virtualenv

Status and License

virtualenv is a successor to workingenv, and an extension of virtual-python.

It is written by Ian Bicking, and sponsored by the Open Planning Project. It is licensed under an MIT-style permissive license.

You can install it with easy_install virtualenv, or from the subversion repository with easy_install virtualenv==dev.

What It Does

virtualenv is a tool to create isolated Python environments.

The basic problem being addressed is one of dependencies and versions, and indirectly permissions. Imagine you have an application that needs version 1 of LibFoo, but another application requires version 2. How can you use both these applications? If you install everything into /usr/lib/python2.4/site-packages (or whatever your platform’s standard location is), it’s easy to end up in a situation where you unintentionally upgrade an application that shouldn’t be upgraded.

Or more generally, what if you want to install an application and leave it be? If an application works, any change in its libraries or the versions of those libraries can break the application.

Also, what if you can’t install packages into the global site-packages directory? For instance, on a shared host.

In all these cases, virtualenv can help you. It creates an environment that has its own installation directories, that doesn’t share libraries with other virtualenv environments (and optionally doesn’t use the globally installed libraries either).

The basic usage is:

$ python virtualenv.py ENV

If you install it you can also just do virtualenv ENV.

This creates ENV/lib/python2.4/site-packages (or ENV/lib/python2.5/site-packages on Python 2.5, etc), where any libraries you install will go. It also creates ENV/bin/python, which is a Python interpreter that uses this environment. Anytime you use that interpreter (including when a script has #!/path/to/ENV/bin/python in it) the libraries in that environment will be used. (Note for Windows: scripts and executables on Windows go in ENV\Scripts\; everywhere you see bin/ replace it with Scripts\)

It also installs Setuptools for you, and if you use ENV/bin/easy_install the packages will be installed into the environment.

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.

To create a script like this, call virtualenv.create_bootstrap_script(extra_text), and write the result to your new bootstrapping script. Here’s the documentation from the docstring:

Creates a bootstrap script, 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.

activate script

In a newly created virtualenv there will be a bin/activate shell script, or a Scripts/activate.bat batch file on Windows.

On Posix systems you can do:

$ source bin/activate

This will change your $PATH to point to the virtualenv bin/ directory, and update your prompt. Unlike workingenv, this is all it does; it’s a convenience. But if you use the complete path like /path/to/env/bin/python script.py you do not need to activate the environment first. You have to use source because it changes the environment in-place. After activating an environment you can use the function deactivate to undo the changes.

On Windows you just do:

> \path\to\env\bin\activate.bat

And use deactivate.bat to undo the changes.

The --no-site-packages Option

If you build with virtualenv --no-site-packages ENV it will not inherit any packages from /usr/lib/python2.5/site-packages (or wherever your global site-packages directory is). This can be used if you don’t have control over site-packages and don’t want to depend on the packages there, or you just want more isolation from the global system.

Using Virtualenv without bin/python

Sometimes you can’t or don’t want to use the Python interpreter created by the virtualenv. For instance, in a mod_python or mod_wsgi environment, there is only one interpreter.

Luckily, it’s easy. You must use the custom Python interpreter to install libraries. But to use libraries, you just have to be sure the path is correct. A script is available to correct the path. You can setup the environment like:

activate_this = '/path/to/env/bin/activate_this.py'
execfile(activate_this, dict(__file__=activate_this))

This will change sys.path and even change sys.prefix, but also allow you to use an existing interpreter. Items in your environment will show up first on sys.path, before global items. However, this cannot undo the activation of other environments, or modules that have been imported. You shouldn’t try to, for instance, activate an environment before a web request; you should activate one environment as early as possible, and not do it again in that process.

Making Environments Relocatable

Note: this option is somewhat experimental, and there are probably caveats that have not yet been identified. Also this does not currently work on Windows.

Normally environments are tied to a specific path. That means that you cannot move an environment around or copy it to another computer. You can fix up an environment to make it relocatable with the command:

$ virtualenv --relocatable ENV

This will make some of the files created by setuptools use relative paths, and will change all the scripts to use activate_this.py instead of using the location of the Python interpreter to select the environment.

