"venv" --- 建立虛擬環境
***********************

在 3.3 版被加入.

**原始碼：**Lib/venv/

======================================================================

The "venv" module supports creating lightweight "virtual
environments", each with their own independent set of Python packages
installed in their "site" directories. A virtual environment is
created on top of an existing Python installation, known as the
virtual environment's "base" Python, and may optionally be isolated
from the packages in the base environment, so only those explicitly
installed in the virtual environment are available.

When used from within a virtual environment, common installation tools
such as pip will install Python packages into a virtual environment
without needing to be told to do so explicitly.

A virtual environment is (amongst other things):

* Used to contain a specific Python interpreter and software libraries
  and binaries which are needed to support a project (library or
  application). These are by default isolated from software in other
  virtual environments and Python interpreters and libraries installed
  in the operating system.

* Contained in a directory, conventionally named ".venv" or "venv" in
  the project directory, or under a container directory for lots of
  virtual environments, such as "~/.virtualenvs".

* Not checked into source control systems such as Git.

* Considered as disposable -- it should be simple to delete and
  recreate it from scratch. You don't place any project code in the
  environment.

* Not considered as movable or copyable -- you just recreate the same
  environment in the target location.

更多關於 Python 虛擬環境的背景資訊請見 **PEP 405**。

也參考:

  Python Packaging User Guide: Creating and using virtual environments

Availability: not Emscripten, not WASI.

此模組在 WebAssembly 平台 "wasm32-emscripten" 和 "wasm32-wasi" 上無法
作用或無法使用。有關更多資訊，請參閱 WebAssembly 平台。


建立虛擬環境
============

Virtual environments are created by executing the "venv" module:

   python -m venv /path/to/new/virtual/environment

This creates the target directory (including parent directories as
needed) and places a "pyvenv.cfg" file in it with a "home" key
pointing to the Python installation from which the command was run. It
also creates a "bin" (or "Scripts" on Windows) subdirectory containing
a copy or symlink of the Python executable (as appropriate for the
platform or arguments used at environment creation time). It also
creates a "lib/pythonX.Y/site-packages" subdirectory (on Windows, this
is "Libsite-packages"). If an existing directory is specified, it will
be re-used.

在 3.5 版的變更: 目前建議使用 "venv" 來建立虛擬環境。

Deprecated since version 3.6, removed in version 3.8: **pyvenv** was
the recommended tool for creating virtual environments for Python 3.3
and 3.4, and replaced in 3.5 by executing "venv" directly.

On Windows, invoke the "venv" command as follows:

   PS> python -m venv C:\path\to\new\virtual\environment

如果使用 "-h" 選項執行該命令，將會顯示可用的選項：

   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.

   options:
     -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) to the latest
                           version in PyPI

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

在 3.4 版的變更: Installs pip by default, added the "--without-pip"
and "--copies" options.

在 3.4 版的變更: 在較早的版本中，如果目標目錄已存在，除非提供了 "--
clear" 或 "--upgrade" 選項，否則會引發錯誤。

在 3.9 版的變更: Add "--upgrade-deps" option to upgrade pip +
setuptools to the latest on PyPI.

在 3.12 版的變更: "setuptools" is no longer a core venv dependency.

備註:

  雖然在 Windows 上支援符號連結，但並不建議使用。特別需要注意的是，在
  檔案總管中按兩下 "python.exe" 會急切地解析符號連結並忽略虛擬環境。

備註:

  在 Microsoft Windows 上，可能需要通過設置使用者的執行策略來啟用
  "Activate.ps1" 腳本。你可以發出以下 PowerShell 命令來執行此操作：

     PS C:\> Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser

  有關更多資訊，請參閱關於執行策略。

The created "pyvenv.cfg" file also includes the "include-system-site-
packages" key, set to "true" if "venv" is run with the "--system-site-
packages" option, "false" otherwise.

除非 "--without-pip" 選項被提供，否則將調用 "ensurepip" 來啟動 "pip"
到虛擬環境中。

可以向 "venv" 提供多個路徑，這樣每個提供的路徑都將根據給定的選項建立一
個相同的虛擬環境。


虛擬環境如何運作
================

當 Python 直譯器跑在虛擬環境時，"sys.prefix" 和 "sys.exec_prefix" 會指
向虛擬環境的目錄，而 "sys.base_prefix" 和 "sys.base_exec_prefix" 會指
向建立虛擬環境的基礎 Python 的目錄。檢查 "sys.prefix !=
sys.base_prefix" 就可以確定目前的直譯器是否跑在虛擬環境中。

虛擬環境可以透過位於二進位檔案目錄中的腳本「啟用」（在 POSIX 上為
"bin"；在 Windows 上為 "Scripts"）這會將該目錄加入到你的 "PATH"，當你
運行 **python** 時就會調用該環境的直譯器並且執行已安裝的腳本，而不需要
使用完整的路徑。啟動腳本的方式因平台而異（"*<venv>*" 需要替換成包含虛
擬環境的目錄路徑）

