New in version 3.3.
Source code:Lib/venv
Thevenv module provides support for creating lightweight “virtualenvironments” with their own site directories, optionally isolated from systemsite directories. Each virtual environment has its own Python binary (allowingcreation of environments with various Python versions) and can have its ownindependent set of installed Python packages in its site directories.
SeePEP 405 for more information about Python virtual environments.
Creation ofvirtual environments is done by executing thepyvenv script:
pyvenv/path/to/new/virtual/environment
Running this command creates the target directory (creating any parentdirectories that don’t exist already) and places apyvenv.cfg file in itwith ahome key pointing to the Python installation the command was runfrom. It also creates abin (orScripts on Windows) subdirectorycontaining a copy of thepython binary (or binaries, in the case ofWindows). It also creates an (initially empty)lib/pythonX.Y/site-packagessubdirectory (on Windows, this isLib\site-packages).
On Windows, you may have to invoke thepyvenv script as follows, if youdon’t have the relevant PATH and PATHEXT settings:
c:\Temp>c:\Python33\python c:\Python33\Tools\Scripts\pyvenv.py myenv
or equivalently:
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, unlessthe--clear or--upgrade option was provided.
The createdpyvenv.cfg file also includes theinclude-system-site-packages key, set totrue ifvenv isrun with the--system-site-packages option,false otherwise.
Multiple paths can be given topyvenv, in which case an identicalvirtualenv will be created, according to the given options, at eachprovided path.
Once a venv has been created, it can be “activated” using a script in thevenv’s binary directory. The invocation of the script is platform-specific: ona Posix platform, you would typically do:
$ source <venv>/bin/activate
whereas on Windows, you might do:
C:\> <venv>/Scripts/activate
if you are using thecmd.exe shell, or perhaps:
PS C:\> <venv>/Scripts/Activate.ps1
if you use PowerShell.
You don’t specificallyneed to activate an environment; activation justprepends the venv’s binary directory to your path, so that “python” invokes thevenv’s Python interpreter and you can run installed scripts without having touse their full path. However, all scripts installed in a venv should berunnable without activating it, and run with the venv’s Python automatically.
You can deactivate a venv by typing “deactivate” in your shell. The exactmechanism is platform-specific: for example, the Bash activation script definesa “deactivate” function, whereas on Windows there are separate scripts calleddeactivate.bat andDeactivate.ps1 which are installed when the venv iscreated.
Note
A virtual environment (also called avenv) is a Pythonenvironment such that the Python interpreter, libraries and scriptsinstalled into it are isolated from those installed in other virtualenvironments, 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 andother files which indicate that it is a venv.
Common installation tools such asSetuptools andpip work asexpected with venvs - i.e. when a venv is active, they install Pythonpackages 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 bedone by runningez_setup.py with the venv activated,followed by runningeasy_installpip. Alternatively, you could downloadthe source tarballs and runpythonsetup.pyinstall after unpacking,with the venv activated.
When a venv is active (i.e. the venv’s Python interpreter is running), theattributessys.prefix andsys.exec_prefix point to the basedirectory of the venv, whereassys.base_prefix andsys.base_exec_prefix point to the non-venv Python installationwhich was used to create the venv. If a venv is not active, thensys.prefix is the same assys.base_prefix andsys.exec_prefix is the same assys.base_exec_prefix (theyall point to a non-venv Python installation).
When a venv is active, any options that change the installation path will beignored from all distutils configuration files to prevent projects beinginadvertently installed outside of the virtual environment.
When working in a command shell, users can make a venv active by running anactivate script in the venv’s executables directory (the precise filenameis shell-dependent), which prepends the venv’s directory for executables tothePATH environment variable for the running shell. There should be noneed in other circumstances to activate a venv – scripts installed intovenvs have a shebang line which points to the venv’s Python interpreter. Thismeans that the script will run with that interpreter regardless of the valueofPATH. On Windows, shebang line processing is supported if you have thePython Launcher for Windows installed (this was added to Python in 3.3 - seePEP 397 for more details). Thus, double-clicking an installed script ina Windows Explorer window should run the script with the correct interpreterwithout there needing to be any reference to its venv inPATH.
