diff -r 2c124e30a324 Lib/inspect.py
--- a/Lib/inspect.py	Fri Aug 07 00:43:39 2015 -0700
+++ b/Lib/inspect.py	Fri Aug 07 12:02:27 2015 -0400
@@ -1907,8 +1907,7 @@
             try:
                 value = eval(s, sys_module_dict)
             except NameError:
-                raise RuntimeError()
-
+                raise RuntimeError('cannot resolve expression {!r}'.format(s))
         if isinstance(value, str):
             return ast.Str(value)
         if isinstance(value, (int, float)):
@@ -1917,7 +1916,11 @@
             return ast.Bytes(value)
         if value in (True, False, None):
             return ast.NameConstant(value)
-        raise RuntimeError()
+        if callable(value):
+            raise MethodEvalNeededError(s)
+        raise RuntimeError('unsupported value type {!r}'.format(type(value)))
+
+    class MethodEvalNeededError(Exception): pass
 
     class RewriteSymbolics(ast.NodeTransformer):
         def visit_Attribute(self, node):
@@ -1947,6 +1950,11 @@
                 o = ast.literal_eval(default_node)
             except ValueError:
                 o = invalid
+            except MethodEvalNeededError as ex:
+                # if a parameter had its default value defined as a function call,
+                # i.e. "encoding=sys.getdefaultencoding()", we will render its
+                # default as a string: "encoding='<sys.getdefaultencoding()>'"
+                o = '<{}()>'.format(ex.args[0])
             if o is invalid:
                 return None
             default = o if o is not invalid else default