New fixer: nested_except_trace

my2to3/fixes/fix_nested_except_trace.py

+from my2to3.fixes import BaseFix
+from my2to3.trace import create_table
+
+
+trace = create_table("nested_except", "filename", "lineno_parent", "lineno_child")
+
+
+class FixNestedExceptTrace(BaseFix):
+  """This fixer detects scope bugs which can occur due to nested except clauses
+  which use the same variable name.
+
+  Try the following code yourself in Python 2 and Python 3...
+
+  try:
+    raise Exception("foo")
+  except Exception as e:
+    try:
+      raise Exception("bar")
+    except Exception as e:
+      print(e)
+    print(e)
+  """
+  # https://github.com/python/cpython/blob/3549ca313a6103a3adb281ef3a849298b7d7f72c/Lib/lib2to3/fixes/fix_except.py#L39-L45
+  PATTERN = """
+  try_stmt< 'try' ':' (simple_stmt | suite)
+                cleanup=(except_clause ':' (simple_stmt | suite))+
+                tail=(['except' ':' (simple_stmt | suite)]
+                      ['else' ':' (simple_stmt | suite)]
+                      ['finally' ':' (simple_stmt | suite)]) >
+  """
+  order = "pre"
+
+  def start_tree(self, tree, filename):
+    super(FixNestedExceptTrace, self).start_tree(tree, filename)
+
+    # Store the results of every node that matches the pattern
+    self.results = []
+
+  def find_parents_results(self, node):
+    parents_results = []
+    parent = node.parent
+    while parent is not None:
+      for r in self.results:
+        if r['node'] is parent:
+          parents_results.append(r)
+      parent = parent.parent
+    return parents_results
+
+  def transform(self, node, results):
+    self.results.append(results)
+
+    # Find if the current node has parents, which are also a try_stmt
+    for parent_results in self.find_parents_results(node):
+      # Check if the node and its parent use the same variable name for the
+      # caught exception
+      parent_exception_node = parent_results['cleanup'][0]
+      exception_node = results['cleanup'][0]
+      try:
+        if exception_node.children[3] == parent_exception_node.children[3]:
+          trace(self.filename, parent_exception_node.get_lineno(), exception_node.get_lineno())
+      except Exception:
+        pass