Add support for handling Uninferable nodes when calling as_string

Some object, for instance List or Tuple can have, after inference,
Uninferable as their elements, happening when their components
weren't couldn't be inferred properly. This means that as_string
needs to cope with expecting Uninferable nodes part of the other
nodes coming for a string transformation. The patch adds a visit
method in AsString and ``accept`` on Yes / Uninferable nodes.
Closes issue #270.

1 files changed, 6 insertions, 0 deletions

diff --git a/astroid/util.py b/astroid/util.py
index 9c20eb4b..6fd59cfb 100644
--- a/astroid/util.py
+++ b/astroid/util.py
@@ -76,17 +76,23 @@ class Uninferable(object):
     """Special inference object, which is returned when inference fails."""
     def __repr__(self):
         return 'Uninferable'
+    __str__ = __repr__
 
     def __getattribute__(self, name):
         if name == 'next':
             raise AttributeError('next method should not be called')
         if name.startswith('__') and name.endswith('__'):
             return object.__getattribute__(self, name)
+        if name == 'accept':
+            return object.__getattribute__(self, name)
         return self
 
     def __call__(self, *args, **kwargs):
         return self
 
+    def accept(self, visitor):
+        func = getattr(visitor, "visit_uninferable")
+        return func(self)
 
 class BadOperationMessage(object):
     """Object which describes a TypeError occurred somewhere in the inference chain