Creating virtual environments¶

Virtual environments are created by executing thevenvmodule:

python-mvenv/path/to/new/virtual/environment

This creates the target directory (including parent directories as needed)and places apyvenv.cfg file in it with ahome keypointing to the Python installation from which the command was run.It also creates abin (orScripts on Windows) subdirectorycontaining a copy or symlink of the Python executable(as appropriate for the platform or arguments used at environment creation time).It also creates alib/pythonX.Y/site-packages subdirectory(on Windows, this isLib\site-packages).If an existing directory is specified, it will be re-used.

On Windows, invoke thevenv command as follows:

PS>python-mvenvC:\path\to\new\virtual\environment

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]            [--without-scm-ignore-files]            ENV_DIR [ENV_DIR ...]Creates virtual Python environments in one or more target directories.Once an environment has been created, you may wish to activate it, e.g. bysourcing an activate script in its bin directory.

ENV_DIR¶

A required argument specifying the directory to create the environment in.

--system-site-packages¶

Give the virtual environment access to the system site-packages directory.

--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) to the latest version in PyPI.

--without-scm-ignore-files¶

Skips adding SCM ignore files to the environment directory (Git is supported by default).

PSC:\>Set-ExecutionPolicy-ExecutionPolicyRemoteSigned-ScopeCurrentUser

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,#!/<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 install manager 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,*,scm_ignore_files=frozenset())¶

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

• scm_ignore_files – Create ignore files based for the specified sourcecontrol managers (SCM) in the iterable. Support is defined by having amethod namedcreate_{scm}_ignore_file. The only value supported bydefault is"git" viacreate_git_ignore_file().

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

Changed in version 3.13:Added thescm_ignore_files parameter

EnvBuilder may be used as a base class.

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.

Changed in version 3.12:The attributelib_path was added to the context, and the contextobject was documented.

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)in the environment. This is done by shelling out to thepip executable in the environment.

Added in version 3.9.

Changed in version 3.12:setuptools is no longer a core venv dependency.

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.

install_scripts(context,path)¶

This method 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 subdirectoriescommon,posix,nt; each containing scripts destined for thebin directory in the environment. The contents ofcommon 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).

create_git_ignore_file(context)¶

Creates a.gitignore file within the virtual environment that causesthe entire directory to be ignored by the Git source control manager.

Added in version 3.13.

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.

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,*,scm_ignore_files=frozenset())¶

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

Added 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

Changed in version 3.13:Added thescm_ignore_files 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://bootstrap.pypa.io/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):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.