* Third attempt to fix lmod "module" for ipython ("fix3")

(dated: 2020-06-09).
* Supports both Turing and Wahab.
* Allows both addition and deletion of paths from sys.path.

* Author's note: Later on, I discovered that these complex steps do not appear
  to be necessary, somehow the lmod Python commands were able to take care
  of additions and deletions through the modification of os.environ['PYTHONPATH']
  alone, so I am not sure if this fix is absolutely needed. (?)

  Sigs:
    # -rw------- 1 wpurwant users 3298 2020-06-09 20:07 lmod_python_fix3.py
    # 78abc7bae90bf23a42aa40285175ca48  lmod_python_fix3.py

lmod/ipython/startup/lmod_python_fix.py

@@ -1,24 +1,73 @@
 # -*- python -*-
+# modified "module" command for Wahab
+
 #from __future__ import print_function
 #from subprocess import PIPE, Popen
 import os, sys 
 
-sys.path.append('/shared/apps/common/lmod/current/init')
+try:
+    with open("/etc/cluster", "r") as _cluster_F:
+        _cluster = next(_cluster_F).strip()
+except:
+    sys.stderr.write("Unable to determine cluster, 'module' command will not be available.\n")
+else:
+    if _cluster == "wahab":
+        sys.path.append('/shared/apps/common/lmod/current/init')
+    elif _cluster == "turing":
+        sys.path.append('/cm/shared/applications/lmod/lmod/init')
+    else:
+        sys.stderr.write("Unknown cluster, 'module' command will not be available.\n")
+    del _cluster
 
-from env_modules_python import module as lmod_module
+try:
+    from env_modules_python import module as lmod_module
+except:
+    pass
+else:
+    def module(command, *arguments):
+        split_path_str = lambda P: P.split(':') if P is not None else []
 
-def module(command, *arguments):
+        PYTHONPATH_ORIG = os.environ.get('PYTHONPATH')
         lmod_module(command, *arguments)
-    # BAD: This will cause alteration of PYTHONPATH in a way that may not be desirable.
-    sys_path_orig = sys.path
         PYTHONPATH = os.environ.get('PYTHONPATH')
-    sys_path_new = PYTHONPATH.split(':') if PYTHONPATH is not None else []
-    for p in sys_path_orig:
-        if p not in sys_path_new:
-            sys_path_new.append(p)
-    print(sys_path_new)
-    sys.path = sys_path_new
 
+        # update sys.path
+        # Must account the *changes* in PYTHONPATH and reflect that to sys.path
+        #
+        # WARNING: This is not perfect!
+        # Due to the algorithm used below, the updated PYTHONPATH
+        # will not be reflected 100% in its order in sys.path
+        #
+        # 1) It assumes that additional paths are prepended.
+        # 2) It will completely delete a path that is remove
+        #    as a result of module command
+        # 3) If there are simultaneous deletion(s) and adition(s),
+        #    then the added paths will take precedence (unfortunately),
+        #    which may result in unexpected behavior in corner cases,
+        #    at least in principle.
+        #    This should not be the case for "well behaved" HPC packages
+        #    where order of loaded modules should NOT cause change in behavior.
+
+        # Save the original in case needed for debugging
+        global sys_path_orig
+        sys_path_orig = sys.path
+
+        python_path_orig = split_path_str(PYTHONPATH_ORIG)
+        python_path = split_path_str(PYTHONPATH)
+
+        sys_path_added = set(python_path) - set(python_path_orig)
+        sys_path_deleted = set(python_path_orig) - set(python_path)
+
+        # delete removed paths
+        sys_path_trimmed = [ p for p in sys_path_orig if p not in sys_path_deleted ]
+
+        # prepend new paths
+        sys_path_prepended = [ p for p in python_path if p in sys_path_added ]
+
+        sys.path = sys_path_prepended + sys_path_trimmed
+
+        # Extra: if LMOD_MANUAL_LD_LIBRARY_PATH is specified, then
+        # the shared library symlinks are updated.
         manual_ld_library_dir = os.environ.get('LMOD_MANUAL_LD_LIBRARY_PATH')
         if manual_ld_library_dir is None:
             return