# `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 either 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

適用：非 Android、非 iOS、非 WASI。

此模組在行動平台或WebAssembly 平台上不支援。

## 建立虛擬環境

建立虛擬環境的方法是透過執行指令 `venv`：

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

執行此命令會建立目標目錄（同時也會建立任何還不存在的父目錄）並在目錄中放置一個名為 `pyvenv.cfg` 的檔案，其中包含一個指向執行該命令的 Python 安裝路徑的 `home` 鍵（目標目錄的常見名稱為 `.venv`）。同時，它會建立一個 `bin` （在 Windows 上為 `Scripts`）子目錄，其中包含一個 Python 二進位檔案的副本/符號連結（根據建立環境時使用的平台或引數而定）。此外，它還會建立一個（最初為空的） `lib/pythonX.Y/site-packages` 子目錄（在 Windows 上為 `Lib\site-packages`）。如果指定的目錄已存在，則將重新使用該目錄。

在 3.5 版的變更: 目前建議使用 `venv` 來建立虛擬環境。

在 3.6 版之後被棄用: `pyvenv` 是在 Python 3.3 和 3.4 中建立虛擬環境的推薦工具，但在 Python 3.6 中已被棄用。

在 Windows 上，執行以下命令以使用 `venv`：

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

或者，如你已經為你的 Python 安裝配置了 `PATH` 和 `PATHEXT` 變數，則可以執行以下命令：

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

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]
                [--without-scm-ignore-file]
                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
    --without-scm-ignore-file
                            Skips adding the default SCM ignore file to the
                            environment directory (the default is a .gitignore
                            file).

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

在 3.13 版的變更: `--without-scm-ignore-file` was added along with creating an ignore file for `git` by default.

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

在 3.9 版的變更: 新增 `--upgrade-deps` 選項以將 pip 和 setuptools 升級至 PyPI 上的最新版本

在 3.4 版的變更: 預設情況下安裝 pip，並新增了 `--without-pip` 和 `--copies` 選項

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。

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。

被建立的 `pyvenv.cfg` 檔案還包括了 `include-system-site-packages` 的鍵，如果使用 `venv` 執行時帶有 `--system-site-packages` 選項，則設置為 `true`，否則設置為 `false`。

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`
PowerShell | `$ <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。

為了實現這一點，安裝在虛擬環境中的腳本會有一個 "shebang" 列，此列指向該環境的 Python 直譯器，例如： `#!/<path-to-venv>/bin/python`。這代表無論 `PATH` 的值為何，該腳本都會在直譯器上運行。在 Windows 上，如果你安裝了 Python Launcher for Windows，則支援 "shebang" 列處理。因此，在 Windows 檔案總管（Windows Explorer）中雙擊已安裝的腳本，應該可以在沒有啟用環境或將其加入 `PATH` 的情況下正確地運行。

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, *, scm_ignore_files=frozenset())

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

  - `system_site_packages` -- 為一個 Boolean （布林值），並表明系統的 Python site-packages 是否可以在環境中可用（預設為 `False` ）。

  - `clear` -- 為一個 Boolean，如果為 true，則在建立環境之前，刪除目標目錄內所有存在的內容。

  - `symlinks` -- 為一個 Boolean，並表明是否嘗試與 Python 二進位檔案建立符號連結而不是複製該檔案。

  - `upgrade` -- 為一個 Boolean，若為 true，則會在執行 Python 時為現有的環境進行升級。目的是讓 Python 可以升級到位（預設為 `False`）。

  - `with_pip` -- 為一個 Boolean，若為 true，則確保 pip 有安裝至虛擬環境之中。當有 `--default-pip` 的選項時，會使用 `ensurepip`。

  - `prompt` -- 為一個 String（字串），該字串會在虛擬環境啟動時被使用。（預設為 `None`，代表該環境的目錄名稱會被使用）倘若出現特殊字串 `"."` ，則當前目錄的 basename 會做為提示路徑使用。

  - `upgrade_deps` -- 更新基礎 venv 模組至 PyPI 的最新版本

  - `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()`.

  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

  在 3.13 版的變更: Added the `scm_ignore_files` parameter

  第三方虛擬環境工具的建立者可以自由地使用 `EnvBuilder` class 作為 base class（基底類別）使用.

  回傳的 env-builder 為一個物件，且帶有一個 method `create`：

  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.

  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.

  In addition, `EnvBuilder` provides this utility method that can be called from `setup_scripts()` or `post_setup()` in subclasses to assist 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 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).

  create_git_ignore_file(context)

    Creates a `.gitignore` file within the virtual environment that causes the entire directory to be ignored by the `git` source control manager.

    在 3.13 版被加入.

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 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: 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

  在 3.13 版的變更: Added the `scm_ignore_files` 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.