[Python-Dev] draft PEP: virtual environments
carl at oddbird.net
Fri Oct 28 20:37:35 CEST 2011
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As has been discussed here previously, Vinay Sajip and I are working on
a PEP for making "virtual Python environments" a la virtualenv  a
built-in feature of Python 3.3.
This idea was first proposed on python-dev by Ian Bicking in February
2010 . It was revived at PyCon 2011 and has seen discussion on
distutils-sig , more recently again on python-dev  , and most
recently on python-ideas .
Full text of the draft PEP is pasted below, and also available on
Bitbucket . The reference implementation is also on Bitbucket .
For known issues in the reference implementation and cases where it does
not yet match the PEP, see the open issues list .
In particular, please note the "Open Questions" section of the draft
PEP. These are areas where we are still unsure of the best approach, or
where we've received conflicting feedback and haven't reached consensus.
We welcome your thoughts on anything in the PEP, but feedback on the
open questions is especially useful.
We'd also especially like to hear from Windows and OSX users, from
authors of packaging-related tools (packaging/distutils2, zc.buildout)
and from Python implementors (PyPy, IronPython, Jython).
If it is easier to review and comment on the PEP after it is published
on python.org, I can submit it to the PEP editors anytime. Otherwise
I'll wait until we've resolved a few more of the open questions, as it's
easier for me to update the PEP on Bitbucket.
Title: Python Virtual Environments
Author: Carl Meyer <carl at oddbird.net>
Type: Standards Track
Post-History: 24-Oct-2011, 28-Oct-2011
This PEP proposes to add to Python a mechanism for lightweight
"virtual environments" with their own site directories, optionally
isolated from system site directories. Each virtual environment has
its own Python binary (allowing creation of environments with various
Python versions) and can have its own independent set of installed
Python packages in its site directories, but shares the standard
library with the base installed Python.
The utility of Python virtual environments has already been well
established by the popularity of existing third-party
virtual-environment tools, primarily Ian Bicking's `virtualenv`_.
Virtual environments are already widely used for dependency management
and isolation, ease of installing and using Python packages without
system-administrator access, and automated testing of Python software
across multiple Python versions, among other uses.
Existing virtual environment tools suffer from lack of support from
the behavior of Python itself. Tools such as `rvirtualenv`_, which do
not copy the Python binary into the virtual environment, cannot
provide reliable isolation from system site directories. Virtualenv,
which does copy the Python binary, is forced to duplicate much of
Python's ``site`` module and manually symlink/copy an ever-changing
set of standard-library modules into the virtual environment in order
to perform a delicate boot-strapping dance at every
startup. (Virtualenv copies the binary because symlinking it does not
provide isolation, as Python dereferences a symlinked executable
before searching for `sys.prefix`.)
The ``PYTHONHOME`` environment variable, Python's only existing
built-in solution for virtual environments, requires
copying/symlinking the entire standard library into every
environment. Copying the whole standard library is not a lightweight
solution, and cross-platform support for symlinks remains inconsistent
(even on Windows platforms that do support them, creating them often
requires administrator privileges).
A virtual environment mechanism integrated with Python and drawing on
years of experience with existing third-party tools can be lower
maintenance, more reliable, and more easily available to all Python
.. _virtualenv: http://www.virtualenv.org
.. _rvirtualenv: https://github.com/kvbik/rvirtualenv
When the Python binary is executed, it attempts to determine its
prefix (which it stores in ``sys.prefix``), which is then used to find
the standard library and other key files, and by the ``site`` module
to determine the location of the site-package directories. Currently
the prefix is found (assuming ``PYTHONHOME`` is not set) by first
walking up the filesystem tree looking for a marker file (``os.py``)
that signifies the presence of the standard library, and if none is
found, falling back to the build-time prefix hardcoded in the binary.
This PEP proposes to add a new first step to this search. If a
``pyvenv.cfg`` file is found either adjacent to the Python executable,
or one directory above it, this file is scanned for lines of the form
``key = value``. If a ``home`` key is found, this signifies that the
Python binary belongs to a virtual environment, and the value of the
``home`` key is the directory containing the Python executable used to
create this virtual environment.
In this case, prefix-finding continues as normal using the value of
the ``home`` key as the effective Python binary location, which
results in ``sys.prefix`` being set to the system installation prefix,
while ``sys.site_prefix`` is set to the directory containing
(If ``pyvenv.cfg`` is not found or does not contain the ``home`` key,
prefix-finding continues normally, and ``sys.site_prefix`` will be
equal to ``sys.prefix``.)
