Add note that explain() is only supported under Python 3.

2 files changed, 6 insertions, 8 deletions

diff --git a/CHANGES b/CHANGES
index eb13d01..675a284 100644
--- a/CHANGES
+++ b/CHANGES
@@ -42,6 +42,8 @@ Version 2.3.1 - January, 2019
   names and parse expressions in the stack trace. This is especially
   useful when an exception is raised in a parse action.
 
+  Note: explain() is only supported under Python 3.
+
 - Fix bug in dictOf which could match an empty sequence, making it
   infinitely loop if wrapped in a OneOrMore.
 
diff --git a/pyparsing.py b/pyparsing.py
index fd7c0fa..ab804d5 100644
--- a/pyparsing.py
+++ b/pyparsing.py
@@ -333,6 +333,8 @@ class ParseException(ParseBaseException):
         that failed to parse. These representations will be more helpful if you use `setName` to
         give identifiable names to your expressions. Otherwise they will use the default string
         forms, which may be cryptic to read.
+
+        explain() is only supported under Python 3.
         """
         import inspect
 
@@ -344,15 +346,9 @@ class ParseException(ParseBaseException):
             ret.append(' ' * (exc.col - 1) + '^')
         ret.append("{0}: {1}".format(type(exc).__name__, exc))
 
-        if hasattr(exc, '__traceback__'):
-            exc_tb = exc.__traceback__
-        else:
-            exc_tb = sys.exc_info()[-1]
-        callers = inspect.getinnerframes(exc_tb, context=depth)
-
-        seen = set()
-
         if depth > 0:
+            callers = inspect.getinnerframes(exc.__traceback__, context=depth)
+            seen = set()
             for i, ff in enumerate(callers[-depth:]):
                 frm = ff.frame