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").

See also: Python Packaging User Guide: Creating and using virtual
  environments

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:\Python35\python c:\Python35\Tools\Scripts\pyvenv.py myenv

or equivalently:

   c:\Temp>c:\Python35\python -m venv myenv

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

   usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear]
               [--upgrade] [--without-pip]
               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 the virtual environment access to the system
                            site-packages dir.
     --symlinks             Try to use symlinks rather than copies, when symlinks
                            are not the default for the platform.
     --copies               Try to use copies rather than symlinks, even when
                            symlinks are the default for the platform.
     --clear                Delete the contents of the environment directory if it
                            already exists, before environment creation.
     --upgrade              Upgrade the environment directory to use this version
                            of Python, assuming Python has been upgraded in-place.
     --without-pip          Skips installing or upgrading pip in the virtual
                            environment (pip is bootstrapped by default)

Depending on how the "venv" functionality has been invoked, the usage
message may vary slightly, e.g. referencing "pyvenv" rather than
"venv".

Changed in version 3.4: Installs pip by default, added the "--without-
pip"  and "--copies" options

Changed in version 3.4: In earlier versions, if the target directory
already existed, an error was raised, unless the "--clear" or "--
upgrade" option was provided. Now, if an existing directory is
specified, its contents are removed and the directory is processed as
if it had been newly created.

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.

Unless the "--without-pip" option is given, "ensurepip" will be
invoked to bootstrap "pip" into the virtual environment.

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:

+---------------+-------------------+-------------------------------------------+
| Platform      | Shell             | Command to activate virtual environment   |
+===============+===================+===========================================+
| Posix         | bash/zsh          | $ source <venv>/bin/activate              |
+---------------+-------------------+-------------------------------------------+
|               | fish              | $ . <venv>/bin/activate.fish              |
+---------------+-------------------+-------------------------------------------+
|               | csh/tcsh          | $ source <venv>/bin/activate.csh          |
+---------------+-------------------+-------------------------------------------+
| Windows       | cmd.exe           | C:\> <venv>\Scripts\activate.bat          |
+---------------+-------------------+-------------------------------------------+
|               | PowerShell        | PS C:\> <venv>\Scripts\Activate.ps1       |
+---------------+-------------------+-------------------------------------------+

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.

New in version 3.4: "fish" and "csh" activation scripts.