The ``site`` and ``sysconfig`` standard-library modules are modified
such that site-package directories ("purelib" and "platlib", in
``sysconfig`` terms) are found relative to ``sys.site_prefix``, while
other directories (the standard library, include files) are still
found relative to ``sys.prefix``.
(Also, ``sys.site_exec_prefix`` is added, and handled similarly with
regard to ``sys.exec_prefix``.)
Thus, a Python virtual environment in its simplest form would consist
of nothing more than a copy or symlink of the Python binary
accompanied by a ``pyvenv.cfg`` file and a site-packages
directory. The ``venv`` module also adds a ``pysetup3`` script into
each venv, as well as necessary DLLs and `.pyd` files on Windows.
In order to allow Python package managers to install packages into the
virtual environment the same way they would install into a normal
Python installation, and avoid special-casing virtual environments in
``sysconfig`` beyond using ``sys.site_prefix`` in place of
``sys.prefix``, the internal virtual environment layout mimics the
layout of the Python installation itself on each platform. So a
typical virtual environment layout on a POSIX system would be::
While on a Windows system::
... other DLLs and pyds...
Third-party packages installed into the virtual environment will have
their Python modules placed in the ``site-packages`` directory, and
their executables placed in ``bin/`` or ``Scripts\``.
On a normal Windows system-level installation, the Python binary
itself wouldn't go inside the "Scripts/" subdirectory, as it does
in the default venv layout. This is useful in a virtual
environment so that a user only has to add a single directory to
their shell PATH in order to effectively "activate" the virtual
On Windows, it is necessary to also copy or symlink DLLs and pyd
files from compiled stdlib modules into the env, because if the
venv is created from a non-system-wide Python installation,
Windows won't be able to find the Python installation's copies of
those files when Python is run from the venv.
Isolation from system site-packages
By default, a virtual environment is entirely isolated from the
system-level site-packages directories.
If the ``pyvenv.cfg`` file also contains a key
``include-system-site-packages`` with a value of ``true`` (not case
sensitive), the ``site`` module will also add the system site
directories to ``sys.path`` after the virtual environment site
directories. Thus system-installed packages will still be importable,
but a package of the same name installed in the virtual environment
will take precedence.
:pep:`370` user-level site-packages are considered part of the system
site-packages for venv purposes: they are not available from an
isolated venv, but are available from an
``include-system-site-packages = true`` venv.
Creating virtual environments
This PEP also proposes adding a new ``venv`` module to the standard
library which implements the creation of virtual environments. This
module can be executed using the ``-m`` flag::
python3 -m venv /path/to/new/virtual/environment
A ``pyvenv`` installed script is also provided to make this more
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 (or symlink) of the
``python3`` executable, and the ``pysetup3`` script from the
``packaging`` standard library module (to facilitate easy installation
of packages from PyPI into the new virtualenv). And it creates an
(initially empty) ``lib/pythonX.Y/site-packages`` (or
``Lib\site-packages`` on Windows) subdirectory.
If the target directory already exists an error will be raised, unless
the ``--clear`` option was provided, in which case the target
directory will be deleted and virtual environment creation will
proceed as usual.
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`` by default.
Multiple paths can be given to ``venv``, in which case an identical
virtualenv will be created, according to the given options, at each
Copies versus symlinks
The technique in this PEP works equally well in general with a copied
or symlinked Python binary (and other needed DLLs on Windows). Some
users prefer a copied binary (for greater isolation from system
changes) and some prefer a symlinked one (so that e.g. security
updates automatically propagate to virtual environments).
There are some cross-platform difficulties with symlinks:
* Not all Windows versions support symlinks, and even on those that
do, creating them often requires administrator privileges.
* On OSX framework builds of Python, sys.executable is just a stub
that executes the real Python binary. Symlinking this stub does not
work with the implementation in this PEP; it must be
copied. (Fortunately the stub is also small, so copying it is not an
Because of these issues, this PEP proposes to copy the Python binary
by default, to maintain cross-platform consistency in the default
The ``pyvenv`` script accepts a ``--symlink`` option. If this option
is provided, the script will attempt to symlink instead of copy. If a
symlink fails (e.g. because they are not supported by the platform, or
additional privileges are needed), the script will warn the user and
fall back to a copy.
