Table of Contents

Motivation

• want to switch major version of Python (i.e. between Python 2 and Python 3).
• want to switch minor version of Python (i.e. between Python3.A and Python3.B).
• want to isolate some packages for every project.
• don’t want to construct in vain a new docker container for each Python project.

How to switch any version of Python with pyenv

Installation

1. Confirm the default Python version.

   $ python --version
   Python 2.7.16

2. Install pyenv by brew on macOS.

   $ brew install pyenv

3. Define the environment variable and add pyenv init to your shell.

   export PYENV_ROOT="${HOME}/.pyenv"
   export PATH=${PYENV_ROOT}/bin:$PATH
   eval "$(pyenv init -)"

4. See installable Python versions on pyenv.

   $ pyenv install -l
   Available versions:
   2.1.3
   2.2.3
   2.3.7
   :
   :

5. Install a specific Python version.

   $ pyenv install 3.9.0
   :
   Installed Python-3.9.0 to /Users/xxxx/.pyenv/versions/3.9.0

6. See the current Python version and installed versions.

   $ pyenv versions
   \* system (set by /Users/xxxx/.pyenv/version)  <=== current Python version.
     3.9.0

Switch the global Python version

Switch to the specific version from the current version as a global configuration.

$ pyenv global 3.9.0

$ pyenv versions
    system
\* 3.9.0 (set by /Users/xxxx/.pyenv/version)

$ python --version
Python 3.9.0

$ pip -V
pip 20.2.3 from /Users/xxxx/.pyenv/versions/3.9.0/lib/python3.9/site-packages/pip (python 3.9)

Switch the local Python version

Apply specific Python version to a specific directory.

$ mkdir sandbox; cd sandbox

$ pyenv versions 
\* system
    3.9.0 (set by /Users/xxxx/.pyenv/version)

$ python --version
Python 2.7.16

$ pyenv local 3.9.0

$ cat .python-version
3.9.0

$ pyenv versions
    system
\* 3.9.0 (set by /Users/xxxx/sandbox/.python-version)

$ python --version
Python 3.9.0

Isolate some packages with venv

1. Switch to a specific Python version with pyenv.

2. See installed packages.

   $ pip list
   Package    Version
   ---------- -------
   pip        20.3.3
   setuptools 49.2.1

3. Create a virtual environment.

   $ python -m venv <target directory>
   
   $ ls <target directory>
   bin     include     lib     pyvenv.cfg

4. Activate the virtual environment.

   $ source <target directory>/bin/activate
   
   (target directory name) $

5. Install a package and confirm that it has been installed in the virtual environment.

   $ pip install selenium
   :
   Successfully installed selenium...
   
   $ pip freeze
   selenium==3.141.0
   urllib3==1.26.2

6. Deactivate the virtual environment.

   (target directory name) $ deactivate
   
   $

7. Some packages have been isolated between the original environment and the virtual environment.

   $ pip freeze
   $

What’s the difference between pip freeze and pip list ?

• pip list shows the list of package names and versions in the environment and pip freeze shows them but leaves out some internal packages like a setuptools.
• pip freeze > requirements.txt outputs the list of packages names vs versions to requirements.txt and they can be installed in bulk by pip install -r requirements.txt.