Virtual Python Environment builder
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
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.
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:
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 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): 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.
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.
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:
And use deactivate.bat to undo the changes.
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. Adding the correct path is easy:
import site site.addsitedir('/path/to/virtualenv/lib/python2.5/site-packages')
Using this you can have your isolated working environment, using the custom Python interpreter, but treat the result as just a simple set of libraries when running your application.
Compare & Contrast with Alternatives
There are several alternatives that create isolated environments:
workingenv (which I do not suggest you use anymore if virtualenv works on your platform) 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 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
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).
Improve ability to create a virtualenv from inside a virtualenv.
Fix a little bug in bin/activate.
Actually get distutils.cfg to work reliably.
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.
Windows installs would sometimes give errors about sys.prefix that were inaccurate.
Slightly prettier output.
Added support for Windows.
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.
Fixed packaging of the library.
Initial release. Everything is changed and new!
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