python-peps/pep-0626.rst

PEP: 626
Title: Precise line numbers for debugging and other tools.
Author: Mark Shannon <mark@hotpy.org>
Status: Active
Type: Process
Content-Type: text/x-rst
Created: 15-Jul-2020
Python-Version: 3.10
Post-History: 17-Jul-2020

Abstract
========

Python should guarantee that when tracing is turned on, "line" tracing events are generated for *all* lines of code executed and *only* for lines of
code that are executed.

The ``f_lineo`` attribute of frame objects should always contain the expected line number.
During frame execution, the expected line number is the line number of source code currently being executed.
After a frame has completed, either by returning or by raising an exception,
the expected line number is the line number of the last line of source that was executed.

A side effect of ensuring correct line numbers, is that some bytecodes will need to be marked as artificial, and not have a meaningful line number.
To assist tools, a new ``co_lines`` attribute will be added that describes the mapping from bytecode to source.

Motivation
==========

Users of ``sys.settrace`` and associated tools should be able to rely on tracing events being
generated for all lines of code, and only for actual code.
They should also be able to assume that the line number in ``f_lineno`` is correct.

The current implementation mostly does this, but fails in a few cases.
This requires workarounds in tooling and is a nuisance for alternative Python implementions.

Having this guarantee also benefits implementers of CPython in the long term, as the current behaviour is not obvious and has some odd corner cases.

Rationale
=========

In order to guarantee that line events are generated when expected, the ``co_lnotab`` attribute, in its current form,
can no longer be the soure of truth for line number information.

Rather than attempt to fix the ``co_lnotab`` attribute, a new method
``co_lines()`` will be added, which returns an iterator over bytecode offsets and source code lines.

Ensuring that the bytecode is annotated correctly to enable accurate line number information means that
some bytecodes must be marked as artificial, and not have a line number.

Some care must be taken not to break existing tooling.
To minimize breakage, the ``co_lnotab`` attribute will be retained, but lazily generated on demand.

Specification
=============

Line events and the ``f_lineno`` attribute should act as an experienced Python user would expect in *all* cases.

Tracing
'''''''

Tracing generates events for calls, returns, exceptions, lines of source code executed, and, under some circumstances, instructions executed.

Only line events are covered by this PEP.

When tracing is turned on, line events will be generated when:

* A new line of source code is reached.
* A backwards jump occurs, even if it jumps to the same line, as may happen in list comprehensions.

Additionally, line events will *never* be generated for source code lines that are not executed.

The f_lineno attribute
''''''''''''''''''''''

* When a frame object is created, the ``f_lineno`` will be set to the line
  at which the function or class is defined. For modules it will be set to zero.
* The ``f_lineno`` attribute will be updated to match the line number about to be executed,
  even if tracing is turned off and no event is generated.

The new co_lines() method of code objects
'''''''''''''''''''''''''''''''''''''''''

The ``co_lines()`` method will return an iterator which yields tuples of values,
each representing the line number of a range of bytecodes. Each tuple will consist of three values:

* ``start`` -- The offset (inclusive) of the start of the bytecode range
* ``end`` -- The offset (exclusive) of the end of the bytecode range
* ``line`` -- The line number, or ``None`` if the the bytecodes in the given range do not have a line number.

The sequence generated will have the following properties:

* The first range in the sequence with have a ``start`` of ``0``
* The ``(start, end)`` ranges will be strictly increasing and consecutive.
  That is, for any pair of tuples the ``start`` of the second
  will equal to the ``end`` of the first.
* No range will be empty, that is ``end`` > ``start`` for all triples.
* The final range in the sequence with have ``end`` equal to the size of the bytecode.
* ``line`` will either be a positive integer, or ``None``

The co_linetable attribute
''''''''''''''''''''''''''

The co_linetable attribute will hold the line number information.
The format is opaque, unspecified and may be changed without notice.
The attribute is public only to support creation of new code objects.

The co_lnotab attribute
'''''''''''''''''''''''

Historically the ``co_lnotab`` attribute held a mapping from bytecode offset to line number, but does not support bytecodes without a line number.
For backward compatibility, the ``co_lnotab`` bytes object will be lazily created when needed.
For ranges of bytecodes without a line number, the line number of the previous bytecode range will be used.

Tools that parse the ``co_lnotab`` table should move to using the new ``co_lines()`` method as soon as is practical.


Backwards Compatibility
=======================

The ``co_lnotab`` attribute will be deprecated in 3.10 and removed in 3.12.

Any tools that parse the ``co_lnotab`` attribute of code objects will need to move to using ``co_lines()`` before 3.12 is released.
Tools that use ``sys.settrace`` will be unaffected, except in cases where the "line" events they receive are more accurate.


Examples of code for which the sequence of trace events will change
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

In the following examples, events are listed as "name", ``f_lineno`` pairs.


``pass`` statement in an ``if`` statement.
------------------------------------------

::

  0.  def spam(a):
  1.      if a:
  2.          eggs()
  3.      else:
  4.          pass

If ``a`` is ``True``, then the sequence of events generated by Python 3.9 is::

  "line" 1
  "line" 2
  "line" 4
  "return" 4

From 3.10 the sequence will be::

  "line" 1
  "line" 2
  "return" 2

Multiple ``pass`` statements.
-----------------------------

::

  0.  def bar():
  1.      pass
  2.      pass
  3.      pass

The sequence of events generated by Python 3.9 is::

  "line" 3
  "return" 3

From 3.10 the sequence will be::

  "line" 1
  "line" 2
  "line" 3
  "return" 3

C API
'''''

Access to the ``f_lineno`` attribute of frame objects through C API functions is unchanged.
``f_lineno`` can be read by ``PyFrame_GetLineNumber``. ``f_lineno`` can only be set via ``PyObject_SetAttr`` and similar functions.

Accessing ``f_lineno`` directly through the underlying data structure is forbidden.


Performance Implications
========================

In general, there should be no change in performance.
When tracing, programs should run a little faster as the new table format can be designed with line number calculation speed in mind.
Code with long sequences of ``pass`` statements will probably become a bit slower.

Reference Implementation
========================

Under development.

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.



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