New in version 3.3.
Source code: Lib/venv.py
The venv module provides support for creating 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.
Creation of virtual environments is done by executing the pyvenv script:
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 of the python binary (or binaries, in the case of Windows). It also creates an (initially empty) lib/pythonX.Y/site-packages subdirectory (on Windows, this is Lib\site-packages).
On Windows, you may have to invoke the pyvenv script as follows, if you don’t have the relevant PATH and PATHEXT settings:
c:\Temp>c:\Python33\python c:\Python33\Tools\Scripts\pyvenv.py myenv
c:\Temp>c:\Python33\python -m venv myenv
The command, if run with -h, will show the available options:
usage: pyvenv [-h] [--system-site-packages] [--symlinks] [--clear] [--upgrade] ENV_DIR [ENV_DIR ...] Creates virtual Python environments in one or more target directories. positional arguments: ENV_DIR A directory to create the environment in. optional arguments: -h, --help show this help message and exit --system-site-packages Give access to the global site-packages dir to the virtual environment. --symlinks Try to use symlinks rather than copies, when symlinks are not the default for the platform. --clear Delete the environment directory if it already exists. If not specified and the directory exists, an error is raised. --upgrade Upgrade the environment directory to use this version of Python, assuming Python has been upgraded in-place.
If the target directory already exists an error will be raised, unless the --clear or --upgrade option was provided.
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 otherwise.
Multiple paths can be given to pyvenv, in which case an identical virtualenv will be created, according to the given options, at each provided path.
Once a venv has been created, it can be “activated” using a script in the venv’s binary directory. The invocation of the script is platform-specific:
|Platform||Shell||Command to activate virtual environment|
|Posix||bash/zsh||$ source <venv>/bin/activate|
|fish||$ . <venv>/bin/activate.fish|
|csh/tcsh||$ source <venv>/bin/activate.csh|
|PowerShell||PS C:> <venv>/Scripts/Activate.ps1|
You don’t specifically need to activate an environment; activation just prepends the venv’s binary directory to your path, so that “python” invokes the venv’s Python interpreter and you can run installed scripts without having to use their full path. However, all scripts installed in a venv should be runnable without activating it, and run with the venv’s Python automatically.
You can deactivate a venv by typing “deactivate” in your shell. The exact mechanism is platform-specific: for example, the Bash activation script defines a “deactivate” function, whereas on Windows there are separate scripts called deactivate.bat and Deactivate.ps1 which are installed when the venv is created.
A virtual environment (also called a venv) is a Python environment such that the Python interpreter, libraries and scripts installed into it are isolated from those installed in other virtual environments, and (by default) any libraries installed in a “system” Python, i.e. one which is installed as part of your operating system.
A venv is a directory tree which contains Python executable files and other files which indicate that it is a venv.
Common installation tools such as Distribute and pip work as expected with venvs - i.e. when a venv is active, they install Python packages into the venv without needing to be told to do so explicitly. Of course, you need to install them into the venv first: this could be done by running distribute_setup.py with the venv activated, followed by running easy_install pip. Alternatively, you could download the source tarballs and run python setup.py install after unpacking, with the venv activated.
When a venv is active (i.e. the venv’s Python interpreter is running), the attributes sys.prefix and sys.exec_prefix point to the base directory of the venv, whereas sys.base_prefix and sys.base_exec_prefix point to the non-venv Python installation which was used to create the venv. If a venv is not active, then sys.prefix is the same as sys.base_prefix and sys.exec_prefix is the same as sys.base_exec_prefix (they all point to a non-venv Python installation).
When a venv is active, any options that change the installation path will be ignored from all distutils configuration files to prevent projects being inadvertently installed outside of the virtual environment.
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 EnvBuilder class.
The EnvBuilder class accepts the following keyword arguments on instantiation:
Creators of third-party virtual environment tools will be free to use the provided EnvBuilder class as a base class.
The returned env-builder is an object which has a method, create:
This method 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.
The create method of the EnvBuilder class illustrates the hooks available for subclass customization:
def create(self, env_dir): """ Create a virtualized Python environment in a directory. env_dir is the target directory to create an environment in. """ env_dir = os.path.abspath(env_dir) context = self.create_directories(env_dir) self.create_configuration(context) self.setup_python(context) self.setup_scripts(context) self.post_setup(context)
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.
Creates the pyvenv.cfg configuration file in the environment.
Creates a copy of the Python executable (and, under Windows, DLLs) in the environment.
Installs activation scripts appropriate to the platform into the virtual environment.
