diff --git a/tools/fnlosrc_install.csh b/tools/fnlosrc_install.csh
index f7ad298cba042bd2e2a0959a4d085f8f6a7f9e03..84f51fc7deb5248d73a18d15b84836c4d17c3b1d 100755
--- a/tools/fnlosrc_install.csh
+++ b/tools/fnlosrc_install.csh
@@ -1190,6 +1190,9 @@ if ( $withpython ) then
    echo 'setenv PYTHONPATH '"${PYTHONPATHADD}" >> fnlosrc_source.csh
    echo 'export PYTHONPATH='"${PYTHONPATHADD}" >> fnlosrc_source.sh
    endif
+   # Unbuffered Python output to avoid mixing C++/Python
+   echo 'setenv PYTHONUNBUFFERED 1' >> fnlosrc_source.csh
+   echo 'export PYTHONUNBUFFERED=1' >> fnlosrc_source.sh
    echo ""
    echo "ATTENTION: PYTHONPATH environment complemented!"
    echo "   PYTHONPATH has been set to:"
diff --git a/v2.5/toolkit/src/fnlo-py-print.py b/v2.5/toolkit/src/fnlo-py-print.py
index f17047ed44cacdd050780cb5abe8ebc16a855c7f..42c93211f9eb60232301a1be4178d38367b0c69f 100755
--- a/v2.5/toolkit/src/fnlo-py-print.py
+++ b/v2.5/toolkit/src/fnlo-py-print.py
@@ -1,5 +1,7 @@
-#!/usr/bin/env -S python3 -u
+#!/usr/bin/env python3
 #-*- coding:utf-8 -*-
+### This shebang line only works with recent enough env version: !/usr/bin/env -S python3 -u
+### Set PYTHONUNBUFFERED environment variable instead to 1.
 #
 ########################################################################
 #