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

Pablo Galindo webhook-mailer at python.org
Sun Jan 12 15:38:57 EST 2020

commit: 6680f4a9f5d15ab82b2ab6266c6f917cb78c919a
branch: master
author: Batuhan Taşkaya <47358913+isidentical at users.noreply.github.com>
committer: Pablo Galindo <Pablogsal at gmail.com>
date: 2020-01-12T20:38:53Z

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

Co-authored-by: Pablo Galindo <Pablogsal at gmail.com>

A Misc/NEWS.d/next/Documentation/2019-11-17-11-53-10.bpo-3530.8zFUMc.rst
M Doc/library/ast.rst

diff --git a/Doc/library/ast.rst b/Doc/library/ast.rst
index c380a81bee6d2..2cee8738e5834 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()),
@@ -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 0000000000000..65f1a6d156a12
--- /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.

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