Creating virtual environments¶

Creation ofvirtual environments is done by executing thecommandvenv:

python-mvenv/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 from which the commandwas run (a common name for the target directory is.venv). It also createsabin (orScripts on Windows) subdirectory containing a copy/symlinkof the Python binary/binaries (as appropriate for the platform or argumentsused at environment creation time). It also creates an (initially empty)lib/pythonX.Y/site-packages subdirectory (on Windows, this isLib\site-packages). If an existing directory is specified, it will bere-used.

On Windows, invoke thevenv command as follows:

c:\>c:\Python35\python -m venv c:\path\to\myenv

Alternatively, if you configured thePATH andPATHEXT variables foryourPython installation:

c:\>python -m venv c:\path\to\myenv

The command, if run with-h, will show the available options:

usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear]            [--upgrade] [--without-pip] [--prompt PROMPT] [--upgrade-deps]            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 the virtual environment access to the system                        site-packages dir.  --symlinks            Try to use symlinks rather than copies, when symlinks                        are not the default for the platform.  --copies              Try to use copies rather than symlinks, even when                        symlinks are the default for the platform.  --clear               Delete the contents of the environment directory if it                        already exists, before environment creation.  --upgrade             Upgrade the environment directory to use this version                        of Python, assuming Python has been upgraded in-place.  --without-pip         Skips installing or upgrading pip in the virtual                        environment (pip is bootstrapped by default)  --prompt PROMPT       Provides an alternative prompt prefix for this                        environment.  --upgrade-deps        Upgrade core dependencies: pip setuptools to the                        latest version in PyPIOnce an environment has been created, you may wish to activate it, e.g. bysourcing an activate script in its bin directory.

The createdpyvenv.cfg file also includes theinclude-system-site-packages key, set totrue ifvenv isrun with the--system-site-packages option,false otherwise.

Unless the--without-pip option is given,ensurepip will beinvoked to bootstrappip into the virtual environment.

Multiple paths can be given tovenv, in which case an identical virtualenvironment will be created, according to the given options, at each providedpath.

How venvs work¶

When a Python interpreter is running from a virtual environment,sys.prefix andsys.exec_prefixpoint to the directories of the virtual environment,whereassys.base_prefix andsys.base_exec_prefixpoint to those of the base Python used to create the environment.It is sufficient to checksys.prefix!=sys.base_prefix to determine if the current interpreter isrunning from a virtual environment.

A virtual environment may be “activated” using a script in its binary directory(bin on POSIX;Scripts on Windows).This will prepend that directory to yourPATH, so that runningpython will invoke the environment’s Python interpreterand you can run installed scripts without having to use their full path.The invocation of the activation script is platform-specific(<venv> must be replaced by the path to the directorycontaining the virtual environment):

You don’t specificallyneed to activate a virtual environment,as you can just specify the full path to that environment’sPython interpreter when invoking Python.Furthermore, all scripts installed in the environmentshould be runnable without activating it.

In order to achieve this, scripts installed into virtual environments havea “shebang” line which points to the environment’s Python interpreter,i.e.#!/<path-to-venv>/bin/python.This means that the script will run with that interpreter regardless of thevalue ofPATH. On Windows, “shebang” line processing is supported ifyou have thePython Launcher for Windows installed. Thus, double-clicking an installedscript in a Windows Explorer window should run it with the correct interpreterwithout the environment needing to be activated or on thePATH.

When a virtual environment has been activated, theVIRTUAL_ENVenvironment variable is set to the path of the environment.Since explicitly activating a virtual environment is not required to use it,VIRTUAL_ENV cannot be relied upon to determinewhether a virtual environment is being used.

You can deactivate a virtual environment by typingdeactivate in your shell.The exact mechanism is platform-specific and is an internal implementationdetail (typically, a script or shell function will be used).

API¶

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.

classvenv.EnvBuilder(system_site_packages=False,clear=False,symlinks=False,upgrade=False,with_pip=False,prompt=None,upgrade_deps=False)¶

TheEnvBuilder class accepts the following keyword arguments oninstantiation:

• system_site_packages – a Boolean value indicating that the system Pythonsite-packages should be available to the environment (defaults toFalse).