On OSX framework builds, where a symlink of the executable would
succeed but create a non-functional virtual environment, the script
will fail with an error message that symlinking is not supported on
OSX framework builds.
The high-level method described above will make use of a simple API
which provides mechanisms for third-party virtual environment creators
to customize environment creation according to their needs.
The ``venv`` module will contain an ``EnvBuilder`` class which accepts
the following keyword arguments on instantiation::
* ``system_site_packages`` - A Boolean value indicating that the
system Python site-packages should be available to the
environment (defaults to ``False``).
* ``clear`` - A Boolean value which, if True, will delete any
existing target directory instead of raising an exception
(defaults to ``False``).
* ``use_symlinks`` - A Boolean value indicating whether to attempt
to symlink the Python binary (and any necessary DLLs or other
binaries, e.g. ``pythonw.exe``), rather than copying. Defaults to
The returned env-builder is an object which is expected to have a
single method, ``create``, which takes as required argument the path
(absolute or relative to the current directory) of the target
directory which is to contain the virtual environment. The ``create``
method will either create the environment in the specified directory,
or raise an appropriate exception.
Creators of third-party virtual environment tools will be free to use
the provided ``EnvBuilder`` class as a base class.
The ``venv`` module will also provide a module-level function as a
system_site_packages=False, clear=False, use_symlinks=True):
builder = EnvBuilder(
The ``create`` method of the ``EnvBuilder`` class illustrates the
hooks available for customization:
def create(self, env_dir):
Create a virtualized Python environment in a directory.
:param env_dir: The target directory to create an environment in.
env_dir = os.path.abspath(env_dir)
context = self.create_directories(env_dir)
Each of the methods ``create_directories``, ``create_configuration``,
``setup_python``, ``setup_packages`` and ``setup_scripts`` can be
overridden. The functions of these methods are::
* ``create_directories`` - creates the environment directory and
all necessary directories, and returns a context object. This is
just a holder for attributes (such as paths), for use by the
* ``create_configuration`` - creates the ``pyvenv.cfg``
configuration file in the environment.
* ``setup_python`` - creates a copy of the Python executable (and,
under Windows, DLLs) in the environment.
* ``setup_packages`` - A placeholder method which can be overridden
in third party implementations to pre-install packages in the
* ``setup_scripts`` - A placeholder methd which can be overridden
in third party implementations to pre-install scripts (such as
activation and deactivation scripts) in the virtual environment.
The ``DistributeEnvBuilder`` subclass in the reference implementation
illustrates how these last two methods can be used in practice. It's
not envisaged that ``DistributeEnvBuilder`` will be actually added to
Python core, but it makes the reference implementation more
immediately useful for testing and exploratory purposes.
* The ``setup_packages`` method installs Distribute in the target
environment. This is needed at the moment in order to actually
install most packages in an environment, since most packages are
not yet packaging / setup.cfg based.
* The ``setup_scripts`` method installs shell activation scripts in
the environment. This is also done in a configurable way: A
``scripts`` property on the builder is expected to provide a
buffer which is a base64-encoded zip file. The zip file contains
directories "common", "linux2", "darwin", "win32", each
containing scripts destined for the bin directory in the
environment. The contents of "common" and the directory
corresponding to ``sys.platform`` are copied after doing some
text replacement of placeholders:
* ``__VIRTUAL_ENV__`` is replaced with absolute path of the
* ``__VIRTUAL_PROMPT__`` is replaced with the environment
* ``__BIN_NAME__`` is replaced with the name of the bin
* ``__ENV_PYTHON__`` is replaced with the absolute path of the
The "shell activation scripts" provided by ``DistributeEnvBuilder``
simply add the virtual environment's ``bin/`` (or ``Scripts\``)
directory to the front of the user's shell PATH. This is not strictly
necessary for use of a virtual environment (as an explicit path to the
venv's python binary or scripts can just as well be used), but it is
convenient. This PEP does not propose that the ``venv`` module in core
Python will add such activation scripts by default, as they are
shell-specific. Adding activation scripts for the wide variety of
possible shells is an added maintenance burden, and is left to
third-party extension tools.
No doubt the process of PEP review will show up any customization
requirements which have not yet been considered.
