"venv" --- 创建虚拟环境
***********************

3.3 版新加入.

**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.

See **PEP 405** for more background on Python virtual environments.

也參考: Python 打包用户指南：创建和使用虚拟环境


创建虚拟环境
============

通过执行 "venv" 指令来创建一个 虚拟环境:

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

运行此命令将创建目标目录（父目录若不存在也将创建），并放置一个
"pyvenv.cfg" 文件在其中，文件中有一个 "home" 键，它的值指向运行此命令
的 Python 安装（目标目录的常用名称是 ".venv"）。它还会创建一个 "bin"
子目录（在 Windows 上是 "Scripts"），其中包含 Python 二进制文件的副本
或符号链接（视创建环境时使用的平台或参数而定）。它还会创建一个（初始为
空的） "lib/pythonX.Y/site-packages" 子目录（在 Windows 上是 "Lib
\site-packages"）。如果指定了一个现有的目录，这个目录就将被重新使用。

3.6 版後已棄用: "pyvenv" was the recommended tool for creating virtual
environments for Python 3.3 and 3.4, and is deprecated in Python 3.6.

3.5 版更變: 现在推荐使用 "venv" 来创建虚拟环境。

在 Windows 上，调用 "venv" 命令如下:

   c:\>c:\Python35\python -m venv c:\path\to\myenv

或者，如果已经为 Python 安装 配置好 "PATH" 和 "PATHEXT" 变量:

   c:\>python -m venv c:\path\to\myenv

本命令如果以 "-h" 参数运行，将显示可用的选项:

   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.

   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.
     --upgrade-deps        Upgrade core dependencies: pip setuptools 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.

3.9 版更變: 添加 "--upgrade-deps" 选项，用于将 pip + setuptools 升级到
PyPI 上的最新版本

3.4 版更變: 默认安装 pip，并添加 "--without-pip" 和 "--copies" 选项

3.4 版更變: 在早期版本中，如果目标目录已存在，将引发错误，除非使用了 "
--clear" 或 "--upgrade" 选项。

備註:

  虽然 Windows 支持符号链接，但不推荐使用它们。特别注意，在文件资源管
  理器中双击 "python.exe" 将立即解析符号链接，并忽略虚拟环境。

備註:

  在 Microsoft Windows 上，为了启用 "Activate.ps1" 脚本，可能需要修改
  用户的执行策略。可以运行以下 PowerShell 命令来执行此操作：PS C:>
  Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser
  参阅 About Execution Policies 以获取更多信息。

生成的 "pyvenv.cfg" 文件还包括 "include-system-site-packages" 键，如果
运行 "venv" 时带有 "--system-site-packages" 选项，则键值为 "true"，否
则为 "false"。

除非采用 "--without-pip" 选项，否则将会调用 "ensurepip" 将 "pip" 引导
到虚拟环境中。

可以向 "venv" 传入多个路径，此时将根据给定的选项，在所给的每个路径上创
建相同的虚拟环境。


How venvs work
==============

When a Python interpreter is running from a virtual environment,
"sys.prefix" and "sys.exec_prefix" point to the directories of the
virtual environment, whereas "sys.base_prefix" and
"sys.base_exec_prefix" point to those of the base Python used to
create the environment. It is sufficient to check "sys.prefix ==
sys.base_prefix" to determine if the current interpreter is running
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 your "PATH", so that running **python** will invoke
the environment's Python interpreter and 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 directory containing the virtual environment):

+---------------+--------------+----------------------------------------------------+
| 平台          | Shell        | 用于激活虚拟环境的命令                             |
|===============|==============|====================================================|
| POSIX         | bash/zsh     | "$ source *<venv>*/bin/activate"                   |
|               +--------------+----------------------------------------------------+
|               | fish         | "$ source *<venv>*/bin/activate.fish"              |
|               +--------------+----------------------------------------------------+
|               | csh/tcsh     | "$ source *<venv>*/bin/activate.csh"               |
|               +--------------+----------------------------------------------------+
|               | PowerShell   | "$ *<venv>*/bin/Activate.ps1"                      |
+---------------+--------------+----------------------------------------------------+
| Windows       | cmd.exe      | "C:\> *<venv>*\Scripts\activate.bat"               |
|               +--------------+----------------------------------------------------+
|               | PowerShell   | "PS C:\> *<venv>*\Scripts\Activate.ps1"            |
+---------------+--------------+----------------------------------------------------+

3.4 版新加入: **fish** and **csh** activation scripts.

3.8 版新加入: 在 POSIX 上安装 PowerShell 激活脚本，以支持 PowerShell
Core。

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, i.e. "#!/*<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 适用于Windows的Python启动器 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.

警告:

  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).