• clear – a Boolean value which, if true, will delete the contents ofany existing target directory, before creating the environment.

• symlinks – a Boolean value indicating whether to attempt to symlink thePython binary rather than copying.

• upgrade – a Boolean value which, if true, will upgrade an existingenvironment with the running Python - for use when that Python has beenupgraded in-place (defaults toFalse).

• with_pip – a Boolean value which, if true, ensures pip isinstalled in the virtual environment. This usesensurepip withthe--default-pip option.

• prompt – a String to be used after virtual environment is activated(defaults toNone which means directory name of the environment wouldbe used). If the special string"." is provided, the basename of thecurrent directory is used as the prompt.

• upgrade_deps – Update the base venv modules to the latest on PyPI

Changed in version 3.4:Added thewith_pip parameter

Changed in version 3.6:Added theprompt parameter

Changed in version 3.9:Added theupgrade_deps parameter

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:

create(env_dir)¶

Create a virtual environment by specifying the target directory(absolute or relative to the current 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 thehooks available 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.

ensure_directories(env_dir)¶

Creates the environment directory and all necessary subdirectories thatdon’t already exist, and returns a context object. This context objectis just a holder for attributes (such as paths) for use by the othermethods. If theEnvBuilder is created with the argclear=True, contents of the environment directory will be clearedand then all necessary subdirectories will be recreated.

The returned context object is atypes.SimpleNamespace with thefollowing attributes:

• env_dir - The location of the virtual environment. Used for__VENV_DIR__ in activation scripts (seeinstall_scripts()).

• env_name - The name of the virtual environment. Used for__VENV_NAME__ in activation scripts (seeinstall_scripts()).

• prompt - The prompt to be used by the activation scripts. Used for__VENV_PROMPT__ in activation scripts (seeinstall_scripts()).

• executable - The underlying Python executable used by the virtualenvironment. This takes into account the case where a virtual environmentis created from another virtual environment.

• inc_path - The include path for the virtual environment.

• lib_path - The purelib path for the virtual environment.

• bin_path - The script path for the virtual environment.

• bin_name - The name of the script path relative to the virtualenvironment location. Used for__VENV_BIN_NAME__ in activationscripts (seeinstall_scripts()).

• env_exe - The name of the Python interpreter in the virtualenvironment. Used for__VENV_PYTHON__ in activation scripts(seeinstall_scripts()).

• env_exec_cmd - The name of the Python interpreter, taking intoaccount filesystem redirections. This can be used to run Python inthe virtual environment.

Changed in version 3.11:Thevenvsysconfig installation schemeis used to construct the paths of the created directories.

create_configuration(context)¶

Creates thepyvenv.cfg configuration file in the environment.

setup_python(context)¶

Creates a copy or symlink to the Python executable in the environment.On POSIX systems, if a specific executablepython3.x was used,symlinks topython andpython3 will be created pointing to thatexecutable, unless files with those names already exist.

setup_scripts(context)¶

Installs activation scripts appropriate to the platform into the virtualenvironment.

upgrade_dependencies(context)¶

Upgrades the core venv dependency packages (currentlypip andsetuptools) in the environment. This is done by shelling out to thepip executable in the environment.

New in version 3.9.

post_setup(context)¶

A placeholder method which can be overridden in third partyimplementations to pre-install packages in the virtual environment orperform other post-creation steps.

Changed in version 3.7.2:Windows now uses redirector scripts forpython[w].exe instead ofcopying the actual binaries. In 3.7.2 onlysetup_python() doesnothing unless running from a build in the source tree.

Changed in version 3.7.3:Windows copies the redirector scripts as part ofsetup_python()instead ofsetup_scripts(). This was not the case in 3.7.2.When using symlinks, the original executables will be linked.

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.

install_scripts(context,path)¶

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:

• __VENV_DIR__ is replaced with the absolute path of the environmentdirectory.

• __VENV_NAME__ is replaced with the environment name (final pathsegment of environment directory).

• __VENV_PROMPT__ is replaced with the prompt (the environmentname surrounded by parentheses and with a following space)

• __VENV_BIN_NAME__ is replaced with the name of the bin directory(eitherbin orScripts).

