diff --git a/Doc/library/ast.rst b/Doc/library/ast.rst index c380a81bee6..2cee8738e58 100644 --- a/Doc/library/ast.rst +++ b/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) diff --git a/Misc/NEWS.d/next/Documentation/2019-11-17-11-53-10.bpo-3530.8zFUMc.rst b/Misc/NEWS.d/next/Documentation/2019-11-17-11-53-10.bpo-3530.8zFUMc.rst new file mode 100644 index 00000000000..65f1a6d156a --- /dev/null +++ b/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.