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 (whichmatches the version of the binary that was used to create this environment) andcan have its own independent set of installed Python packages in its sitedirectories.
SeePEP 405 for more information about Python virtual environments.
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 (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.
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] [--prompt PROMPT] 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.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.
Note
While symlinks are supported on Windows, they are not recommended. Ofparticular note is that double-clickingpython.exe in File Explorerwill resolve the symlink eagerly and ignore the virtual environment.
Note
On Microsoft Windows, it may be required to enable theActivate.ps1script by setting the execution policy for the user. You can do this byissuing the following PowerShell command:
PS C:> Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
SeeAbout Execution Policiesfor more information.
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 | |
PowerShell Core | $ <venv>/bin/Activate.ps1 | |
Windows | cmd.exe | C:\> <venv>\Scripts\activate.bat |
PowerShell | PS C:\> <venv>\Scripts\Activate.ps1 |
When a virtual environment is active, theVIRTUAL_ENV environmentvariable is set to the path of the virtual environment. This can be used tocheck if one is running inside a virtual environment.
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 and is an internal implementationdetail (typically a script or shell function will be used).
New in version 3.4:fish andcsh activation scripts.
New in version 3.8:PowerShell activation scripts installed under POSIX for PowerShell Coresupport.
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 alldistutils configurationfiles to prevent projects being inadvertently installed outside of thevirtual environment.
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 and command to use the file isshell-dependent), which prepends the virtual environment’s directory forexecutables to thePATH environment variable for the running shell. Thereshould be no need in other circumstances to activate a virtualenvironment; scripts installed into virtual environments have a “shebang”line which points to the virtual environment’s Python interpreter. This meansthat the script will run with that interpreter regardless of the value ofPATH. 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 in aWindows Explorer window should run the script with the correct interpreterwithout there needing to be any reference to its virtual environment inPATH.
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.
- class
venv.EnvBuilder(system_site_packages=False,clear=False,symlinks=False,upgrade=False,with_pip=False,prompt=None)¶ The
EnvBuilderclass 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 usesensurepipwiththe--default-pipoption.prompt– a String to be used after virtual environment is activated(defaults toNonewhich means directory name of the environment wouldbe used).
Changed in version 3.4:Added the
with_pipparameterNew in version 3.6:Added the
promptparameterCreators of third-party virtual environment tools will be free to use theprovided
EnvBuilderclass 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. The
createmethod will either create theenvironment in the specified directory, or raise an appropriateexception.The
createmethod of theEnvBuilderclass 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 methods
ensure_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 either
clearorupgradewerespecified to allow operating on an existing environment directory.
create_configuration(context)¶Creates the
pyvenv.cfgconfiguration 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 executable
python3.xwas used,symlinks topythonandpython3will 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.
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 for
python[w].exeinstead 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 of
setup_python()instead ofsetup_scripts(). This was not the case in 3.7.2.When using symlinks, the original executables will be linked.In addition,
EnvBuilderprovides 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 to
os.nameare 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(eitherbinorScripts).__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)¶Create an
EnvBuilderwith the given keyword arguments, and call itscreate()method with theenv_dir argument.New in version 3.3.
Changed in version 3.4:Added the
with_pipparameterChanged in version 3.6:Added the
promptparameter
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.