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.