One of the most frustrating source of errors with Python is a result of environment mismanagement. So let’s review some pain points, and how to correctly set up Python on macOS, and use virtual environments.


Mixing Python version

One could be about mixing python2 and python3 environment which leads to weird error:

For example on exception can exist in Python3:

# Python 3.9.4 
>>> FileNotFoundError
<class 'FileNotFoundError'>

However, yield an error in Python2:

# Python 2.7.16 
>>> FileNotFoundError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'FileNotFoundError' is not defined

Since python2 is getting deprecated it should occur less.

Mixing python environment

Even within the same python distribution it can happen, when you multiply the virtual environment: When you install a module but get:

Traceback (most recent call last):
File "./", line 3, in <module>
  import matplotlib.pyplot as plt
ImportError: No module named matplotlib.pyplot

In fact, you may have installed it in your global python environment instead of the current virtual one. In that case, it’s always helpful to run:

which python

To see who you’re calling, because python3 could not be the same as python or python2.
Same as when you install a package:

python -m pip install <package>
pip3 install <package>

Those two may not be the same depending on your environment.

Managing your python environments 🌍

Python Installation (macOS)

I would recommend a python version Manager like pyenv that will allow us to manage multiple versions of python on Mac. It works great, and you’ll have a definite knowledge of which python you are actually using.

Here is the installation process, with python 3.9.4:

brew install pyenv
pyenv install 3.9.4
pyenv global 3.9.4

echo -e 'eval "$(pyenv init -)"' >> ~/.zshrc

To see the versions installed you can use, and see with the * which version is installed:

$ pyenv versions
* 3.9.4 (set by /Users/sylhare/.pyenv/version)

You can set a new version using the pyenv global <version> if you have it already installed.

Remember that if you have installed a package on 3.7.3, you will need to reinstall it when you switch to 3.9.4 to have access to it.

Typing pyenv will list the help, and you can better see for yourself all you can do with it.

Set up the Virtual Environment

Virtual environment are great, because you can install weird package or more experimental packages that require you to change some internal mechanisms. You can then just discard it without messing your whole system.

To create a python virtual environment you can use venv which is a core component of Python3:

python -m venv venv
source venv/bin/activate

This way you create a virtual environment in a folder named venv, then you can activate that environment. You can make sure it’s activated by your terminal having (venv) prefixed:

(venv) which python

To stop the virtual environment just type deactivate, and your shell will turn back to normal.