+---------------+--------------+----------------------------------------------------+
| 平台          | Shell        | 啟動虛擬環境的指令                                 |
|===============|==============|====================================================|
| POSIX         | bash/zsh     | "$ source *<venv>*/bin/activate"                   |
|               +--------------+----------------------------------------------------+
|               | fish         | "$ source *<venv>*/bin/activate.fish"              |
|               +--------------+----------------------------------------------------+
|               | csh/tcsh     | "$ source *<venv>*/bin/activate.csh"               |
|               +--------------+----------------------------------------------------+
|               | pwsh         | "$ *<venv>*/bin/Activate.ps1"                      |
+---------------+--------------+----------------------------------------------------+
| Windows       | cmd.exe      | "C:\> *<venv>*\Scripts\activate.bat"               |
|               +--------------+----------------------------------------------------+
|               | PowerShell   | "PS C:\> *<venv>*\Scripts\Activate.ps1"            |
+---------------+--------------+----------------------------------------------------+

在 3.4 版被加入: **fish** 和 **csh** 啟動腳本。

在 3.8 版被加入: PowerShell 的啟動腳本安裝在 POSIX 上支援 PowerShell
Core。

你不用特別開啟虛擬環境，你可以在調用 Python 時指定該環境下 Python 直譯
器的完整路徑。此外，所有安裝在環境裡的腳本都應該都可以在未啟用虛擬環境
的情況下運行。

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 Launcher for Windows 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".

當虛擬環境被啟用時，"VIRTUAL_ENV" 環境變數會被設置為該環境的路徑。由於
不需要明確啟用虛擬環境才能使用它。因此，無法依賴 "VIRTUAL_ENV" 來判斷
是否正在使用虛擬環境。

警告:

  因為安裝在環境中的腳本不應該預期該環境已經被啟動，所以它們的 shebang
  列會包含環境直譯器的絕對路徑。因此，在一般情況下，環境本質上是不可攜
  帶的。你應該使用一個簡單的方法來重新建立一個環境（例如：如果你有一個
  名為 "requirements.txt" 的需求檔案，你可以使用環境的 "pip install -r
  requirements.txt" 來安裝環境所需的所有套件）。如果出於某種原因，你需
  要將環境移至新位置，你應該在所需位置重新建立它，並刪除舊位置的環境。
  如果你移動環境是因為移動了其父目錄，你應該在新位置重新建立環境。否則
  ，安裝在該環境中的軟體可能無法正常運作。

你可以在 shell 輸入 "deactivate" 來關閉虛擬環境。具體的使用方式因平台
而異，是內部實作的細節（通常會使用腳本或是 shell 函式）


API
===

上述提到的高階 method（方法）透過簡單的 API 使用， 為第三方虛擬環境建
立者提供可以依據他們需求來建立環境的客製化機制： "EnvBuilder" class。

class venv.EnvBuilder(system_site_packages=False, clear=False, symlinks=False, upgrade=False, with_pip=False, prompt=None, upgrade_deps=False)

   進行實例化時，class "EnvBuilder" 接受下列的關鍵字引數：

   * *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

   在 3.4 版的變更: 新增 "with_pip" 參數

   在 3.6 版的變更: 新增 "prompt" 參數

   在 3.9 版的變更: 新增 "upgrade_deps" 參數

   "EnvBuilder" may be used as a base class.

   create(env_dir)

      透過指定將會容納虛擬環境的目標目錄來建立一個虛擬環境（絕對路徑或
      相對路徑到該目錄），也就是在該目錄中容納虛擬環境。"create"
      method 將會在指定的目錄下建立環境，或是觸發適當的例外。

      "EnvBuilder" class 的 "create" method 會闡述可用的 Hooks 以客製
      化 subclass （子類別）:

         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)

      每個 methods "ensure_directories()"、"create_configuration()"、
      "setup_python()"、"setup_scripts()" 及 "post_setup()" 都可以被覆
      寫。

   ensure_directories(env_dir)

      建立還不存在的環境目錄及必要的子目錄，並回傳一個情境物件（
      context object）。這個情境物件只是一個屬性 (例如：路徑) 的所有者
      ，可被其他 method 使用。如果 "EnvBuilder" 已被建立且帶有
      "clear=True" 的引數，該環境目錄下的內容將被清空，以及所有必要的
      子目錄將被重新建立。

      回傳的情境物件（context object）其型別會是
      "types.SimpleNamespace"，並包含以下屬性：

      * "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.

      * "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.

      在 3.11 版的變更: The *venv* sysconfig installation scheme is
      used to construct the paths of the created directories.

      在 3.12 版的變更: The attribute "lib_path" was added to the
      context, and the context object was documented.

   create_configuration(context)

      Creates the "pyvenv.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 executable
      "python3.x" was used, symlinks to "python" and "python3" will be
      created pointing to that executable, unless files with those
      names already exist.

   setup_scripts(context)

      Installs activation scripts appropriate to the platform into the
      virtual environment.

   upgrade_dependencies(context)

      Upgrades the core venv dependency packages (currently pip) in
      the environment. This is done by shelling out to the "pip"
      executable in the environment.

      在 3.9 版被加入.

      在 3.12 版的變更: setuptools is no longer a core venv
      dependency.

   post_setup(context)

      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.

   install_scripts(context, path)

      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).

   在 3.7.2 版的變更: 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.

   在 3.7.3 版的變更: 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:

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

   Create an "EnvBuilder" with the given keyword arguments, and call
   its "create()" method with the *env_dir* argument.

   在 3.3 版被加入.

   在 3.4 版的變更: Added the *with_pip* parameter

   在 3.6 版的變更: Added the *prompt* parameter

   在 3.9 版的變更: Added the *upgrade_deps* parameter


An example of extending "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 urlparse
   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 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)

       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, _, _, _ = urlparse(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.
