28.3.venv — Creation of virtual environments

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.

Note

Thepyvenv script has been deprecated as of Python 3.6 in favor of usingpython3-mvenv to help prevent any potential confusion as to whichPython interpreter a virtual environment will be based on.

28.3.1.Creating virtual environments

Creation ofvirtual environments is done by executing thecommandvenv:

python3-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. It also creates abin (orScripts on Windows) subdirectorycontaining a copy/symlink of the Python binary/binaries (as appropriate for theplatform or arguments used at environment creation time). It also creates an(initially empty)lib/pythonX.Y/site-packages subdirectory(on Windows, this isLib\site-packages). If an existingdirectory is specified, it will be re-used.

Deprecated since version 3.6:pyvenv was the recommended tool for creating virtual environments forPython 3.3 and 3.4, and isdeprecated in Python 3.6.

Changed in version 3.5:The use ofvenv is now recommended for creating virtual environments.

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]            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)Once an environment has been created, you may wish to activate it, e.g. bysourcing an activate script in its bin directory.

Changed in version 3.4:Installs pip by default, added the--without-pip and--copiesoptions

Changed in version 3.4:In earlier versions, if the target directory already existed, an error wasraised, unless the--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.

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.

Once a virtual environment has been created, it can be “activated” using ascript in the virtual environment’s binary directory. The invocation of thescript is platform-specific (<venv> must be replaced by the path of thedirectory containing the virtual environment):

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

Windows

cmd.exe

C:\> <venv>\Scripts\activate.bat

PowerShell

PS C:\> <venv>\Scripts\Activate.ps1

You don’t specificallyneed to activate an environment; activation justprepends the virtual environment’s binary directory to your path, so that“python” invokes the virtual environment’s Python interpreter and you can runinstalled scripts without having to use their full path. However, all scriptsinstalled in a virtual environment should be runnable without activating it,and run with the virtual environment’s Python automatically.

You can deactivate a virtual environment by typing “deactivate” in your shell.The exact mechanism is platform-specific: for example, the Bash activationscript defines a “deactivate” function, whereas on Windows there are separatescripts calleddeactivate.bat andDeactivate.ps1 which are installedwhen the virtual environment is created.

New in version 3.4:fish andcsh activation scripts.

Note

A virtual environment is a Python environment such that the Pythoninterpreter, libraries and scripts installed into it are isolated from thoseinstalled in other virtual environments, and (by default) any librariesinstalled in a “system” Python, i.e., one which is installed as part of youroperating system.

A virtual environment is a directory tree which contains Python executablefiles and other files which indicate that it is a virtual environment.

Common installation tools such asSetuptools andpip work asexpected with virtual environments. In other words, when a virtualenvironment is active, they install Python packages into the virtualenvironment without needing to be told to do so explicitly.

When a virtual environment is active (i.e., the virtual environment’s Pythoninterpreter is running), the attributessys.prefix andsys.exec_prefix point to the base directory of the virtualenvironment, whereassys.base_prefix andsys.base_exec_prefix point to the non-virtual environment Pythoninstallation which was used to create the virtual environment. If a virtualenvironment is not active, thensys.prefix is the same assys.base_prefix andsys.exec_prefix is the same assys.base_exec_prefix (they all point to a non-virtual environmentPython installation).

When a virtual environment is active, any options that change theinstallation path will be ignored from all distutils configuration files toprevent projects being inadvertently installed outside of the virtualenvironment.

When working in a command shell, users can make a virtual environment activeby running anactivate script in the virtual environment’s executablesdirectory (the precise filename is shell-dependent), which prepends thevirtual environment’s directory for executables to thePATH environmentvariable for the running shell. There should be no need in othercircumstances to activate a virtual environment—scripts installed intovirtual environments have a “shebang” line which points to the virtualenvironment’s Python interpreter. This means that the script will run withthat interpreter regardless of the value ofPATH. On Windows, “shebang”line processing is supported if you have the Python Launcher for Windowsinstalled (this was added to Python in 3.3 - seePEP 397 for moredetails). Thus, double-clicking an installed script in a Windows Explorerwindow should run the script with the correct interpreter without thereneeding to be any reference to its virtual environment inPATH.

28.3.2.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)

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 (and any necessary DLLs or other binaries,e.g.pythonw.exe), 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).

Changed in version 3.4:Added thewith_pip parameter

New in version 3.6:Added theprompt 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)

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.

ensure_directories(env_dir)

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.

create_configuration(context)

Creates thepyvenv.cfg configuration file in the environment.

setup_python(context)

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.

setup_scripts(context)

Installs activation scripts appropriate to the platform into the virtualenvironment.

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.

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)

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

Changed in version 3.4:Added thewith_pip parameter

28.3.3.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://raw.github.com/pypa/pip/master/contrib/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.