diff --git a/jedi/common/utils.py b/jedi/common/utils.py
index 72726a46..92ff52e2 100644
--- a/jedi/common/utils.py
+++ b/jedi/common/utils.py
@@ -1,4 +1,5 @@
 import os
+from contextlib import contextmanager
 
 
 def traverse_parents(path, include_current=False):
@@ -10,3 +11,16 @@ def traverse_parents(path, include_current=False):
         yield path
         previous = path
         path = os.path.dirname(path)
+
+
+@contextmanager
+def monkeypatch(obj, attribute_name, new_value):
+    """
+    Like pytest's monkeypatch, but as a context manager.
+    """
+    old_value = getattr(obj, attribute_name)
+    try:
+        setattr(obj, attribute_name, new_value)
+        yield
+    finally:
+        setattr(obj, attribute_name, old_value)
diff --git a/jedi/evaluate/lazy_context.py b/jedi/evaluate/lazy_context.py
index 9380212e..97e4b068 100644
--- a/jedi/evaluate/lazy_context.py
+++ b/jedi/evaluate/lazy_context.py
@@ -1,4 +1,5 @@
 from jedi.evaluate.base_context import ContextSet, NO_CONTEXTS
+from jedi.common.utils import monkeypatch
 
 class AbstractLazyContext(object):
     def __init__(self, data):
@@ -40,12 +41,8 @@ class LazyTreeContext(AbstractLazyContext):
         self._predefined_names = dict(context.predefined_names)
 
     def infer(self):
-        old, self._context.predefined_names = \
-            self._context.predefined_names, self._predefined_names
-        try:
+        with monkeypatch(self._context, 'predefined_names', self._predefined_names):
             return self._context.eval_node(self.data)
-        finally:
-            self._context.predefined_names = old
 
 
 def get_merged_lazy_context(lazy_contexts):