diff --git a/py/introspection.py b/py/introspection.py
index 810bd7f..28767f6 100644
--- a/py/introspection.py
+++ b/py/introspection.py
@@ -15,6 +15,9 @@ def name_rlookup(val, namespace, prefix=None, rx_match=None, lookup="dict"):
 
   This is useful for example to reverse-lookup an object's name, but only in
   restricted contexts (e.g. when the value to look for is unique).
+
+  `prefix` or `rx_match` can be given to narrow the scope of names to look for.
+  The two cannot be given at once; `prefix` will take precedence in that case.
   """
   # Imported 20150819
   # Original subroutine name: search_var_name (from Cr2_analysis_cbs.py).
@@ -23,9 +26,9 @@ def name_rlookup(val, namespace, prefix=None, rx_match=None, lookup="dict"):
   else:
     # attribute lookup
     names = dir(namespace)
-  if prefix != None:
+  if prefix is not None:
     names_filt = [ n for n in names if n.startswith(prefix) ]
-  elif rx_match != None:
+  elif rx_match is not None:
     # TODO later: for my regexp object?
     if True:
       if isinstance(rx_match, basestring):