make docstring handling more robust and work with -OO python interpreter option

Author: danxshap

pandas/core/format.py

@@ -156,6 +156,7 @@ class DataFrameFormatter(object):
 
     """
 
+    __doc__ = __doc__ if __doc__ else ''
     __doc__ += docstring_to_string
 
     def __init__(self, frame, buf=None, columns=None, col_space=None,

pandas/util/decorators.py

@@ -93,7 +93,9 @@ def __init__(self, addendum, join='', indents=0):
         self.join = join
 
     def __call__(self, func):
-        docitems = [func.__doc__ if func.__doc__ else '', self.addendum]
+        func.__doc__ = func.__doc__ if func.__doc__ else ''
+        self.addendum = self.addendum if self.addendum else ''
+        docitems = [func.__doc__, self.addendum]
         func.__doc__ = ''.join(docitems)
         return func