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GeezerVison is showing docs.python.org — venv — Creation of virtual environments
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Platform |
Shell |
Command to activate virtual environment |
|---|---|---|
POSIX |
bash/zsh |
|
fish |
|
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csh/tcsh |
|
|
pwsh |
|
|
Windows |
cmd.exe |
|
PowerShell |
|
Added in version 3.4: fish and csh activation scripts.
Added in version 3.8: PowerShell activation scripts installed under POSIX for PowerShell Core support.
You don’t specifically need to activate a virtual environment, as you can just specify the full path to that environment’s Python interpreter when invoking Python. Furthermore, all scripts installed in the environment should be runnable without activating it.
In order to achieve this, scripts installed into virtual environments have
a “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 the
value of PATH. On Windows, “shebang” line processing is supported if
you have the Python install manager installed. Thus, double-clicking an installed
script in a Windows Explorer window should run it with the correct interpreter
without the environment needing to be activated or on the PATH.
When a virtual environment has been activated, the VIRTUAL_ENV
environment 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 determine
whether a virtual environment is being used.
Warning
Because scripts installed in environments should not expect the
environment to be activated, their shebang lines contain the absolute paths
to their environment’s interpreters. Because of this, environments are
inherently non-portable, in the general case. You should always have a
simple means of recreating an environment (for example, if you have a
requirements file requirements.txt, you can invoke pip install -r
requirements.txt using the environment’s pip to install all of the
packages needed by the environment). If for any reason you need to move the
environment to a new location, you should recreate it at the desired
location and delete the one at the old location. If you move an environment
because you moved a parent directory of it, you should recreate the
environment in its new location. Otherwise, software installed into the
environment may not work as expected.
You can deactivate a virtual environment by typing deactivate in your shell.
The exact mechanism is platform-specific and is an internal implementation
detail (typically, a script or shell function will be used).
The high-level method described above makes use of a simple API which provides
mechanisms for third-party virtual environment creators to customize environment
creation according to their needs, the EnvBuilder class.
The EnvBuilder class accepts the following keyword arguments on
instantiation:
system_site_packages – a boolean value indicating that the system Python
site-packages should be available to the environment (defaults to False).
clear – a boolean value which, if true, will delete the contents of any existing target directory, before creating the environment.
symlinks – a boolean value indicating whether to attempt to symlink the Python binary rather than copying.
upgrade – a boolean value which, if true, will upgrade an existing
environment with the running Python - for use when that Python has been
upgraded in-place (defaults to False).
with_pip – a boolean value which, if true, ensures pip is
installed in the virtual environment. This uses ensurepip with
the --default-pip option.
prompt – a string to be used after virtual environment is activated
(defaults to None which means directory name of the environment would
be used). If the special string "." is provided, the basename of the
current 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 source
control managers (SCM) in the iterable. Support is defined by having a
method named create_{scm}_ignore_file. The only value supported by
default is "git" via create_git_ignore_file().
Changed in version 3.4: Added the with_pip parameter
Changed in version 3.6: Added the prompt parameter
Changed in version 3.9: Added the upgrade_deps parameter
Changed in version 3.13: Added the scm_ignore_files parameter
EnvBuilder may be used as a base class.
Create a virtual environment by specifying the target directory
(absolute or relative to the current directory) which is to contain the
virtual environment. The create method will either create the
environment in the specified directory, or raise an appropriate
exception.
The create method of the EnvBuilder class illustrates the
hooks available for subclass customization:
def create(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() and post_setup() can be overridden.
Creates the environment directory and all necessary subdirectories that
don’t already exist, and returns a context object. This context object
is just a holder for attributes (such as paths) for use by the other
methods. If the EnvBuilder is created with the arg
clear=True, contents of the environment directory will be cleared
and then all necessary subdirectories will be recreated.
The returned context object is a types.SimpleNamespace with the
following attributes:
env_dir - The location of the virtual environment. Used for
__VENV_DIR__ in activation scripts (see install_scripts()).
env_name - The name of the virtual environment. Used for
__VENV_NAME__ in activation scripts (see install_scripts()).
prompt - The prompt to be used by the activation scripts. Used for
__VENV_PROMPT__ in activation scripts (see install_scripts()).
executable - The underlying Python executable used by the virtual
environment. This takes into account the case where a virtual environment
is created from another virtual environment.
inc_path - The include path for the virtual environment.
lib_path - The purelib path for the virtual environment.
platlib_path - The platlib 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 virtual
environment location. Used for __VENV_BIN_NAME__ in activation
scripts (see install_scripts()).
env_exe - The name of the Python interpreter in the virtual
environment. Used for __VENV_PYTHON__ in activation scripts
(see install_scripts()).
env_exec_cmd - The name of the Python interpreter, taking into
account filesystem redirections. This can be used to run Python in
the virtual environment.
Changed in version 3.11: The venv sysconfig installation scheme is used to construct the paths of the created directories.
Changed in version 3.12: The attribute lib_path was added to the context, and the context
object was documented.
Changed in version 3.15: The attribute platlib_path was added to the context.
Creates the pyvenv.cfg configuration file in the environment.
Creates a copy or symlink to the Python executable in the environment.
On POSIX systems, if a specific executable python3.x was used,
symlinks to python and python3 will be created pointing to that
executable, unless files with those names already exist.
Installs activation scripts appropriate to the platform into the virtual environment.
Upgrades the core venv dependency packages (currently pip)
in the environment. This is done by shelling out to the
pip executable in the environment.
Added in version 3.9.
Changed in version 3.12: setuptools is no longer a core venv dependency.
A placeholder method which can be overridden in third party implementations to pre-install packages in the virtual environment or perform other post-creation steps.
This method can be
called from setup_scripts() or post_setup() in subclasses to
assist 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
bin directory in the environment. The contents of common and the
directory corresponding to os.name are copied after some text
replacement of placeholders:
__VENV_DIR__ is replaced with the absolute path of the environment
directory.
__VENV_NAME__ is replaced with the environment name (final path
segment of environment directory).
__VENV_PROMPT__ is replaced with the prompt (the environment
name surrounded by parentheses and with a following space)
__VENV_BIN_NAME__ is replaced with the name of the bin directory
(either bin or Scripts).
__VENV_PYTHON__ is replaced with the absolute path of the
environment’s executable.
The directories are allowed to exist (for when an existing environment is being upgraded).
Creates a .gitignore file within the virtual environment that causes
the 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 for python[w].exe instead of
copying the actual binaries. In 3.7.2 only setup_python() does
nothing 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 of setup_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:
Create an EnvBuilder with the given keyword arguments, and call its
create() method with the env_dir argument.
Added in version 3.3.
Changed in version 3.4: Added the with_pip parameter
Changed in version 3.6: Added the prompt parameter
Changed in version 3.9: Added the upgrade_deps parameter
Changed in version 3.13: Added the scm_ignore_files parameter
EnvBuilder¶The following script shows how to extend EnvBuilder by implementing a
subclass which installs setuptools and pip into a created virtual environment:
import os import os.path from subprocess import Popen, PIPE import sys from threading import Thread from urllib.parse import urlsplit from urllib.request import urlretrieve import venv class ExtendedEnvBuilder(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 install_script() 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 virtual environment being created. :param context: The information for the virtual 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, _, _ = urlsplit(url) fn = os.path.split(path)[-1] binpath = context.bin_path distpath = os.path.join(binpath, fn) # Download script into the virtual environment'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 virtual environment 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 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 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 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) def main(args=None): 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 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.') 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 ' '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) 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 download online.