Fix a crash caused by a lookup of a monkey-patched method (#803)2.4

Close PyCQA/pylint#3686

3 files changed, 24 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index cdabb7f1..83c09682 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,10 @@ What's New in astroid 2.4.3?
 ============================
 Release Date: TBA
 
+* Fix a crash caused by a lookup of a monkey-patched method
+
+  Close PyCQA/pylint#3686
+
 
 What's New in astroid 2.4.2?
 ============================
diff --git a/astroid/node_classes.py b/astroid/node_classes.py
index 621dc5f2..dfe85b04 100644
--- a/astroid/node_classes.py
+++ b/astroid/node_classes.py
@@ -1149,10 +1149,9 @@ class LookupMixIn:
         _stmts = []
         _stmt_parents = []
         statements = self._get_filtered_node_statements(stmts)
-
         for node, stmt in statements:
             # line filtering is on and we have reached our location, break
-            if stmt.fromlineno > mylineno > 0:
+            if stmt.fromlineno and stmt.fromlineno > mylineno > 0:
                 break
             # Ignore decorators with the same name as the
             # decorated function
diff --git a/tests/unittest_regrtest.py b/tests/unittest_regrtest.py
index 7171941b..17668edd 100644
--- a/tests/unittest_regrtest.py
+++ b/tests/unittest_regrtest.py
@@ -324,5 +324,24 @@ class Whatever:
     a = property(lambda x: x, lambda x: x)
 
 
+def test_ancestor_looking_up_redefined_function():
+    code = """
+    class Foo:
+        def _format(self):
+            pass
+
+        def format(self):
+            self.format = self._format
+            self.format()
+    Foo
+    """
+    node = extract_node(code)
+    inferred = next(node.infer())
+    ancestor = next(inferred.ancestors())
+    _, found = ancestor.lookup("format")
+    assert len(found) == 1
+    assert isinstance(found[0], nodes.FunctionDef)
+
+
 if __name__ == "__main__":
     unittest.main()