Install and set a specific Python version for a directory

On a Mac (M1 / arm64), to install an older version of Python (say 2.7), follow these steps:

1. Install the required Python version using pyenv:
   pyenv install 2.7.18
   That should install Python-2.7.18 to ~/.pyenv/versions/2.7.18
   Check other versions of Python installed:
   ls ~/.pyenv/versions

2. To run that version of Python:
   ~/.pyenv/versions/2.7.18/bin/python
   Output: Python 2.7.18 (default, Jul 14 2024, 15:15:48) [GCC Apple LLVM 15.0.0 (clang-1500.3.9.4)] on Darwin

3. Check which version of Python pyenv is using:
   pyenv version
   Output: 3.10.12 (set by /Users/username/.pyenv/version)

4. Set up a particular version for a directory (say 2.7), change to that directory, and run:
   pyenv local 2.7.18
   This creates a .python-version file in the current directory. Whenever you change to this directory or its subdirectories, pyenv will automatically switch to Python 2.7.18 for that session.
   The .python-version simply has the Python version:
   cat .python-version
   Output: 2.7.18

5. Check the Python version in that directory:
   pyenv version
   Output: 2.7.18 (set by /Users/username/somedirectory/.python-version)

6. Try to change to the parent directory and check the Python version again:
   cd ..
pyenv version
   Output: 3.10.12 (set by /Users/username/.pyenv/version)

7. To explicitly run Python 2.7.18 that is set by pyenv in the local directory, use pyenv’s exec command:
   pyenv exec python
   Output: Python 2.7.18 (default, Jul 14 2024, 15:15:48) ...

Install a virtual environment in a directory using a specific Python version

It's good to install an app's dependencies in a local/virtual environment instead of polluting the global Python environment on your system. virtualenv available globally on the system might be set to use another version of Python than what was set using pyenv for a specific directory. To set up a virtual environment in a directory, first install virtualenv in the pyenv's required Python version. Then create the virtual environment using that virtualenv. Follow these steps:

1. Use pip to install virtualenv using the Python 2.7 interpreter provided by pyenv:
   ~/.pyenv/versions/2.7.18/bin/pip install virtualenv
   This command ensures virtualenv is installed using Python 2.7.18.

2. Now create a virtual environment (say 'myenv') using the Python 2.7 interpreter:
   ~/.pyenv/versions/2.7.18/bin/virtualenv myenv

3. Activate 'myenv' and verify the Python version:
   source myenv/bin/activate
python --version
   Output: Python 2.7.18

4. Now go ahead and install the app's dependencies. If it has a 'requirements.txt':
   pip install -r requirements.txt