150 lines
4.9 KiB
Plaintext
150 lines
4.9 KiB
Plaintext
PEP: 486
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Title: Make the Python Launcher aware of virtual environments
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Version: $Revision$
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Last-Modified: $Date$
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Author: Paul Moore <p.f.moore@gmail.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 12-Feb-2015
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Python-Version: 3.5
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Post-History:
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Abstract
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========
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The Windows installers for Python include a launcher that locates the
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correct Python interpreter to run (see PEP 397). However, the
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launcher is not aware of virtual environments (virtualenv [1]_ or PEP
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405 based), and so cannot be used to run commands from the active
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virtualenv.
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This PEP proposes making the launcher "virtualenv aware". This means
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that when run without specifying an explicit Python interpreter to
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use, the launcher will use the currently active virtualenv, if any,
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before falling back to the configured default Python.
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Rationale
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=========
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Windows users with multiple copies of Python installed need a means of
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selecting which one to use. The Python launcher provides this
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facility by means of a ``py`` command that can be used to run either a
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configured "default" Python or a specific interpreter, by means of
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command line arguments. So typical usage would be::
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# Run the Python interactive interpreter
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py
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# Execute an installed module
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py -m pip install pytest
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py -m pytest
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When using virtual environments, the ``py`` launcher is unaware that a
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virtualenv is active, and will continue to use the system Python. So
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different command invocations are needed to run the same commands in a
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virtualenv::
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# Run the Python interactive interpreter
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python
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# Execute an installed module (these could use python -m,
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# which is longer to type but is a little mopre similar to the
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# launcher approach)
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pip install pytest
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py.test
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Having to use different commands is is error-prone, and in many cases
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the error is difficult to spot immediately. The PEP proposes making
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the ``py`` command usable with virtual environments, so that the first
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form of command can be used in all cases.
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Implementation
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==============
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Both ``virtualenv`` and the core ``venv`` module set an environment
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variable ``VIRTUAL_ENV`` when activating a virtualenv. This PEP
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proposes that the launcher checks for the ``VIRTUAL_ENV`` environment
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variable whenever it would run the "default" Python interpreter for
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the system (i.e., when no specific version flags such as ``py -2.7``
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are used) and if present, run the Python interpreter for the
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virtualenv rather than the default system Python.
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The "default" Python interpreter referred to above is (as per PEP 397)
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either the latest version of Python installed on the system, or
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a version configured via the ``py.ini`` configuration file. When the
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user specifies an explicit Python version on the command line, this
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will always be used (as at present).
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Impact on Script Launching
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==========================
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As well as interactive use, the launcher is used as the Windows file
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association for Python scripts. In that case, a "shebang" (``#!``)
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line at the start of the script is used to identify the interpreter to
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run. A fully-qualified path can be used, or a version-specific Python
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(``python3`` or ``python2``, or even ``python3.5``), or the generic
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``python``, which means to use the default interpreter.
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The launcher also looks for the specific shebang line
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``#!/usr/bin/env python``. On Unix, the ``env`` program searches for a
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command on ``$PATH`` and runs the command so located. Similarly, with
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this shebang line, the launcher will look for a copy of ``python.exe``
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on the user's current ``%PATH%`` and will run that copy.
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As activating a virtualenv means that it is added to ``PATH``, no
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special handling is needed to run scripts with the active virtualenv -
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they just need to use the ``#!/usr/bin/env python`` shebang line,
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exactly as on Unix. (If there is no activated virtualenv, and no
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``python.exe`` on ``PATH``, the launcher will look for a default
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Python exactly as if the shebang line had said ``#!python``).
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Exclusions
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==========
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The PEP makes no attempt to promote the use of the launcher for
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running Python on Windows. Most existing documentation assumes the
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user of ``python`` as the command to run Python, and (for example)
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``pip`` to run an installed Python command. This documentation is not
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expected to change, and users who choose to manage their ``PATH``
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environment variable can continue to use this form. The focus of this
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PEP is purely on allowing users who prefer to use the launcher when
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dealing with their system Python installations, to be able to continue
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to do so when using virtual environments.
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Reference Implementation
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========================
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A patch implementing the proposed behaviour is available at
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http://bugs.python.org/issue23465
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References
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==========
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.. [1] https://virtualenv.pypa.io/
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Copyright
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=========
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This document has been placed in the public domain.
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..
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End:
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