If set, cache PYTHONPATH to _PYTHONPATH for Rose to use later

cylc/flow/etc/cylc

@@ -2,7 +2,7 @@
 
 # THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
 # Copyright (C) NIWA & British Crown (Met Office) & Contributors.
-# 
+#
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
 # the Free Software Foundation, either version 3 of the License, or
@@ -96,6 +96,12 @@ CYLC_HOME_ROOT="${CYLC_HOME_ROOT:-/opt}"
 # export ROSE_SITE_CONF_PATH="${ROSE_SITE_CONF_PATH:-/etc/rose}"
 ###############################################################################
 
+# Prevent user's PyhtonPath interfering with Cylc:
+if [[ -n "${PYTHONPATH}" ]]; then
+    export _PYTHONPATH="${PYTHONPATH}"
+    unset PYTHONPATH
+fi
+
 if [[ -z "${CYLC_HOME}" ]]; then
     if [[ -z "${CYLC_ENV_NAME}" ]]; then
         if [[ -n "${CYLC_VERSION}" ]]; then