[Python-Dev] PEP 486: Make the Python Launcher aware of virtual environments (original) (raw)

Paul Moore p.f.moore at gmail.com
Thu Feb 12 20:44:58 CET 2015

On 12 February 2015 at 14:46, Paul Moore <p.f.moore at gmail.com> wrote:

If py.exe detected when the environment variable VIRTUALENV was set, and used that virtualenv as the default Python rather than the "system" python it normally used, this would be perfect.

I think I'll write a PEP for this. Do people think it's a reasonable idea?

OK, here we go. https://www.python.org/dev/peps/pep-0486/

Reproduced inline below for ease of reading / commenting. It's also available on github at https://github.com/pfmoore/peps should people want to submit corrections or anything there.

PEP: 486 Title: Make the Python Launcher aware of virtual environments Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Paul Moore <p.f.moore at gmail.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 12-Feb-2015 Python-Version: 3.5 Post-History:

Abstract

The Windows installers for Python include a launcher that locates the correct Python interpreter to run (see PEP 397). However, the launcher is not aware of virtual environments (virtualenv [1]_ or PEP 405 based), and so cannot be used to run commands from the active virtualenv.

This PEP proposes making the launcher "virtualenv aware". This means that when run without specifying an explicit Python interpreter to use, the launcher will use the currently active virtualenv, if any, before falling back to the configured default Python.

Rationale

Windows users with multiple copies of Python installed need a means of selecting which one to use. The Python launcher provides this facility by means of a py command that can be used to run either a configured "default" Python or a specific interpreter, by means of command line arguments. So typical usage would be::

# Run the Python interactive interpreter
py

# Execute an installed module
py -m pip install pytest
py -m pytest

When using virtual environments, the py launcher is unaware that a virtualenv is active, and will continue to use the system Python. So different command invocations are needed to run the same commands in a virtualenv::

# Run the Python interactive interpreter
python

# Execute an installed module (these could use python -m,
# which is longer to type but is a little mopre similar to the
# launcher approach)
pip install pytest
py.test

Having to use different commands is is error-prone, and in many cases the error is difficult to spot immediately. The PEP proposes making the py command usable with virtual environments, so that the first form of command can be used in all cases.

Implementation

Both virtualenv and the core venv module set an environment variable VIRTUAL_ENV when activating a virtualenv. This PEP proposes that the launcher checks for the VIRTUAL_ENV environment variable whenever it would run the "default" Python interpreter for the system (i.e., when no specific version flags such as py -2.7 are used) and if present, run the Python interpreter for the virtualenv rather than the default system Python.

The "default" Python interpreter referred to above is (as per PEP 397) either the latest version of Python installed on the system, or a version configured via the py.ini configuration file. When the user specifies an explicit Python version on the command line, this will always be used (as at present).

Impact on Script Launching

As well as interactive use, the launcher is used as the Windows file association for Python scripts. In that case, a "shebang" (#!) line at the start of the script is used to identify the interpreter to run. A fully-qualified path can be used, or a version-specific Python (python3 or python2, or even python3.5), or the generic python, which means to use the default interpreter.

With the proposed change, scripts that start with #!python (or one of its equivalent forms) will be run using an active virtualenv. This is a change in behaviour, although it will only affect users running scripts from the command line with a virtualenv activated.

Under Unix, the #!/usr/bin/env python shebang line will use the version of Python found on $PATH, whereas #!/usr/bin/python will use the system Python. In order to match Unix behaviour as closely as possible, it is proposed that the two shebang forms::

#!/usr/bin/env python
#!python

use an active virtualenv, if present, whereas the forms::

#!/usr/bin/python
#!/usr/local/bin/python

use only the default system Python, and ignore the VIRTUAL_ENV environment variable.

Exclusions

The PEP makes no attempt to promote the use of the launcher for running Python on Windows. Most existing documentation assumes the user of python as the command to run Python, and (for example) pip to run an installed Python command. This documentation is not expected to change, and users who choose to manage their PATH environment variable can continue to use this form. The focus of this PEP is purely on allowing users who prefer to use the launcher when dealing with their system Python installations, to be able to continue to do so when using virtual environments.

References

.. [1] https://virtualenv.pypa.io/

Copyright

This document has been placed in the public domain.

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