A placeholder method which can be overridden in third party implementations to pre-install packages in the virtual environment or perform other post-creation steps.
path is the path to a directory that 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 os.name are copied after some text replacement of placeholders:
There is also a module-level convenience function:
The following script shows how to extend EnvBuilder by implementing a subclass which installs Distribute and pip into a created venv:
import os import os.path from subprocess import Popen, PIPE import sys from threading import Thread from urllib.parse import urlparse from urllib.request import urlretrieve import venv class DistributeEnvBuilder(venv.EnvBuilder): """ This builder installs Distribute and pip so that you can pip or easy_install other packages into the created environment. :param nodist: If True, Distribute is not installed into the created environment. :param nopip: If True, pip is not installed into the created environment. :param progress: If Distribute or pip are installed, the progress of the installation can be monitored by passing a progress callable. If specified, it is called with two arguments: a string indicating some progress, and a context indicating where the string is coming from. The context argument can have one of three values: 'main', indicating that it is called from virtualize() itself, and 'stdout' and 'stderr', which are obtained by reading lines from the output streams of a subprocess which is used to install the app. If a callable is not specified, default progress information is output to sys.stderr. """ def __init__(self, *args, **kwargs): self.nodist = kwargs.pop('nodist', False) self.nopip = kwargs.pop('nopip', False) self.progress = kwargs.pop('progress', None) self.verbose = kwargs.pop('verbose', False) super().__init__(*args, **kwargs) def post_setup(self, context): """ Set up any packages which need to be pre-installed into the environment being created. :param context: The information for the environment creation request being processed. """ if not self.nodist: self.install_distribute(context) if not self.nopip: self.install_pip(context) def reader(self, stream, context): """ Read lines from a subprocess' output stream and either pass to a progress callable (if specified) or write progress information to sys.stderr. """ progress = self.progress while True: s = stream.readline() if not s: break if progress is not None: progress(s, context) else: if not self.verbose: sys.stderr.write('.') else: sys.stderr.write(s.decode('utf-8')) sys.stderr.flush() def install_script(self, context, name, url): _, _, path, _, _, _ = urlparse(url) fn = os.path.split(path)[-1] binpath = context.bin_path distpath = os.path.join(binpath, fn) # Download script into the env's binaries folder urlretrieve(url, distpath) progress = self.progress if progress is not None: progress('Installing %s' %name, 'main') else: sys.stderr.write('Installing %s ' % name) sys.stderr.flush() # Install in the env args = [context.env_exe, fn] p = Popen(args, stdout=PIPE, stderr=PIPE, cwd=binpath) t1 = Thread(target=self.reader, args=(p.stdout, 'stdout')) t1.start() t2 = Thread(target=self.reader, args=(p.stderr, 'stderr')) t2.start() p.wait() t1.join() t2.join() if progress is not None: progress('done.', 'main') else: sys.stderr.write('done.\n') # Clean up - no longer needed os.unlink(distpath) def install_distribute(self, context): """ Install Distribute in the environment. :param context: The information for the environment creation request being processed. """ url = 'http://python-distribute.org/distribute_setup.py' self.install_script(context, 'distribute', url) # clear up the distribute archive which gets downloaded pred = lambda o: o.startswith('distribute-') and o.endswith('.tar.gz') files = filter(pred, os.listdir(context.bin_path)) for f in files: f = os.path.join(context.bin_path, f) os.unlink(f) def install_pip(self, context): """ Install pip in the environment. :param context: The information for the environment creation request being processed. """ url = 'https://raw.github.com/pypa/pip/master/contrib/get-pip.py' self.install_script(context, 'pip', url) def main(args=None): compatible = True if sys.version_info < (3, 3): compatible = False elif not hasattr(sys, 'base_prefix'): compatible = False if not compatible: raise ValueError('This script is only for use with ' 'Python 3.3 or later') else: import argparse parser = argparse.ArgumentParser(prog=__name__, description='Creates virtual Python ' 'environments in one or ' 'more target ' 'directories.') parser.add_argument('dirs', metavar='ENV_DIR', nargs='+', help='A directory to create the environment in.') parser.add_argument('--no-distribute', default=False, action='store_true', dest='nodist', help="Don't install Distribute in the virtual " "environment.") parser.add_argument('--no-pip', default=False, action='store_true', dest='nopip', help="Don't install pip in the virtual " "environment.") parser.add_argument('--system-site-packages', default=False, action='store_true', dest='system_site', help='Give the virtual environment access to the ' 'system site-packages dir.') if os.name == 'nt': use_symlinks = False else: use_symlinks = True parser.add_argument('--symlinks', default=use_symlinks, action='store_true', dest='symlinks', help='Try to use symlinks rather than copies, ' 'when symlinks are not the default for ' 'the platform.') parser.add_argument('--clear', default=False, action='store_true', dest='clear', help='Delete the contents of the ' 'environment directory if it ' 'already exists, before ' 'environment creation.') parser.add_argument('--upgrade', default=False, action='store_true', dest='upgrade', help='Upgrade the environment ' 'directory to use this version ' 'of Python, assuming Python ' 'has been upgraded in-place.') parser.add_argument('--verbose', default=False, action='store_true', dest='verbose', help='Display the output ' 'from the scripts which ' 'install Distribute and pip.') options = parser.parse_args(args) if options.upgrade and options.clear: raise ValueError('you cannot supply --upgrade and --clear together.') builder = DistributeEnvBuilder(system_site_packages=options.system_site, clear=options.clear, symlinks=options.symlinks, upgrade=options.upgrade, nodist=options.nodist, nopip=options.nopip, verbose=options.verbose) for d in options.dirs: builder.create(d) if __name__ == '__main__': rc = 1 try: main() rc = 0 except Exception as e: print('Error: %s' % e, file=sys.stderr) sys.exit(rc)
This script is also available for download online.