> Perhaps the dummy trick needs to be done by doctest itself?

The issue really comes from the readline module. The workaround is to set an environment variable, which affects the whole process (all threads).

Only setting TERM for test_CLI() is fine, because it's the only test running doctest in a subprocess.

Hum, we may set TERM=dummy in script_helper directly, because this module tries to run Python in a fresh environment. It uses "python -E" for example.