Python 2 is an ex-Python, it is no more 
|> python
bash: python: command not found
I relent and make a symbolic link:
|> which python
/home/users/russel/Built/bin/python
518 lynet:~
|> python
Python 3.8.6 (default, Sep 25 2020, 09:36:53)
[GCC 10.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
and now I get:
|> mason clean
520 lynet:~/Progs/Applications/Factorial/Chapel (git:master)
|> mason build
Skipping registry update since no dependency found in manifest file.
Compiling [debug] target: Factorial
Build Successful
521 lynet:~/Progs/Applications/Factorial/Chapel (git:master)
|> mason test
Skipping registry update since no dependency found in manifest file.
----------------------------------------------------------------------
Ran 2 tests in 11.3921 seconds
OK (passed = 2 )
So clearly the Chapel system requires Python 2 for some of the hidden Python scripts.
As for the environment:
|> /home/linuxbrew/.linuxbrew/Cellar/chapel/1.23.0/libexec/util/printchplenv
machine info: Linux lynet 5.8.0-3-amd64 #1 SMP Debian 5.8.14-1 (2020-10-10) x86_64
CHPL_HOME: /home/linuxbrew/.linuxbrew/Cellar/chapel/1.23.0/libexec
script location: /home/linuxbrew/.linuxbrew/Cellar/chapel/1.23.0/libexec/util/chplenv
CHPL_TARGET_PLATFORM: linux64
CHPL_TARGET_COMPILER: gnu
CHPL_TARGET_ARCH: x86_64
CHPL_TARGET_CPU: native
CHPL_LOCALE_MODEL: flat
CHPL_COMM: none
CHPL_TASKS: qthreads
CHPL_LAUNCHER: none
CHPL_TIMERS: generic
CHPL_UNWIND: none
CHPL_MEM: jemalloc
CHPL_ATOMICS: cstdlib
CHPL_GMP: system *
CHPL_HWLOC: hwloc
CHPL_REGEXP: re2
CHPL_LLVM: none
CHPL_AUX_FILESYS: none
|> /home/linuxbrew/.linuxbrew/Cellar/chapel/1.23.0/libexec/util/chplenv/chpl_comm.py
none