Raise exceptions on LookupErrors in ErrorTrees

Closes #92

2 files changed, 17 insertions, 0 deletions

diff --git a/jsonschema.py b/jsonschema.py
index d1cb280..9b528d2 100644
--- a/jsonschema.py
+++ b/jsonschema.py
@@ -1228,6 +1228,8 @@ class ErrorTree(object):
 
     """
 
+    _instance = _unset
+
     def __init__(self, errors=()):
         self.errors = {}
         self._contents = collections.defaultdict(self.__class__)
@@ -1238,6 +1240,8 @@ class ErrorTree(object):
                 container = container[element]
             container.errors[error.validator] = error
 
+            self._instance = error.instance
+
     def __contains__(self, k):
         return k in self._contents
 
@@ -1245,8 +1249,14 @@ class ErrorTree(object):
         """
         Retrieve the child tree with key ``k``.
 
+        If the key is not in the instance that this tree corresponds to,
+        whatever error would be raised by ``instance.__getitem__`` will be
+        propagated (usually this is some subclass of :class:`LookupError`.
+
         """
 
+        if self._instance is not _unset:
+            self._instance[k]
         return self._contents[k]
 
     def __setitem__(self, k, v):
diff --git a/tests.py b/tests.py
index 05ef02f..3160f90 100644
--- a/tests.py
+++ b/tests.py
@@ -608,6 +608,13 @@ class TestErrorTree(unittest.TestCase):
         tree = ErrorTree([e1, e2])
         self.assertEqual(tree["bar"][0].errors, {"foo" : e1, "quux" : e2})
 
+    def test_it_does_not_contain_subtrees_that_are_not_in_the_instance(self):
+        error = ValidationError("a message", validator="foo", instance=[])
+        tree = ErrorTree([error])
+
+        with self.assertRaises(IndexError):
+            tree[0]
+
 
 class ValidatorTestMixin(object):
     def setUp(self):