Splitting the meanings of ``sys.prefix``
Any virtual environment tool along these lines (which attempts to
isolate site-packages, while still making use of the base Python's
standard library with no need for it to be symlinked into the virtual
environment) is proposing a split between two different meanings
(among others) that are currently both wrapped up in ``sys.prefix``:
the answers to the questions "Where is the standard library?" and
"Where is the site-packages location where third-party modules should
This split could be handled by introducing a new ``sys`` attribute for
either the former prefix or the latter prefix. Either option
potentially introduces some backwards-incompatibility with software
written to assume the other meaning for ``sys.prefix``. (Such software
should preferably be using the APIs in the ``site`` and ``sysconfig``
modules to answer these questions rather than using ``sys.prefix``
directly, in which case there is no backwards-compatibility issue, but
in practice ``sys.prefix`` is sometimes used.)
The `documentation`__ for ``sys.prefix`` describes it as "A string
giving the site-specific directory prefix where the platform
independent Python files are installed," and specifically mentions the
standard library and header files as found under ``sys.prefix``. It
does not mention ``site-packages``.
This PEP currently proposes to leave ``sys.prefix`` pointing to the
base system installation (which is where the standard library and
header files are found), and introduce a new value in ``sys``
(``sys.site_prefix``) to point to the prefix for
``site-packages``. This maintains the documented semantics of
``sys.prefix``, but risks breaking isolation if third-party code uses
``sys.prefix`` rather than ``sys.site_prefix`` or the appropriate
``site`` API to find site-packages directories.
The most notable case is probably `setuptools`_ and its fork
`distribute`_, which mostly use ``distutils``/``sysconfig`` APIs, but
do use ``sys.prefix`` directly to build up a list of site directories
for pre-flight checking where ``pth`` files can usefully be placed.
It would be trivial to modify these tools (currently only
`distribute`_ is Python 3 compatible) to check ``sys.site_prefix`` and
fall back to ``sys.prefix`` if it doesn't exist (for earlier versions
of Python). If Distribute is modified in this way and released before
Python 3.3 is released with the ``venv`` module, there would be no
likely reason for an older version of Distribute to ever be installed
in a virtual environment.
In terms of other third-party usage, a `Google Code Search`_ turns up
what appears to be a roughly even mix of usage between packages using
``sys.prefix`` to build up a site-packages path and packages using it
to e.g. eliminate the standard-library from code-execution
tracing. Either choice that's made here will require one or the other
of these uses to be updated.
.. _setuptools: http://peak.telecommunity.com/DevCenter/setuptools
.. _distribute: http://packages.python.org/distribute/
.. _Google Code Search:
Naming of the new ``sys`` prefix attributes
The name ``sys.site_prefix`` was chosen with the following
considerations in mind:
* Inasmuch as "site" has a meaning in Python, it means a combination
of Python version, standard library, and specific set of
site-packages. This is, fundamentally, what a venv is (although it
shares the standard library with its "base" site).
* It is the Python ``site`` module which implements adding
site-packages directories to ``sys.path``, so ``sys.site_prefix`` is
a prefix used (and set) primarily by the ``site`` module.
A concern has been raised that the term ``site`` in Python is already
overloaded and of unclear meaning, and this usage will increase the
One proposed alternative is ``sys.venv_prefix``, which has the
advantage of being clearly related to the venv implementation. The
downside of this proposal is that it implies the attribute is only
useful/relevant when in a venv and should be absent or ``None`` when
not in a venv. This imposes an unnecessary extra burden on code using
the attribute: ``sys.venv_prefix if sys.venv_prefix else
sys.prefix``. The prefix attributes are more usable and general if
they are always present and set, and split by meaning (stdlib vs
site-packages, roughly), rather than specifically tied to venv. Also,
third-party code should be encouraged to not know or care whether it
is running in a virtual environment or not; this option seems to work
against that goal.
Another option would be ``sys.local_prefix``, which has both the
advantage and disadvantage, depending on perspective, that it
introduces the new term "local" rather than drawing on existing
associations with the term "site".
Why not modify sys.prefix?
As discussed above under `Backwards Compatibility`_, this PEP proposes
to add ``sys.site_prefix`` as "the prefix relative to which
site-package directories are found". This maintains compatibility with
the documented meaning of ``sys.prefix`` (as the location relative to
which the standard library can be found), but means that code assuming
that site-packages directories are found relative to ``sys.prefix``
will not respect the virtual environment correctly.
