Virtual environments setup

venv is a low level environment in python.

The point of an environment is isolation. Each one gets its own site-packages directory, so two projects on the same machine can depend on different versions of the same library without fighting each other, and nothing you install ends up in the system Python that your OS depends on.

$ pip install virtualenv  # low level virtual environment, less prefereable to use
$ pip3 install virtualenvwrapper  # high level environment with abstraction, more preferred
$ python<version> -m venv <virtual-environment-name>

To activate and deactivate environment:
source env/bin/activate
and
~ deactivate

Creating and activating with venv

venv ships with Python, so there is nothing to install first. Create the environment inside the project folder and keep the name consistent across projects:

python3 -m venv .venv

Activation depends on the shell you are in:

source .venv/bin/activate     # Linux / macOS, bash or zsh
.venv\Scripts\Activate.ps1    # Windows PowerShell
.venv\Scripts\activate.bat    # Windows cmd.exe

Once it is active your prompt is prefixed with the environment name, and which python (where python on Windows) points inside .venv. That is the quickest way to confirm you are not installing into the system interpreter by mistake.

On Windows PowerShell the activation script often fails the first time with:

.venv\Scripts\Activate.ps1 cannot be loaded because running scripts is disabled on this system.

That is PowerShell's execution policy, not a Python problem. Allow local scripts for your own account and run the activation again:

Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser

Do not commit the environment

Add the environment directory to .gitignore:

.venv/

The directory holds installed packages plus scripts with absolute paths baked into them, so it is not portable between machines — moving or renaming the folder is enough to break activation. What you commit is requirements.txt. Anyone cloning the repo rebuilds the environment from that, which is both smaller and reproducible. If an environment gets into a bad state, delete the folder and create a new one; nothing of value is lost.

Virtualenvwrapper is a set of extensions built on top of virtualenv. It provides a more convenient and
streamlined way to manage your virtual environments.

$ pip3 install virtualenvwrapper
export WORKON_HOME=~/.virtualenvs
export VIRTUALENVWRAPPER_PYTHON=/Library/Frameworks/Python.framework/Versions/3.6/bin/python3
source /usr/local/bin/virtualenvwrapper.sh

mkvirtualenv first-env
mkvirtualenv second-env
workon first-env
workon second-env
deactivate

Find dependencies of a Project:

• pip list

• pip freeze

How to find a current shell is bash or zsh

• $ echo $0
• $ echo $SHELL

echo $0 is more accurate and if you type bash in zsh shell

To change prompt to include a new line
PS1="$PS1\n"

Uninstall existing versions

pip uninstall werkzeug Flask-Login

Reinstall all dependencies

pip freeze > requirements.txt

pip install -r requirements.txt

pip freeze prints every installed package with an exact version, which is why it is the one you redirect into requirements.txt. pip list is for reading — its table layout is not valid input for pip install -r.

Regenerate the file whenever you add a dependency, and do it with the environment activated. Freezing from a deactivated shell captures whatever the system interpreter happens to have installed, which is almost never what you want.

Useful Links:

• how-to-setup-virtual-environments