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[doc] Refresh the venv introduction documentation, and correct the statement about VIRTUAL_ENV (GH-98350)
Co-authored-by: C.A.M. Gerlach <[email protected]>
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‎Doc/library/venv.rst‎

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--------------
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The :mod:`venv` module provides support for creating lightweight "virtual
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environments" with their own site directories, optionally isolated from system
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site directories. Each virtual environment has its own Python binary (which
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matches the version of the binary that was used to create this environment) and
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can have its own independent set of installed Python packages in its site
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directories.
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.. _venv-def:
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.. _venv-intro:
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The :mod:`!venv` module supports creating lightweight "virtual environments",
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each with their own independent set of Python packages installed in
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their :mod:`site` directories.
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A virtual environment is created on top of an existing
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Python installation, known as the virtual environment's "base" Python, and may
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optionally be isolated from the packages in the base environment,
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so only those explicitly installed in the virtual environment are available.
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See :pep:`405` for more information about Python virtual environments.
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When used from within a virtual environment, common installation tools such as
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`pip`_ will install Python packages into a virtual environment
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without needing to be told to do so explicitly.
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See :pep:`405` for more background on Python virtual environments.
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.. seealso::
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.. include:: /using/venv-create.inc
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.. _venv-explanation:
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.. _venv-def:
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How venvs work
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--------------
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.. note:: A virtual environment is a Python environment such that the Python
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interpreter, libraries and scripts installed into it are isolated from those
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installed in other virtual environments, and (by default) any libraries
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installed in a "system" Python, i.e., one which is installed as part of your
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operating system.
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A virtual environment is a directory tree which contains Python executable
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files and other files which indicate that it is a virtual environment.
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Common installation tools such as setuptools_ and pip_ work as
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expected with virtual environments. In other words, when a virtual
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environment is active, they install Python packages into the virtual
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environment without needing to be told to do so explicitly.
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When a virtual environment is active (i.e., the virtual environment's Python
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interpreter is running), the attributes :attr:`sys.prefix` and
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:attr:`sys.exec_prefix` point to the base directory of the virtual
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environment, whereas :attr:`sys.base_prefix` and
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:attr:`sys.base_exec_prefix` point to the non-virtual environment Python
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installation which was used to create the virtual environment. If a virtual
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environment is not active, then :attr:`sys.prefix` is the same as
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:attr:`sys.base_prefix` and :attr:`sys.exec_prefix` is the same as
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:attr:`sys.base_exec_prefix` (they all point to a non-virtual environment
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Python installation).
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When a virtual environment is active, any options that change the
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installation path will be ignored from all ``setuptools`` configuration
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files to prevent projects being inadvertently installed outside of the
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virtual environment.
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When working in a command shell, users can make a virtual environment active
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by running an ``activate`` script in the virtual environment's executables
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directory (the precise filename and command to use the file is
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shell-dependent), which prepends the virtual environment's directory for
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executables to the ``PATH`` environment variable for the running shell. There
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should be no need in other circumstances to activate a virtual
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environment; scripts installed into virtual environments have a "shebang"
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line which points to the virtual environment's Python interpreter. This means
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that the script will run with that interpreter regardless of the value of
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``PATH``. On Windows, "shebang" line processing is supported if you have the
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Python Launcher for Windows installed (this was added to Python in 3.3 - see
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:pep:`397` for more details). Thus, double-clicking an installed script in a
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Windows Explorer window should run the script with the correct interpreter
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without there needing to be any reference to its virtual environment in
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``PATH``.
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When a Python interpreter is running from a virtual environment,
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:data:`sys.prefix` and :data:`sys.exec_prefix`
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point to the directories of the virtual environment,
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whereas :data:`sys.base_prefix` and :data:`sys.base_exec_prefix`
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point to those of the base Python used to create the environment.
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It is sufficient to check
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``sys.prefix == sys.base_prefix`` to determine if the current interpreter is
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running from a virtual environment.
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A virtual environment may be "activated" using a script in its binary directory
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(``bin`` on POSIX; ``Scripts`` on Windows).
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This will prepend that directory to your :envvar:`!PATH`, so that running
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:program:`!python` will invoke the environment's Python interpreter
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and you can run installed scripts without having to use their full path.
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The invocation of the activation script is platform-specific
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(:samp:`{<venv>}` must be replaced by the path to the directory
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containing the virtual environment):
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+-------------+------------+--------------------------------------------------+
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| Platform | Shell | Command to activate virtual environment |
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+=============+============+==================================================+
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| POSIX | bash/zsh | :samp:`$ source {<venv>}/bin/activate` |
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| +------------+--------------------------------------------------+
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| | fish | :samp:`$ source {<venv>}/bin/activate.fish` |
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| +------------+--------------------------------------------------+
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| | csh/tcsh | :samp:`$ source {<venv>}/bin/activate.csh` |
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| +------------+--------------------------------------------------+
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| | PowerShell | :samp:`$ {<venv>}/bin/Activate.ps1` |
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+-------------+------------+--------------------------------------------------+
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| Windows | cmd.exe | :samp:`C:\\> {<venv>}\\Scripts\\activate.bat` |
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| +------------+--------------------------------------------------+
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| | PowerShell | :samp:`PS C:\\> {<venv>}\\Scripts\\Activate.ps1` |
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+-------------+------------+--------------------------------------------------+
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.. versionadded:: 3.4
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:program:`!fish` and :program:`!csh` activation scripts.
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.. versionadded:: 3.8
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PowerShell activation scripts installed under POSIX for PowerShell Core
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support.
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You don't specifically *need* to activate a virtual environment,
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as you can just specify the full path to that environment's
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Python interpreter when invoking Python.
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Furthermore, all scripts installed in the environment
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should be runnable without activating it.
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In order to achieve this, scripts installed into virtual environments have
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a "shebang" line which points to the environment's Python interpreter,
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i.e. :samp:`#!/{<path-to-venv>}/bin/python`.
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This means that the script will run with that interpreter regardless of the
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value of :envvar:`!PATH`. On Windows, "shebang" line processing is supported if
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you have the :ref:`launcher` installed. Thus, double-clicking an installed
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script in a Windows Explorer window should run it with the correct interpreter
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without the environment needing to be activated or on the :envvar:`!PATH`.
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When a virtual environment has been activated, the :envvar:`!VIRTUAL_ENV`
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environment variable is set to the path of the environment.
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Since explicitly activating a virtual environment is not required to use it,
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:envvar:`!VIRTUAL_ENV` cannot be relied upon to determine
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whether a virtual environment is being used.
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.. warning:: Because scripts installed in environments should not expect the
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environment to be activated, their shebang lines contain the absolute paths
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environment in its new location. Otherwise, software installed into the
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environment may not work as expected.
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You can deactivate a virtual environment by typing ``deactivate`` in your shell.
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The exact mechanism is platform-specific and is an internal implementation
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detail (typically, a script or shell function will be used).
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.. _venv-api:
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API

