Add support for indexing containers with instances which provides an __index__ returning-int method.

This patch moves _class_as_index to helpers, where it becames class_instance_as_index.
Also, it instantiates its own call context, which makes certain idioms with lambdas
to work.

1 files changed, 4 insertions, 0 deletions

diff --git a/astroid/inference.py b/astroid/inference.py
index b1c81da..3253acb 100644
--- a/astroid/inference.py
+++ b/astroid/inference.py
@@ -252,6 +252,10 @@ def infer_subscript(self, context=None):
             step = _slice_value(index.step, context)
             if all(elem is not _SLICE_SENTINEL for elem in (lower, upper, step)):
                 index_value = slice(lower, upper, step)
+        elif isinstance(index, bases.Instance):
+            index = helpers.class_instance_as_index(index)
+            if index:
+                index_value = index.value
         else:
             raise exceptions.InferenceError()