The high-level method described above makes use of a simple API which providesmechanisms for third-party virtual environment creators to customize environmentcreation according to their needs, theEnvBuilder class.
TheEnvBuilder class accepts the following keyword arguments oninstantiation:
Creators of third-party virtual environment tools will be free to use theprovidedEnvBuilder 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 tothe current directory) of the target directory which is to contain thevirtual environment. Thecreate method will either create theenvironment in the specified directory, or raise an appropriateexception.
Thecreate method of theEnvBuilder class illustrates the hooksavailable for subclass customization:
defcreate(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.ensure_directories(env_dir)self.create_configuration(context)self.setup_python(context)self.setup_scripts(context)self.post_setup(context)
Each of the methodsensure_directories(),create_configuration(),setup_python(),setup_scripts() andpost_setup() can be overridden.
Creates the environment directory and all necessary directories, andreturns a context object. This is just a holder for attributes (such aspaths), for use by the other methods. The directories are allowed toexist already, as long as eitherclear orupgrade werespecified to allow operating on an existing environment directory.
Creates thepyvenv.cfg configuration file in the environment.
Creates a copy of the Python executable (and, under Windows, DLLs) inthe environment. On a POSIX system, if a specific executablepython3.x was used, symlinks topython andpython3 will becreated pointing to that executable, unless files with those namesalready exist.
Installs activation scripts appropriate to the platform into the virtualenvironment.
A placeholder method which can be overridden in third partyimplementations to pre-install packages in the virtual environment orperform other post-creation steps.
In addition,EnvBuilder provides this utility method that can becalled fromsetup_scripts() orpost_setup() in subclasses toassist in installing custom scripts into the virtual environment.
path is the path to a directory that should contain subdirectories“common”, “posix”, “nt”, each containing scripts destined for the bindirectory in the environment. The contents of “common” and thedirectory corresponding toos.name are copied after some textreplacement of placeholders:
The directories are allowed to exist (for when an existing environmentis being upgraded).
There is also a module-level convenience function:
Create anEnvBuilder with the given keyword arguments, and call itscreate() method with theenv_dir argument.
The following script shows how to extendEnvBuilder by implementing asubclass which installs setuptools and pip into a created venv:
import osimport os.pathfrom subprocess import Popen, PIPEimport sysfrom threading import Threadfrom urllib.parse import urlparsefrom urllib.request import urlretrieveimport venvclass ExtendedEnvBuilder(venv.EnvBuilder): """ This builder installs setuptools and pip so that you can pip or easy_install other packages into the created environment. :param nodist: If True, setuptools and pip are not installed into the created environment. :param nopip: If True, pip is not installed into the created environment. :param progress: If setuptools 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. """ os.environ['VIRTUAL_ENV'] = context.env_dir if not self.nodist: self.install_setuptools(context) # Can't install pip without setuptools if not self.nopip and not self.nodist: 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() stream.close() 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 self.verbose: term = '\n' else: term = '' if progress is not None: progress('Installing %s ...%s' % (name, term), 'main') else: sys.stderr.write('Installing %s ...%s' % (name, term)) 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_setuptools(self, context): """ Install setuptools in the environment. :param context: The information for the environment creation request being processed. """ url = 'https://bitbucket.org/pypa/setuptools/downloads/ez_setup.py' self.install_script(context, 'setuptools', url) # clear up the setuptools archive which gets downloaded pred = lambda o: o.startswith('setuptools-') 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-setuptools', default=False, action='store_true', dest='nodist', help="Don't install setuptools or pip 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 setuptools and pip.') options = parser.parse_args(args) if options.upgrade and options.clear: raise ValueError('you cannot supply --upgrade and --clear together.') builder = ExtendedEnvBuilder(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 downloadonline.
26.7.test — Regression tests package for Python
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