[Python-checkins] bpo-44964: Add a note explaining the new semantics of f_last_i in frame objects (GH-28200)

[miss-islington]

https://github.com/python/cpython/commit/fc840736e54da0557616882012f362b809490165
commit: fc840736e54da0557616882012f362b809490165
branch: 3.10
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: miss-islington <31488909+miss-islington at users.noreply.github.com>
date: 2021-09-07T04:36:43-07:00
summary:

bpo-44964: Add a note explaining the new semantics of f_last_i in frame objects (GH-28200)

(cherry picked from commit fa2c0b85a8d5c9486661083afdf38cbaadb3432a)

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

files:
M Doc/reference/datamodel.rst
M Doc/whatsnew/3.10.rst

diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index eefdc3d5100b56..477daaed8d3b1f 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1034,8 +1034,9 @@ Internal types
       :attr:`f_code` is the code object being executed in this frame; :attr:`f_locals`
       is the dictionary used to look up local variables; :attr:`f_globals` is used for
       global variables; :attr:`f_builtins` is used for built-in (intrinsic) names;
-      :attr:`f_lasti` gives the precise instruction (this is an index into the
-      bytecode string of the code object).
+      :attr:`f_lasti` gives the precise instruction (it represents a wordcode index, which
+      means that to get an index into the bytecode string of the code object it needs to be
+      multiplied by 2).
 
       Accessing ``f_code`` raises an :ref:`auditing event <auditing>`
       ``object.__getattr__`` with arguments ``obj`` and ``"f_code"``.
diff --git a/Doc/whatsnew/3.10.rst b/Doc/whatsnew/3.10.rst
index 068eb7676324d5..dbf89239d652ef 100644
--- a/Doc/whatsnew/3.10.rst
+++ b/Doc/whatsnew/3.10.rst
@@ -1944,6 +1944,12 @@ Changes in the C API
        source_buf = PyBytes_AsString(source_bytes_object);
        code = Py_CompileString(source_buf, filename, Py_file_input);
 
+  * For ``FrameObject`` objects, the ``f_lasti`` member now represents a wordcode
+    offset instead of a simple offset into the bytecode string. This means that this
+    number needs to be multiplied by 2 to be used with APIs that expect a byte offset
+    instead (like :c:func:`PyCode_Addr2Line` for example). Notice as well that the
+    ``f_lasti`` member of ``FrameObject`` objects is not considered stable.
+
 CPython bytecode changes
 ========================