Both postgres and python3 are already installed on my High Sierra system - so the following was attempted:
brew upgrade postgres --with-python
I had thought that the --with-python would have installed just the postgres plpython3u language support . However it tried to [re]install python itself.
/usr/local/Cellar/sqlite/3.26.0: 11 files, 3.7MB
==> Installing postgresql dependency: python
==> Downloading https://homebrew.bintray.com/bottles/python-3.7.2.high_sierra.bottle.tar.gz
######################################################################## 100.0%
==> Pouring python-3.7.2.high_sierra.bottle.tar.gz
Error: An unexpected error occurred during the `brew link` step
The formula built, but is not symlinked into /usr/local
Directory not empty @ dir_s_rmdir - /usr/local/opt/python3
Error: Directory not empty @ dir_s_rmdir - /usr/local/opt/python3
Then when attempting to load the plpython3u we see:
19:44:24/sbgmaster2 $psql
psql (10.3)
Type "help" for help.
myuser=# CREATE LANGUAGE plpython3u;
ERROR: could not access file "$libdir/plpython3": No such file or directory
So how can plpyton3u be installed then?
Note: this is about programming (on topic) because this is for using UDF's - user defined functions that are purely for programming.