5. Additional Tools and Scripts

5.1. pyvenv - Creating virtual environments

Creation of virtual environments is done by executing the pyvenv script:

pyvenv /path/to/new/virtual/environment

Running this command creates the target directory (creating any parent directories that don’t exist already) and places a pyvenv.cfg file in it with a home key pointing to the Python installation the command was run from. It also creates a bin (or Scripts on Windows) subdirectory containing a copy of the python binary (or binaries, in the case of Windows). It also creates an (initially empty) lib/pythonX.Y/site-packages subdirectory (on Windows, this is Lib\site-packages).

On Windows, you may have to invoke the pyvenv script as follows, if you don’t have the relevant PATH and PATHEXT settings:

c:\Temp>c:\Python33\python c:\Python33\Tools\Scripts\pyvenv.py myenv

or equivalently:

c:\Temp>c:\Python33\python -m venv myenv

The command, if run with -h, will show the available options:

usage: pyvenv [-h] [--system-site-packages] [--symlinks] [--clear]
              [--upgrade] ENV_DIR [ENV_DIR ...]

Creates virtual Python environments in one or more target directories.

positional arguments:
  ENV_DIR             A directory to create the environment in.

optional arguments:
  -h, --help             show this help message and exit
  --system-site-packages Give access to the global site-packages dir to the
                         virtual environment.
  --symlinks             Try to use symlinks rather than copies, when symlinks
                         are not the default for the platform.
  --clear                Delete the environment directory if it already exists.
                         If not specified and the directory exists, an error is
                         raised.
  --upgrade              Upgrade the environment directory to use this version
                         of Python, assuming Python has been upgraded in-place.

If the target directory already exists an error will be raised, unless the --clear or --upgrade option was provided.

The created pyvenv.cfg file also includes the include-system-site-packages key, set to true if venv is run with the --system-site-packages option, false otherwise.

Multiple paths can be given to pyvenv, in which case an identical virtualenv will be created, according to the given options, at each provided path.

Once a venv has been created, it can be “activated” using a script in the venv’s binary directory. The invocation of the script is platform-specific: on a Posix platform, you would typically do:

$ source <venv>/bin/activate

whereas on Windows, you might do:

C:\> <venv>/Scripts/activate

if you are using the cmd.exe shell, or perhaps:

PS C:\> <venv>/Scripts/Activate.ps1

if you use PowerShell.

You don’t specifically need to activate an environment; activation just prepends the venv’s binary directory to your path, so that “python” invokes the venv’s Python interpreter and you can run installed scripts without having to use their full path. However, all scripts installed in a venv should be runnable without activating it, and run with the venv’s Python automatically.

You can deactivate a venv by typing “deactivate” in your shell. The exact mechanism is platform-specific: for example, the Bash activation script defines a “deactivate” function, whereas on Windows there are separate scripts called deactivate.bat and Deactivate.ps1 which are installed when the venv is created.