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.

3 files changed, 19 insertions, 0 deletions

diff --git a/astroid/as_string.py b/astroid/as_string.py
index 0d7ba51..b2b2c95 100644
--- a/astroid/as_string.py
+++ b/astroid/as_string.py
@@ -435,6 +435,9 @@ class AsStringVisitor(object):
     def visit_super(self, node):
         return node.parent.accept(self)
 
+    def visit_uninferable(self, node):
+        return str(node)
+
 
 class AsStringVisitor3(AsStringVisitor):
     """AsStringVisitor3 overwrites some AsStringVisitor methods"""
diff --git a/astroid/tests/unittest_nodes.py b/astroid/tests/unittest_nodes.py
index 3677bfb..e2e8338 100644
--- a/astroid/tests/unittest_nodes.py
+++ b/astroid/tests/unittest_nodes.py
@@ -62,6 +62,16 @@ class AsStringTest(resources.SysPathSetup, unittest.TestCase):
         node = parse(code)
         self.assertEqual(node.as_string().strip(), code.strip())
 
+    def test_as_string_for_list_containing_uninferable(self):
+        node = test_utils.extract_node('''
+        def foo():
+            bar = [arg] * 1
+        ''')
+        binop = node.body[0].value
+        inferred = next(binop.infer())
+        self.assertEqual(inferred.as_string(), '[Uninferable]')
+        self.assertEqual(binop.as_string(), '([arg]) * (1)')
+
     def test_frozenset_as_string(self):
         nodes = test_utils.extract_node('''
         frozenset((1, 2, 3)) #@
diff --git a/astroid/util.py b/astroid/util.py
index 1d50f7b..40c0fb2 100644
--- a/astroid/util.py
+++ b/astroid/util.py
@@ -37,17 +37,23 @@ 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