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

Close pylint-dev/pylint#3686

Author: PCManticore

ChangeLog

@@ -24,6 +24,15 @@ Release Date: TBA
   Close PyCQA/pylint#3583
 
 
+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?
 ============================
 Release Date: 2020-06-08

astroid/node_classes.py

@@ -1149,10 +1149,9 @@ def _filter_stmts(self, stmts, frame, offset):
         _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

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()