There is probably a better way of doing this, but I decided to install all the needed python dependencies into python_modules, ala npm for python. This should also work if you have installed the needed modules system or user wide, though you might like to mkdir python_modules so the path still exists.
I had to change the code to some of the dependencies, so it is useful to have them installed into a subdirectory of the mutatormathtools checkout. In particular some of the code that MutatorMath uses tries to do the right thing by prepending the local path to the path to a file to open, sometimes this results in disaster with /path/to/mutatormathtools/fonts/Merriweather-Regular.ufo being doubled up at the start of a path to reference a file like metainfo.plist. Worse still the code actually hides those errors, raising coarser errors without a hint that the issue stemmed from a file path issue rather than something deeper. When you see a complaint on the command line about metainfo.plist being invalid it might just be that the path eventually used to read that file is invalid rather than the file itself.
The first example takes the Merriweather font using Bold as a weight of 1.0 and Regular as a weight of 0.0 and allows you to export an instance that is somewhere in between those two extremes. To do this in the most simple of ways just execute the following command which will generate out.ufo in the current directory for an instance 50% the way between the normal and bold masters.
./run-merriweather.sh
In the above run script you will see that the 3rd parameter to ./simpleblend.py is weight which is the axis we are hoping to change. The fourth parameter is 0.5 which is where in the range of 0 (normal) to 1 (bold) you would like the instance to be.
Note that the script also shows you the project designspace XML file to compare with that on the MutatorMath site: https://github.com/LettError/MutatorMath
A final touchup to out.ufo/fontinfo.plist allows FontForge (and other applications) to make sense of the out.ufo instance font file.
I hope that this example is doing things correctly. It was created independent of the MutatorMath project from RTFM & RTFS. (Constructive) Comments, especially constructive criticism, feedback, and good pull requests very welcome!
Use the metapolator-interpolate.js command to run metapolator and generate output. The script requires two font names and an output, and by default generates an instance 50% between the two masters. You can select masters just by name if they are in the ./fonts top level directory of mutatormathtools. So the following command should be valid:
$ ./metapolator-interpolate.js
-a Khula-Light-090x-456.ufo
-b Khula-ExtraBold-090x-456.ufo
-o /tmp/foo.ufo
A master script ./update-example-output.sh will update all the example output that is contained in your example-output subdirectory. Note that the example-output directory is itself under git control, so you probably want to make a throw away branch before running update-example-output.sh.