:

conda create -n py37 python=3.7

would for example give you an environment called py37 with python3.7 installed. It can be activated and used with

conda activate py37

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