API
===

上述的高级方法使用了一个简单的 API，该 API 提供了一种机制，第三方虚拟
环境创建者可以根据其需求自定义环境创建过程，该 API 为 "EnvBuilder" 类
。

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

   "EnvBuilder" 类在实例化时接受以下关键字参数：

   * "system_site_packages" -- 一个布尔值，要求系统 Python 的 site-
     packages 对环境可用（默认为 "False"）。

   * "clear" -- 一个布尔值，如果为 true，则在创建环境前将删除目标目录
     的现有内容。

   * "symlinks" -- 一个布尔值，指示是否尝试符号链接 Python 二进制文件
     ，而不是进行复制。

   * "upgrade" -- 一个布尔值，如果为 true，则将使用当前运行的 Python
     去升级一个现有的环境，这主要在原位置的 Python 更新后使用（默认为
     "False"）。

   * "with_pip" -- 一个布尔值，如果为 true，则确保在虚拟环境中已安装
     pip。这使用的是带有 "--default-pip" 选项的 "ensurepip"。

   * "prompt" -- 激活虚拟环境后显示的提示符（默认为 "None"，表示使用环
     境所在的目录名称）。如果使用了 ""."" 这一特殊字符串，则使用当前目
     录的基本名称作为提示符。

   * "upgrade_deps" -- 将基本 venv 模块更新为 PyPI 上的最新版本。

   3.4 版更變: 新增 "with_pip" 參數

   3.6 版新加入: 新增 "prompt" 參數

   3.9 版新加入: 新增 "upgrade_deps" 參數

   第三方虚拟环境工具的创建者可以自由地将此处提供的 "EnvBuilder" 类作
   为基类。

   返回的 env-builder 是一个对象，包含一个 "create" 方法：

   create(env_dir)

      指定要建立虚拟环境的目标目录（绝对路径或相对于当前路径）来创建虚
      拟环境。"create" 方法将在指定目录中创建环境，或者引发对应的异常
      。

      "EnvBuilder" 类的 "create" 方法定义了可用于定制子类的钩子:

         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)

      每个方法 "ensure_directories()", "create_configuration()",
      "setup_python()", "setup_scripts()" 和 "post_setup()" 都可以被重
      写。

   ensure_directories(env_dir)

      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.

      * "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.

      3.12 版更變: The attribute "lib_path" was added to the context,
      and the context object was documented.

      3.11 版更變: The *venv* sysconfig installation scheme is used to
      construct the paths of the created directories.

   create_configuration(context)

      在环境中创建 "pyvenv.cfg" 配置文件。

   setup_python(context)

      在环境中创建 Python 可执行文件的拷贝或符号链接。在 POSIX 系统上
      ，如果给定了可执行文件 "python3.x"，将创建指向该可执行文件的
      "python" 和 "python3" 符号链接，除非相同名称的文件已经存在。

   setup_scripts(context)

      将适用于平台的激活脚本安装到虚拟环境中。

   upgrade_dependencies(context)

      升级环境中 venv 依赖的核心软件包（当前为 "pip" 和 "setuptools"）
      。通过在环境中使用 "pip" 可执行文件来完成。

      3.9 版新加入.

   post_setup(context)

      占位方法，可以在第三方实现中重写，用于在虚拟环境中预安装软件包，
      或是其他创建后要执行的步骤。

   3.7.2 版更變: Windows 现在为 "python[w].exe" 使用重定向脚本，而不是
   复制实际的二进制文件。仅在 3.7.2 中，除非运行的是源码树中的构建，否
   则 "setup_python()" 不会执行任何操作。

   3.7.3 版更變: Windows 将重定向脚本复制为 "setup_python()" 的一部分
   而非 "setup_scripts()"。在 3.7.2 中不是这种情况。使用符号链接时，将
   链接至原始可执行文件。

   此外，"EnvBuilder" 提供了如下实用方法，可以从子类的
   "setup_scripts()" 或 "post_setup()" 调用，用来将自定义脚本安装到虚
   拟环境中。

   install_scripts(context, path)

      *path* 是一个目录的路径，该目录应包含子目录 "common", "posix",
      "nt"，每个子目录存有发往对应环境中 bin 目录的脚本。在下列占位符
      替换完毕后，将复制 "common" 的内容和与 "os.name" 对应的子目录：

      * "__VENV_DIR__" 会被替换为环境目录的绝对路径。

      * "__VENV_NAME__" 会被替换为环境名称（环境目录的最后一个字段）。

      * "__VENV_PROMPT__" 会被替换为提示符（用括号括起来的环境名称紧跟
        着一个空格）。

      * "__VENV_BIN_NAME__" 会被替换为 bin 目录的名称（ "bin" 或
        "Scripts" ）。

      * "__VENV_PYTHON__" 会被替换为环境可执行文件的绝对路径。

      允许目录已存在（用于升级现有环境时）。

有一个方便实用的模块级别的函数:

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

   通过关键词参数来创建一个 "EnvBuilder"，并且使用 *env_dir* 参数来调
   用它的 "create()" 方法。

   3.3 版新加入.

   3.4 版更變: 新增 "with_pip" 參數

   3.6 版更變: 新增 "prompt" 參數

   3.9 版更變: 新增 "upgrade_deps" 參數


一个扩展 "EnvBuilder" 的例子
============================

下面的脚本展示了如何通过实现一个子类来扩展 "EnvBuilder"。这个子类会安
装 setuptools 和 pip 到被创建的虚拟环境中。

   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://bitbucket.org/pypa/setuptools/downloads/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):
       compatible = True
       if sys.version_info < (3, 3):
           compatible = False
       elif not hasattr(sys, 'base_prefix'):
           compatible = False
       if not compatible:
           raise ValueError('This script is only for use with '
                            'Python 3.3 or later')
       else:
           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)

这个脚本同样可以 在线下载。
