bpo-3530: Add advice on when to correctly use fix_missing_locations in the AST docs (GH-17172)

Co-authored-by: Pablo Galindo <[email protected]>

Author: isidentical

Doc/library/ast.rst

@@ -316,7 +316,7 @@ and classes for traversing abstract syntax trees:
       class RewriteName(NodeTransformer):
 
           def visit_Name(self, node):
-              return copy_location(Subscript(
+              return Subscript(
                   value=Name(id='data', ctx=Load()),
                   slice=Index(value=Constant(value=node.id)),
                   ctx=node.ctx
@@ -330,6 +330,14 @@ and classes for traversing abstract syntax trees:
    statement nodes), the visitor may also return a list of nodes rather than
    just a single node.
 
+   If :class:`NodeTransformer` introduces new nodes (that weren't part of
+   original tree) without giving them location information (such as
+   :attr:`lineno`), :func:`fix_missing_locations` should be called with
+   the new sub-tree to recalculate the location information::
+
+      tree = ast.parse('foo', mode='eval')
+      new_tree = fix_missing_locations(RewriteName().visit(tree))
+
    Usually you use the transformer like this::
 
       node = YourTransformer().visit(node)

Misc/NEWS.d/next/Documentation/2019-11-17-11-53-10.bpo-3530.8zFUMc.rst

@@ -0,0 +1,2 @@
+In the :mod:`ast` module documentation, fix a misleading ``NodeTransformer`` example and add
+advice on when to use the ``fix_missing_locations`` function.