Reference Guide

`virtualenv` Command

Usage

virtualenv [OPTIONS] ENV_DIR

Where ENV_DIR is an absolute or relative path to a directory to create the virtual environment in.

Options

--version: show program’s version number and exit

-h, --help: show this help message and exit

-v, --verbose: Increase verbosity.

-q, --quiet: Decrease verbosity.

-p PYTHON_EXE, --python=PYTHON_EXE: The Python interpreter to use, e.g., --python=python2.5 will use the python2.5 interpreter to create the new environment. The default is the interpreter that virtualenv was installed with (like /usr/bin/python)

--clear: Clear out the non-root install and start from scratch.

--system-site-packages: Give the virtual environment access to the global site-packages.

--always-copy: Always copy files rather than symlinking.

--relocatable: Make an EXISTING virtualenv environment relocatable. This fixes up scripts and makes all .pth files relative.

--unzip-setuptools: Unzip Setuptools when installing it.

--no-setuptools: Do not install setuptools in the new virtualenv.

--no-pip: Do not install pip in the new virtualenv.

--no-wheel: Do not install wheel in the new virtualenv.

--extra-search-dir=DIR: Directory to look for setuptools/pip distributions in. This option can be specified multiple times.

--prompt=PROMPT: Provides an alternative prompt prefix for this environment.

--download: Download preinstalled packages from PyPI.

--no-download: Do not download preinstalled packages from PyPI.

--no-site-packages: DEPRECATED. Retained only for backward compatibility. Not having access to global site-packages is now the default behavior.

--distribute

--setuptools: Legacy; now have no effect. Before version 1.10 these could be used to choose whether to install Distribute or Setuptools into the created virtualenv. Distribute has now been merged into Setuptools, and the latter is always installed.

Configuration

Environment Variables

Each command line option is automatically used to look for environment variables with the name format VIRTUALENV_<UPPER_NAME>. That means the name of the command line options are capitalized and have dashes ('-') replaced with underscores ('_').

For example, to automatically use a custom Python binary instead of the one virtualenv is run with you can also set an environment variable:

$ export VIRTUALENV_PYTHON=/opt/python-3.3/bin/python
$ virtualenv ENV

It’s the same as passing the option to virtualenv directly:

$ virtualenv --python=/opt/python-3.3/bin/python ENV

This also works for appending command line options, like --find-links. Just leave an empty space between the passed values, e.g.:

$ export VIRTUALENV_EXTRA_SEARCH_DIR="/path/to/dists /path/to/other/dists"
$ virtualenv ENV

is the same as calling:

$ virtualenv --extra-search-dir=/path/to/dists --extra-search-dir=/path/to/other/dists ENV

VIRTUAL_ENV_DISABLE_PROMPT: Any virtualenv activated when this is set to a non-empty value will leave the shell prompt unchanged during processing of the activate script, rather than modifying it to indicate the newly activated environment.

Configuration File

virtualenv also looks for a standard ini config file. On Unix and Mac OS X that’s $HOME/.virtualenv/virtualenv.ini and on Windows, it’s %APPDATA%\virtualenv\virtualenv.ini.

The names of the settings are derived from the long command line option, e.g. the option --python would look like this:

[virtualenv]
python = /opt/python-3.3/bin/python

Appending options like --extra-search-dir can be written on multiple lines:

[virtualenv]
extra-search-dir =
    /path/to/dists
    /path/to/other/dists

Please have a look at the output of --help for a full list of supported options.

Extending Virtualenv

Creating Your Own Bootstrap Scripts

While this creates an environment, it doesn’t put anything into the environment. Developers may find it useful to distribute a script that sets up a particular environment, for example a script that installs a particular web application.

Note

A bootstrap script requires a virtualenv_support directory containing pip and setuptools wheels alongside it, just like the actual virtualenv script. Running a bootstrap script without a virtualenv_support directory is unsupported (but if you use --no-setuptools and manually install pip and setuptools in your virtualenv, it will work).

To create a script like this, call virtualenv.create_bootstrap_script(), and write the result to your new bootstrapping script.

create_bootstrap_script(extra_text): Creates a bootstrap script from extra_text, which is like this script but with extend_parser, adjust_options, and after_install hooks.

