postinst is in openmpipython 2.9.4-1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 | #! /bin/sh -e
update-alternatives \
    --install /usr/bin/mpipython mpipython /usr/bin/mpipython.openmpi 30 \
    --slave /usr/share/man/man1/mpipython.1.gz mpipython.1.gz \
	/usr/share/man/man1/mpipython.openmpi.1.gz
exit 0
 |