Note: you must run this after you’ve installed any packages into the environment. If you make an environment relocatable, then install a new package, you must run virtualenv --relocatable again.

Also, this does not make your packages cross-platform. You can move the directory around, but it can only be used on other similar computers. Some known environmental differences that can cause incompatibilities: a different version of Python, when one platform uses UCS2 for its internal unicode representation and another uses UCS4 (a compile-time option), obvious platform changes like Windows vs. Linux, or Intel vs. ARM, and if you have libraries that bind to C libraries on the system, if those C libraries are located somewhere different (either different versions, or a different filesystem layout).

Currently the --no-site-packages option will not be honored if you use this on an environment.

Compare & Contrast with Alternatives

There are several alternatives that create isolated environments:

  • workingenv (which I do not suggest you use anymore) is the predecessor to this library. It used the main Python interpreter, but relied on setting $PYTHONPATH to activate the environment. This causes problems when running Python scripts that aren’t part of the environment (e.g., a globally installed hg or bzr). It also conflicted a lot with Setuptools.

  • virtual-python is also a predecessor to this library. It uses only symlinks, so it couldn’t work on Windows. It also symlinks over the entire standard library and global site-packages. As a result, it won’t see new additions to the global site-packages.

    This script only symlinks a small portion of the standard library into the environment, and so on Windows it is feasible to simply copy these files over. Also, it creates a new/empty site-packages and also adds the global site-packages to the path, so updates are tracked separately. This script also installs Setuptools automatically, saving a step and avoiding the need for network access.

  • zc.buildout doesn’t create an isolated Python environment in the same style, but achieves similar results through a declarative config file that sets up scripts with very particular packages. As a declarative system, it is somewhat easier to repeat and manage, but more difficult to experiment with. zc.buildout includes the ability to setup non-Python systems (e.g., a database server or an Apache instance).

I strongly recommend anyone doing application development or deployment use one of these tools.

Changes & News

1.3.4

  • Virtualenv now copies the actual embedded Python binary on Mac OS X to fix a hang on Snow Leopard (10.6).
  • Fail more gracefully on Windows when win32api is not installed.
  • Fix site-packages taking precedent over Jython’s __classpath__ and also specially handle the new __pyclasspath__ entry in sys.path.
  • Now copies Jython’s registry file to the virtualenv if it exists.
  • Better find libraries when compiling extensions on Windows.
  • Create Scripts\pythonw.exe on Windows.
  • Added support for the Debian/Ubuntu /usr/lib/pythonX.Y/dist-packages directory.
  • Set distutils.sysconfig.get_config_vars()['LIBDIR'] (based on sys.real_prefix) which is reported to help building on Windows.
  • Make deactivate work on ksh
  • Fixes for --python: make it work with --relocatable and the symlink created to the exact Python version.

1.3.3

  • Use Windows newlines in activate.bat, which has been reported to help when using non-ASCII directory names.
  • Fixed compatibility with Jython 2.5b1.
  • Added a function virtualenv.install_python for more fine-grained access to what virtualenv.create_environment does.
  • Fix a problem with Windows and paths that contain spaces.
  • If /path/to/env/.pydistutils.cfg exists (or /path/to/env/pydistutils.cfg on Windows systems) then ignore ~/.pydistutils.cfg and use that other file instead.
  • Fix ` a problem <https://bugs.launchpad.net/virtualenv/+bug/340050>`_ picking up some .so libraries in /usr/local.

1.3.2

  • Remove the [install] prefix = ... setting from the virtualenv distutils.cfg – this has been causing problems for a lot of people, in rather obscure ways.
  • If you use a boot script it will attempt to import virtualenv and find a pre-downloaded Setuptools egg using that.
  • Added platform-specific paths, like /usr/lib/pythonX.Y/plat-linux2

1.3.1

  • Real Python 2.6 compatibility. Backported the Python 2.6 updates to site.py, including user directories (this means older versions of Python will support user directories, whether intended or not).
  • Always set [install] prefix in distutils.cfg – previously on some platforms where a system-wide distutils.cfg was present with a prefix setting, packages would be installed globally (usually in /usr/local/lib/pythonX.Y/site-packages).
  • Sometimes Cygwin seems to leave .exe off sys.executable; a workaround is added.
  • Fix --python option.
  • Fixed handling of Jython environments that use a jython-complete.jar.

