Creating your first venv

Creating an environment is quite easy. The basic command comes down to pyvenv PATH_TO_THE_NEW_VIRTUAL_ENVIRONMENT, so let's give it a try. Note that this command works on Linux, Unix, and Mac; the Windows command will follow shortly:

# pyvenv test_venv
# . ./test_venv/bin/activate
(test_venv) #

This creates an environment called test_venv, and the second line activates the environment.

On Windows, everything is slightly different but similar overall. By default, the pyvenv command won't be in your PATH, so running the command is slightly different. The three options are as follows:

For convenience, I would recommend that you add the Scripts directory to your PATH anyhow, since many other applications/scripts (such as pip) will be installed there as well.

Here is the full example for Windows:

C:\envs>python -m venv test_venv
C:\envs>test_venv\Scripts\activate.bat
(test_venv) C:\envs>

venv arguments

So far, we have just created a plain and regular venv, but there are a few, really useful flags for customizing your venv specifically to your needs.

First, let's look at the venv help:

The most important argument to note is --system-site-packages, which enables the global site packages within the environment. This means that if you have a package installed in your global Python version, it will be available within your environment as well. However, if you try to update it to a different version, it will be installed locally. Whenever possible, I would recommend disabling the --system-site-packages flag because it gives you a simple environment without too many variables. A simple update of the system packages could break your virtual environment otherwise, but worse, there is no way to know which packages are needed locally and which ones are just installed for other purposes.

To enable this for an existing environment, you can simply run the environment creation command again, but this time adding the --system-site-packages flag to enable the global site packages.

To disable it again, you can simply run the environment creation command without the flag. This will keep the locally (within the environment) installed packages available but will remove the global packages from your Python scope.

The --symlinks and --copies arguments can generally be ignored, but it is important to know the difference. These arguments decide whether the files will be copied from the base python directory or whether they will be symlinked.

By default, venv will try to symlink the files, and if that fails, it will fall back to copying. Since Windows Vista and Python 3.2, this is also supported on Windows, so unless you're using a very old system, you will most likely be using symlinks in your environment. The benefit of symlinks is that it saves disk space and stays in sync with your Python installation. The downside is that if your system's Python version undergoes an upgrade, it can break the packages installed within your environment, but that can easily be fixed by reinstalling the packages using pip.

Finally, the --upgrade argument is useful if your system Python version has been upgraded in-place. The most common use case for this argument is for repairing broken environments after upgrading the system Python with a copied (as opposed to symlinked) environment.

Differences between virtualenv and venv

Since the venv module is essentially a simpler version of virtualenv, they are mostly the same, but some things are different. Also, since virtualenv is a package that is distributed separately from Python, it does have some advantages.

The following are the advantages of venv over virtualenv:

Advantages of virtualenv over venv:

In short, if venv is enough for you, use it. If you are using an old Python version or want some extra convenience, such as virtualenvwrapper, use virtualenv instead. Both projects essentially do the same thing, and efforts have been made to easily switch between them. The biggest and most significant difference between the two is the wide variety of Python versions that virtualenv supports.