• __VENV_PYTHON__ is replaced with the absolute path of theenvironment’s executable.

The directories are allowed to exist (for when an existing environmentis being upgraded).

There is also a module-level convenience function:

venv.create(env_dir,system_site_packages=False,clear=False,symlinks=False,with_pip=False,prompt=None,upgrade_deps=False)¶

Create anEnvBuilder with the given keyword arguments, and call itscreate() method with theenv_dir argument.

New in version 3.3.

Changed in version 3.4:Added thewith_pip parameter

Changed in version 3.6:Added theprompt parameter

Changed in version 3.9:Added theupgrade_deps parameter

An example of extendingEnvBuilder¶

The following script shows how to extendEnvBuilder by implementing asubclass which installs setuptools and pip into a created virtual environment:

importosimportos.pathfromsubprocessimportPopen,PIPEimportsysfromthreadingimportThreadfromurllib.parseimporturlparsefromurllib.requestimporturlretrieveimportvenvclassExtendedEnvBuilder(venv.EnvBuilder):"""    This builder installs setuptools and pip so that you can pip or    easy_install other packages into the created virtual environment.    :param nodist: If true, setuptools and pip are not installed into the                   created virtual environment.    :param nopip: If true, pip is not installed into the created                  virtual 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)defpost_setup(self,context):"""        Set up any packages which need to be pre-installed into the        virtual environment being created.        :param context: The information for the virtual environment                        creation request being processed.        """os.environ['VIRTUAL_ENV']=context.env_dirifnotself.nodist:self.install_setuptools(context)# Can't install pip without setuptoolsifnotself.nopipandnotself.nodist:self.install_pip(context)defreader(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.progresswhileTrue:s=stream.readline()ifnots:breakifprogressisnotNone:progress(s,context)else:ifnotself.verbose:sys.stderr.write('.')else:sys.stderr.write(s.decode('utf-8'))sys.stderr.flush()stream.close()definstall_script(self,context,name,url):_,_,path,_,_,_=urlparse(url)fn=os.path.split(path)[-1]binpath=context.bin_pathdistpath=os.path.join(binpath,fn)# Download script into the virtual environment's binaries folderurlretrieve(url,distpath)progress=self.progressifself.verbose:term='\n'else:term=''ifprogressisnotNone:progress('Installing%s ...%s'%(name,term),'main')else:sys.stderr.write('Installing%s ...%s'%(name,term))sys.stderr.flush()# Install in the virtual environmentargs=[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()ifprogressisnotNone:progress('done.','main')else:sys.stderr.write('done.\n')# Clean up - no longer neededos.unlink(distpath)definstall_setuptools(self,context):"""        Install setuptools in the virtual environment.        :param context: The information for the virtual 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 downloadedpred=lambdao:o.startswith('setuptools-')ando.endswith('.tar.gz')files=filter(pred,os.listdir(context.bin_path))forfinfiles:f=os.path.join(context.bin_path,f)os.unlink(f)definstall_pip(self,context):"""        Install pip in the virtual environment.        :param context: The information for the virtual environment                        creation request being processed.        """url='https://bootstrap.pypa.io/get-pip.py'self.install_script(context,'pip',url)defmain(args=None):compatible=Trueifsys.version_info<(3,3):compatible=Falseelifnothasattr(sys,'base_prefix'):compatible=Falseifnotcompatible:raiseValueError('This script is only for use with ''Python 3.3 or later')else:importargparseparser=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 in which to create the ''virtual environment.')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.')ifos.name=='nt':use_symlinks=Falseelse:use_symlinks=Trueparser.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 ''virtual environment ''directory if it already ''exists, before virtual ''environment creation.')parser.add_argument('--upgrade',default=False,action='store_true',dest='upgrade',help='Upgrade the virtual ''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)ifoptions.upgradeandoptions.clear:raiseValueError('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)fordinoptions.dirs:builder.create(d)if__name__=='__main__':rc=1try:main()rc=0exceptExceptionase:print('Error:%s'%e,file=sys.stderr)sys.exit(rc)

This script is also available for downloadonline.