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tox plugin to allow specification of interpreter locationspaths to use

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  • tox --scan shows previously found name to executable path mappings.

  • tox --scan PATTERN1 PATTERN2 searches the patterns for python executables and stores paths found in that way for future use by tox

This allows you to easily specify multiple python executables for tox to use. Without the need for permanently or temporarily adding their paths to your $PATH, which does have unacceptable side effects in all cases, except for direct commandline invocation of tox via an alias (as you can guess not my typical usage).

On Windows the executables can be found via the registry. Therefore specifying PATTERNS to find executables is unnecessary and relatively cumbersome. tox --scan registry will scan the registry for executable paths and add provide these for future runs. This finally allows it to run tox on windows without the need to clutter C:\ with python installs.


Since tox 2.0 the plugin mechanism based on pluggy is included in tox. You can just do:

pip install tox_globinterpreter

If you install pytest (pip install pytest) to test this module itself, and you use the bash shell, make sure you rehash bash (hash -r or re-activate the virtual environment.

Default tox interpreter finding behaviour


tox searches through the $PATH specified paths for python executables (ToDo: check).

For many Linux distributions the system python (and more recently pythons, as both 2.7.x and 3.4.X are used by system utilities as Linux distributions slowly moves to Python3) cannot be replaced by a newer micro version without a lot of hassle. These newer micro versions should be installed in some “other” directory e.g. /opt/python/2.7.9. Its bin directory cannot be added to the normal PATH during startup as this will break some system programs relying on the older python micro version (and its additionally installed libraries).

This directory can temporarily be added to each invocation of tox by making an alias for tox, but that doesn’t work if tox is called from a Makefile. And aliasing make to include that path can break invocations of system utilities from the same Makefile.


On Windows tox looks for installations in C:\\python?.? and determines the version based on the directory name (ToDo: check).

This doesn’t work when you have the python interpreters installed in the non default location, e.g. necessary when you want 64 and 32 bit versions installed, or when you have them installed under the more correct C:\Program Files\ or similar directory.

tox_globinterpreter plugin extensions


tox_globinterpreter adds one commandline option to tox: --scan. If invoked with arguments, these arguments should be a pattern, or list of patterns that will be globb-ed and shlex-ed to form a mapping of base names to paths to python binaries that is then stored globally per user for future usage (under $XDG_CONFIG_HOME/tox, defaulting to ~/.config/tox). E.g.:

tox --scan '/opt/python/2.?/bin/python?.?' ../../../opt/python/pypy*/bin/pypy

(please note that only the second argument is not expanded by the shell, the first is quoted and expanded by tox_globinterpreter).

If tox --scan is invoked without arguments, then the currently stored mapping (base name to executable)is printed out. E.g.:

python2.7 /opt/python/2.7/bin/python2.7
pypy /opt/python/pypy-2.5.0/bin/pypy

(if the same base name is found multiple times, the first one in the list is used).

If the environment variable TOX_INTERPRETER_GLOBS is set this will cause the python binaries to be searched for using the patterns specified by that environment variable (expanded and searched every time).

If TOX_INTERPRETER_GLOBS is set it prevents the use of the –scan-ned list, and either of them is set, the normal search through PATH is not done.

During --scan-ning the base name (py27) to be used in the envlist in tox.ini, is determined based on the name of the binary if this includes a version number (python2.7). if the binary equals python the base name (including version) is determined by invoking the interpreter. When expanding TOX_INTERPRETER_GLOBS, this invocation to determine version information is currently considered too expensive.


tox_globinterpreter adds commandline options to tox, the primary one of which is: --scan.

On windows you should specify --scan c:/python/*/python.exe c:/pypy*/*/*/pyp.exe first to create a mapping of base names to paths. This doesn’t get them from the registry as the registry is incomplete (only one of both 64 and 32 bit version of a particular CPython is registered, pypy is not registered at all).

If tox --scan is invoked without arguments, then the currently stored mapping is printed out. E.g.:

python2.7 c:\python\2.7\python2.7.exe
python2.7 c:\python\2.7-32\python2.7.exe

If you have 64 and 32 bit versions installed select the -32 version by doing:

tox -r --32

(This might be supported in tox itself at some point, currently you cannot specify py27-32 as the 32 part is never handed to tox_get_python_executable)


I originally implemented the possibility to specify the list of interpreters as a patch for tox for which I put in a PR that lingered for two years (with repeated updates) until I was asked to update it (once more, but this time by the author of tox). Shortly after the plugin interface was provided and this plugin for tox replaces the earlier PR requests tox.

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