1.3

  • Update to Setuptools 0.6c9
  • Added an option virtualenv --relocatable EXISTING_ENV, which will make an existing environment “relocatable” – the paths will not be absolute in scripts, .egg-info and .pth files. This may assist in building environments that can be moved and copied. You have to run this after any new packages installed.
  • Added bin/activate_this.py, a file you can use like execfile("path_to/activate_this.py", dict(__file__="path_to/activate_this.py")) – this will activate the environment in place, similar to what the mod_wsgi example does.
  • For Mac framework builds of Python, the site-packages directory /Library/Python/X.Y/site-packages is added to sys.path, from Andrea Rech.
  • Some platform-specific modules in Macs are added to the path now (plat-darwin/, plat-mac/, plat-mac/lib-scriptpackages), from Andrea Rech.
  • Fixed a small Bashism in the bin/activate shell script.
  • Added __future__ to the list of required modules, for Python 2.3. You’ll still need to backport your own subprocess module.
  • Fixed the __classpath__ entry in Jython’s sys.path taking precedent over virtualenv’s libs.

1.2

  • Added a --python option to select the Python interpreter.
  • Add warnings to the modules copied over, for Python 2.6 support.
  • Add sets to the module copied over for Python 2.3 (though Python 2.3 still probably doesn’t work).

1.1.1

  • Added support for Jython 2.5.

1.1

  • Added support for Python 2.6.
  • Fix a problem with missing DLLs/zlib.pyd on Windows. Create
  • bin/python (or bin/python.exe) even when you run virtualenv with an interpreter named, e.g., python2.4
  • Fix MacPorts Python
  • Added –unzip-setuptools option
  • Update to Setuptools 0.6c8
  • If the current directory is not writable, run ez_setup.py in /tmp
  • Copy or symlink over the include directory so that packages will more consistently compile.

1.0

  • Fix build on systems that use /usr/lib64, distinct from /usr/lib (specifically CentOS x64).
  • Fixed bug in --clear.
  • Fixed typos in deactivate.bat.
  • Preserve $PYTHONPATH when calling subprocesses.

0.9.2

  • Fix include dir copying on Windows (makes compiling possible).
  • Include the main lib-tk in the path.
  • Patch distutils.sysconfig: get_python_inc and get_python_lib to point to the global locations.
  • Install distutils.cfg before Setuptools, so that system customizations of distutils.cfg won’t effect the installation.
  • Add bin/pythonX.Y to the virtualenv (in addition to bin/python).
  • Fixed an issue with Mac Framework Python builds, and absolute paths (from Ronald Oussoren).

0.9.1

  • Improve ability to create a virtualenv from inside a virtualenv.
  • Fix a little bug in bin/activate.
  • Actually get distutils.cfg to work reliably.

0.9

  • Added lib-dynload and config to things that need to be copied over in an environment.
  • Copy over or symlink the include directory, so that you can build packages that need the C headers.
  • Include a distutils package, so you can locally update distutils.cfg (in lib/pythonX.Y/distutils/distutils.cfg).
  • Better avoid downloading Setuptools, and hitting PyPI on environment creation.
  • Fix a problem creating a lib64/ directory.
  • Should work on MacOSX Framework builds (the default Python installations on Mac). Thanks to Ronald Oussoren.

0.8.4

  • Windows installs would sometimes give errors about sys.prefix that were inaccurate.
  • Slightly prettier output.

0.8.3

  • Added support for Windows.

0.8.2

  • Give a better warning if you are on an unsupported platform (Mac Framework Pythons, and Windows).
  • Give error about running while inside a workingenv.
  • Give better error message about Python 2.3.

0.8.1

Fixed packaging of the library.

0.8

Initial release. Everything is changed and new!