This returns a string that (written to disk of course) can be used as a bootstrap script with your own customizations. The script will be the standard virtualenv.py script, with your extra text added (your extra text should be Python code).

If you include these functions, they will be called:

extend_parser(optparse_parser): You can add or remove options from the parser here.

adjust_options(options, args): You can change options here, or change the args (if you accept different kinds of arguments, be sure you modify args so it is only [DEST_DIR]).

after_install(options, home_dir)

After everything is installed, this function is called. This is probably the function you are most likely to use. An example would be:

def after_install(options, home_dir):
    if sys.platform == 'win32':
        bin = 'Scripts'
    else:
        bin = 'bin'
    subprocess.call([join(home_dir, bin, 'easy_install'),
                     'MyPackage'])
    subprocess.call([join(home_dir, bin, 'my-package-script'),
                     'setup', home_dir])

This example immediately installs a package, and runs a setup script from that package.

Bootstrap Example

Here’s a more concrete example of how you could use this:

import virtualenv, textwrap
output = virtualenv.create_bootstrap_script(textwrap.dedent("""
import os, subprocess
def after_install(options, home_dir):
    etc = join(home_dir, 'etc')
    if not os.path.exists(etc):
        os.makedirs(etc)
    subprocess.call([join(home_dir, 'bin', 'easy_install'),
                     'BlogApplication'])
    subprocess.call([join(home_dir, 'bin', 'paster'),
                     'make-config', 'BlogApplication',
                     join(etc, 'blog.ini')])
    subprocess.call([join(home_dir, 'bin', 'paster'),
                     'setup-app', join(etc, 'blog.ini')])
"""))
f = open('blog-bootstrap.py', 'w').write(output)

Another example is available here.

Compatibility with the stdlib venv module

Starting with Python 3.3, the Python standard library includes a venv module that provides similar functionality to virtualenv - however, the mechanisms used by the two modules are very different.

Problems arise when environments get “nested” (a virtual environment is created from within another one - for example, running the virtualenv tests using tox, where tox creates a virtual environment to run the tests, and the tests themselves create further virtual environments).

virtualenv supports creating virtual environments from within another one (the sys.real_prefix variable allows virtualenv to locate the “base” environment) but stdlib-style venv environments don’t use that mechanism, so explicit support is needed for those environments.

A standard library virtual environment is most easily identified by checking sys.prefix and sys.base_prefix. If these differ, the interpreter is running in a virtual environment and the base interpreter is located in the directory specified by sys.base_prefix. Therefore, when sys.base_prefix is set, virtualenv gets the interpreter files from there rather than from sys.prefix (in the same way as sys.real_prefix is used for virtualenv-style environments). In practice, this is sufficient for all platforms other than Windows.

On Windows from Python 3.7.2 onwards, a stdlib-style virtual environment does not contain an actual Python interpreter executable, but rather a “redirector” which launches the actual interpreter from the base environment (this redirector is based on the same code as the standard py.exe launcher). As a result, the virtualenv approach of copying the interpreter from the starting environment fails. In order to correctly set up the virtualenv, therefore, we need to be running from a “full” environment. To ensure that, we re-invoke the virtualenv.py script using the “base” interpreter, in the same way as we do with the --python command line option.

The process of identifying the base interpreter is complicated by the fact that the implementation changed between different Python versions. The logic used is as follows:

If the (private) attribute sys._base_executable is present, this is the base interpreter. This is the long-term solution and should be stable in the future (the attribute may become public, and have the leading underscore removed, in a Python 3.8, but that is not confirmed yet).
In the absence of sys._base_executable (only the case for Python 3.7.2) we check for the existence of the environment variable __PYVENV_LAUNCHER__. This is used by the redirector, and if it is present, we know that we are in a stdlib-style virtual environment and need to locate the base Python. In most cases, the base environment is located at sys.base_prefix - however, in the case where the user creates a virtualenv, and then creates a venv from that virtualenv, sys.base_prefix is not correct - in that case, though, we have sys.real_prefix (set by virtualenv) which is correct.

There is one further complication - as noted above, the environment variable __PYVENV_LAUNCHER__ affects how the interpreter works, so before we re-invoke the virtualenv script, we remove this from the environment.