Since it is unable to modify ``distutils``/``sysconfig``,
`virtualenv`_ is forced to instead re-point ``sys.prefix`` at the
An argument could be made that this PEP should follow virtualenv's
lead here (and introduce something like ``sys.base_prefix`` to point
to the standard library and header files), since virtualenv already
does this and it doesn't appear to have caused major problems with
Another argument in favor of this is that it would be preferable to
err on the side of greater, rather than lesser, isolation. Changing
``sys.prefix`` to point to the virtual environment and introducing a
new ``sys.base_prefix`` attribute would err on the side of greater
isolation in the face of existing code's use of ``sys.prefix``.
What about include files?
For example, ZeroMQ installs zmq.h and zmq_utils.h in $VE/include,
whereas SIP (part of PyQt4) installs sip.h by default in
$VE/include/pythonX.Y. With virtualenv, everything works because the
PythonX.Y include is symlinked, so everything that's needed is in
$VE/include. At the moment the reference implementation doesn't do
anything with include files, besides creating the include directory;
this might need to change, to copy/symlink $VE/include/pythonX.Y.
As in Python there's no abstraction for a site-specific include
directory, other than for platform-specific stuff, then the user
expectation would seem to be that all include files anyone could ever
want should be found in one of just two locations, with sysconfig
labels "include" & "platinclude".
There's another issue: what if includes are Python-version-specific?
For example, SIP installs by default into $VE/include/pythonX.Y rather
than $VE/include, presumably because there's version-specific stuff in
there - but even if that's not the case with SIP, it could be the case
with some other package. And the problem that gives is that you can't
just symlink the include/pythonX.Y directory, but actually have to
provide a writable directory and symlink/copy the contents from the
system include/pythonX.Y. Of course this is not hard to do, but it
does seem inelegant. OTOH it's really because there's no supporting
concept in Python/sysconfig.
Interface with packaging tools
Some work will be needed in packaging tools (Python 3.3 packaging,
Distribute) to support implementation of this PEP. For example:
* How Distribute and packaging use sys.prefix and/or sys.site_prefix.
in practice we'll need to use Distribute for a while, until packages have
migrated over to usage of setup.cfg.
* How packaging and Distribute set up shebang lines in scripts which they
install in virtual environments.
Testability and Source Build Issues
Currently in the reference implementation, virtual environments must
be created with an installed Python, rather than a source build, as
the base installation. In order to be able to fully test the ``venv``
module in the Python regression test suite, some anomalies in how
sysconfig data is configured in source builds will need to be
removed. For example, sysconfig.get_paths() in a source build gives
'libdir': '/usr/lib ; or /usr/lib64 on a multilib system',
Need for ``install_name_tool`` on OSX?
`Virtualenv uses`_ ``install_name_tool``, a tool provided in the Xcode
developer tools, to modify the copied executable on OSX. We need input
from OSX developers on whether this is actually necessary in this
PEP's implementation of virtual environments, and if so, if there is
an alternative to ``install_name_tool`` that would allow ``venv`` to
not require that Xcode is installed.
.. _Virtualenv uses: https://github.com/pypa/virtualenv/issues/168
Activation and Utility Scripts
Virtualenv provides shell "activation" scripts as a user convenience,
to put the virtual environment's Python binary first on the shell
PATH. This is a maintenance burden, as separate activation scripts
need to be provided and maintained for every supported shell. For this
reason, this PEP proposes to leave such scripts to be provided by
third-party extensions; virtual environments created by the core
functionality would be used by directly invoking the environment's
Python binary or scripts.
If we are going to rely on external code to provide these
conveniences, we need to check with existing third-party projects in
this space (virtualenv, zc.buildout) and ensure that the proposed API
meets their needs.
(Virtualenv would be fine with the proposed API; it would become a
relatively thin wrapper with a subclass of the env builder that adds
shell activation and automatic installation of ``pip`` inside the
Provide a mode that is isolated only from user site packages?
Is there sufficient rationale for providing a mode that isolates the
venv from :pep:`370` user site packages, but not from the system-level
Other Python implementations?
We should get feedback from Jython, IronPython, and PyPy about whether
there's anything in this PEP that they foresee as a difficulty for
The in-progress reference implementation is found in `a clone of the
CPython Mercurial repository`_. To test it, build and install it (the
virtual environment tool currently does not run from a source tree).
- From the installed Python, run ``bin/python3 -m venv
/path/to/new/virtualenv`` to create a virtual environment.
The reference implementation (like this PEP!) is a work in progress.
.. _a clone of the CPython Mercurial repository:
This document has been placed in the public domain.
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