Fix the lambda issues.

parso/python/normalizer.py

@@ -47,7 +47,10 @@ class PEP8Normalizer(Normalizer):
                 for name in names[:1]:
                     self.add_issue(401, 'Multiple imports on one line', name)
         elif typ == 'lambdef':
-            if node.parent.type == 'expr_stmt':
+            expr_stmt = node.parent
+            # Check if it's simply defining a single name, not something like
+            # foo.bar or x[1], where using a lambda could make more sense.
+            if expr_stmt.type == 'expr_stmt' and any(n.type == 'name' for n in expr_stmt.children[:-2:2]):
                 self.add_issue(731, 'Do not assign a lambda expression, use a def', node)
         elif typ == 'try_stmt':
             for child in node.children:

test/normalizer_issue_files/E73.py

@@ -0,0 +1,16 @@
+#: E731:4
+f = lambda x: 2 * x
+while False:
+    #: E731:6
+    foo = lambda y, z: 2 * x
+# Okay
+f = object()
+f.method = lambda: 'Method'
+
+f = {}
+f['a'] = lambda x: x ** 2
+
+f = []
+f.append(lambda x: x ** 2)
+
+lambda: 'no-op'