New submission from Tim Smith:
Homebrew, the OS X package manager, distributes python3 as a framework build.
We like to be able to control the shebang that gets written to scripts
installed with pip. [1]
The path we prefer for invoking the python3 interpreter is like
/usr/local/opt/python3/bin/python3.4, which is symlinked to the framework stub
launcher at
/usr/local/Cellar/python3/3.4.1_1/Frameworks/Python.framework/Versions/3.4/bin/python3.4.
For Python 2.x, we discovered that assigning
"/usr/local/opt/python/bin/python2.7" to sys.executable in sitecustomize.py
resulted in correct shebangs for scripts installed by pip. The same approach
doesn't work with Python 3.
A very helpful conversation with Vinay Sajip [2] led us to consider how the
python3 stub launcher sets __PYVENV_LAUNCHER__, which distlib uses in
preference to sys.executable to discover the intended interpreter when pip
writes shebangs.
Roughly, __PYVENV_LAUNCHER__ is set from argv[0], so it mimics the invocation
of the stub, except that symlinks in the directory component of the path to the
identified interpreter are resolved to a "real" path. For us, this means that
__PYVENV_LAUNCHER__ (and therefore the shebangs of installed scripts) always
points to the Cellar path, not the preferred opt path, even when python is
invoked via the opt path.
Avoiding this symlink resolution would allow us to control pip's shebang (which
sets the shebangs of all pip-installed scripts) by controlling the way we
invoke python3 when we use ensurepip during installation.
Building python3 with the attached diff removes the symlink resolution.
[1] This is important to Homebrew because packages are physically installed to
versioned prefixes, like /usr/local/Cellar/python3/3.4.1_1/. References to
these real paths are fragile and break when the version number changes or when
the revision number ("_1") changes, when can happen when e.g. openssl is
released and Python needs to be recompiled against the new library. To avoid
this breakage, Homebrew maintains a version-independent symlink to each
package, like /usr/local/opt/python3, which points to the
.../Cellar/python3/3.4.1_1/ location.
[2] https://github.com/pypa/pip/issues/2031
----------
assignee: ronaldoussoren
components: Macintosh
files: dont-realpath-venv-dirname.diff
keywords: patch
messages: 227505
nosy: ned.deily, ronaldoussoren, tdsmith, vinay.sajip
priority: normal
severity: normal
status: open
title: Using realpath for __PYVENV_LAUNCHER__ makes Homebrew installs fragile
type: behavior
versions: Python 3.4, Python 3.5
Added file: http://bugs.python.org/file36718/dont-realpath-venv-dirname.diff
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