‎Doc/using/venv-create.inc‎

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environment will be created, according to the given options, at each provided
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path.
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Once a virtual environment has been created, it can be "activated" using a
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script in the virtual environment's binary directory. The invocation of the
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script is platform-specific (`<venv>` must be replaced by the path of the
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directory containing the virtual environment):
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+-------------+-----------------+-----------------------------------------+
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| Platform | Shell | Command to activate virtual environment |
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+=============+=================+=========================================+
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| POSIX | bash/zsh | $ source <venv>/bin/activate |
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+-------------+-----------------+-----------------------------------------+
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| | fish | $ source <venv>/bin/activate.fish |
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+-------------+-----------------+-----------------------------------------+
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| | csh/tcsh | $ source <venv>/bin/activate.csh |
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+-------------+-----------------+-----------------------------------------+
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| | PowerShell Core | $ <venv>/bin/Activate.ps1 |
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+-------------+-----------------+-----------------------------------------+
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| Windows | cmd.exe | C:\\> <venv>\\Scripts\\activate.bat |
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+-------------+-----------------+-----------------------------------------+
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| | PowerShell | PS C:\\> <venv>\\Scripts\\Activate.ps1 |
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+-------------+-----------------+-----------------------------------------+
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When a virtual environment is active, the :envvar:`VIRTUAL_ENV` environment
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variable is set to the path of the virtual environment. This can be used to
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check if one is running inside a virtual environment.
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You don't specifically *need* to activate an environment; activation just
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prepends the virtual environment's binary directory to your path, so that
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"python" invokes the virtual environment's Python interpreter and you can run
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installed scripts without having to use their full path. However, all scripts
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installed in a virtual environment should be runnable without activating it,
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and run with the virtual environment's Python automatically.
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You can deactivate a virtual environment by typing "deactivate" in your shell.
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The exact mechanism is platform-specific and is an internal implementation
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detail (typically a script or shell function will be used).
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.. versionadded:: 3.4
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``fish`` and ``csh`` activation scripts.
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.. versionadded:: 3.8
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PowerShell activation scripts installed under POSIX for PowerShell Core
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support.

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