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Python Enhancement Proposals

PEP 405 – Python Virtual Environments

Carl Meyer <carl at>
Alyssa Coghlan
Standards Track
24-Oct-2011, 28-Oct-2011, 06-Mar-2012, 24-May-2012
Python-Dev message

Table of Contents


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 must copy the binary in order to 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 lower maintenance, raise reliability, and be more easily available to all Python users.


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 ( 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 (if the executable is a symlink, it is not dereferenced), 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 finds the prefix of the base installation. sys.base_prefix is set to this value, while sys.prefix is set to the directory containing pyvenv.cfg.

(If pyvenv.cfg is not found or does not contain the home key, prefix-finding continues normally, and sys.prefix will be equal to sys.base_prefix.)

Also, sys.base_exec_prefix is added, and handled similarly with regard to sys.exec_prefix. (sys.exec_prefix is the equivalent of sys.prefix, but for platform-specific files; by default it has the same value as sys.prefix.)

The site and sysconfig standard-library modules are modified such that the standard library and header files are found relative to sys.base_prefix / sys.base_exec_prefix, while site-package directories (“purelib” and “platlib”, in sysconfig terms) are still found relative to sys.prefix / 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.

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 convenient:

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 (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 venv). 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 pyvenv is run with the --system-site-packages option, false by default.

Multiple paths can be given to pyvenv, in which case an identical venv will be created, according to the given options, at each provided path.

The venv module also places “shell activation scripts” for POSIX and Windows systems in the bin or Scripts directory of the venv. These scripts 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.

In order to allow pysetup and other 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.base_prefix in place of sys.prefix where appropriate, 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 environment.


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.

Sysconfig install schemes and user-site

This approach explicitly chooses not to introduce a new sysconfig install scheme for venvs. Rather, by modifying sys.prefix we ensure that existing install schemes which base locations on sys.prefix will simply work in a venv. Installation to other install schemes (for instance, the user-site schemes) whose paths are not relative to sys.prefix, will not be affected by a venv at all.

It may be feasible to create an alternative implementation of Python virtual environments based on a virtual-specific sysconfig scheme, but it would be less robust, as it would require more code to be aware of whether it is operating within a virtual environment or not.

Include files

Current virtualenv handles include files in this way:

On POSIX systems where the installed Python’s include files are found in ${base_prefix}/include/pythonX.X, virtualenv creates ${venv}/include/ and symlinks ${base_prefix}/include/pythonX.X to ${venv}/include/pythonX.X. On Windows, where Python’s include files are found in {{ sys.prefix }}/Include and symlinks are not reliably available, virtualenv copies {{ sys.prefix }}/Include to ${venv}/Include. This ensures that extension modules built and installed within the virtualenv will always find the Python header files they need in the expected location relative to sys.prefix.

This solution is not ideal when an extension module installs its own header files, as the default installation location for those header files may be a symlink to a system directory that may not be writable. One installer, pip, explicitly works around this by installing header files to a nonstandard location ${venv}/include/site/pythonX.X/, as in Python there’s currently no standard abstraction for a site-specific include directory.

This PEP proposes a slightly different approach, though one with essentially the same effect and the same set of advantages and disadvantages. Rather than symlinking or copying include files into the venv, we simply modify the sysconfig schemes so that header files are always sought relative to base_prefix rather than prefix. (We also create an include/ directory within the venv, so installers have somewhere to put include files installed within the env).

Better handling of include files in distutils/packaging and, by extension, pyvenv, is an area that may deserve its own future PEP. For now, we propose that the behavior of virtualenv has thus far proved itself to be at least “good enough” in practice.


The high-level method described above makes 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 contains 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.
  • 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 False.

The instantiated env-builder has a create method, 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 either creates the environment in the specified directory, or raises an appropriate exception.

The venv module also provides a module-level create function as a convenience:

def create(env_dir,
           system_site_packages=False, clear=False, use_symlinks=False):
    builder = EnvBuilder(

Creators of third-party virtual environment tools are free to use the provided EnvBuilder class as a base class.

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, and post_setup 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 other methods.
  • 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.
  • post_setup - A (no-op by default) hook method which can be overridden in third party subclasses to pre-install packages or install scripts in the virtual environment.

In addition, EnvBuilder provides a utility method that can be called from post_setup in subclasses to assist in installing custom scripts into the virtual environment. The method install_scripts accepts as arguments the context object (see above) and a path to a directory. The directory should contain subdirectories “common”, “posix”, “nt”, each containing scripts destined for the bin directory in the environment. The contents of “common” and the directory corresponding to are copied after doing some text replacement of placeholders:

  • __VENV_DIR__ is replaced with absolute path of the environment directory.
  • __VENV_NAME__ is replaced with the environment name (final path segment of environment directory).
  • __VENV_BIN_NAME__ is replaced with the name of the bin directory (either bin or Scripts).
  • __VENV_PYTHON__ is replaced with the absolute path of the environment’s executable.

The DistributeEnvBuilder subclass in the reference implementation illustrates how the customization hook can be used in practice to pre-install Distribute into the virtual environment. 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.

Backwards Compatibility

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 be installed?”

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.

Maintaining this documented definition would mean leaving sys.prefix pointing to the base system installation (which is where the standard library and header files are found), and introducing a new value in sys (something like sys.site_prefix) to point to the prefix for site-packages. This would maintain the documented semantics of sys.prefix, but risk 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 and 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.

Otherwise, 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.

Although it requires modifying the documented definition of sys.prefix, this PEP prefers to have sys.prefix point to the virtual environment (where site-packages is found), and introduce sys.base_prefix to point to the standard library and Python header files. Rationale for this choice:

  • It is preferable to err on the side of greater isolation of the virtual environment.
  • Virtualenv already modifies sys.prefix to point at the virtual environment, and in practice this has not been a problem.
  • No modification is required to setuptools/distribute.

Impact on other Python implementations

The majority of this PEP’s changes occur in the standard library, which is shared by other Python implementations and should not present any problem.

Other Python implementations will need to replicate the new sys.prefix-finding behavior of the interpreter bootstrap, including locating and parsing the pyvenv.cfg file, if it is present.

Reference Implementation

The reference implementation is found in a clone of the CPython Mercurial repository. To test it, build and run bin/pyvenv /path/to/new/venv to create a virtual environment.


Last modified: 2023-10-11 12:05:51 GMT