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PEP 626 -- Precise line numbers for debugging and other tools.

PEP:626
Title:Precise line numbers for debugging and other tools.
Author:Mark Shannon <mark at hotpy.org>
BDFL-Delegate:Pablo Galindo <pablogsal at python.org>
Status:Accepted
Type:Standards Track
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 source 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.

What is considered to be code for the purposes of tracing

All expressions and parts of expressions are considered to be executable code.

In general, all statements are also considered to be executable code. However, when a statement is spread over several lines, we must consider which parts of a statement are considered to be executable code.

Statements are made up of keywords and expressions. Not all keywords have a direct runtime effect, so not all keywords are considered to be executable code. For example, else, is a necessary part of an if statement, but there is no runtime effect associated with an else.

For the purposes of tracing, the following keywords will not be considered to be executable code:

  • del -- The expression to be deleted is treated as the executable code.
  • else -- No runtime effect
  • finally -- No runtime effect
  • global -- Purely declarative
  • nonlocal -- Purely declarative

All other keywords are considered to be executable code.

Example event sequences

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

The code

1.     global x
2.     x = a

generates the following event:

"line" 2

The code

1.     try:
2.        pass
3.     finally:
4.        pass

generates the following events:

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

The code

1.      for (
2.          x) in [1]:
3.          pass
4.      return

generates the following events:

"line" 2       # evaluate [1]
"line" 1       # for
"line" 2       # store to x
"line" 3       # pass
"line" 1       # for
"line" 4       # return
"return" 1

The f_lineno attribute

  • When a frame object is created, the f_lineno attribute will be set to the line at which the function or class is defined; that is the line on which the def or class keyword appears. 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.

Out of process debuggers and profilers

Out of process tools, such as py-spy [1], cannot use the C-API, and must parse the line number table themselves. Although the line number table format may change without warning, it will not change during a release unless absolutely necessary for a bug fix.

To reduce the work required to implement these tools, the following C struct and utility functions are provided. Note that these functions are not part of the C-API, so will be need to be linked into any code that needs to use them.

typedef struct addressrange {
    int ar_start;
    int ar_end;
    int ar_line;
    int opaque1;
    void *opaque2;
} PyCodeAddressRange;

void PyLineTable_InitAddressRange(char *linetable, int firstlineno, PyCodeAddressRange *range);
int PyLineTable_NextAddressRange(PyCodeAddressRange *range);
int PyLineTable_PreviousAddressRange(PyCodeAddressRange *range);

PyLineTable_InitAddressRange initializes the PyCodeAddressRange struct from the line number table and first line number.

PyLineTable_NextAddressRange advances the range to the next entry, returning non-zero if valid.

PyLineTable_PreviousAddressRange retreats the range to the previous entry, returning non-zero if valid.

Note

The data in linetable is immutable, but its lifetime depends on its code object. For reliable operation, linetable should be copied into a local buffer before calling PyLineTable_InitAddressRange.

Although these functions are not part of C-API, they will provided by all future versions of CPython. The PyLineTable_ functions do not call into the C-API, so can be safely copied into any tool that needs to use them. The PyCodeAddressRange struct may acquire additional opaque fields in future versions, but the ar_ fields will remain unchanged.

For example, the following code prints out all the address ranges:

void print_address_ranges(char *linetable, int firstlineno)
{
    PyCodeAddressRange range;
    PyLineTable_InitAddressRange(linetable, firstlineno, &range);
    while (PyLineTable_NextAddressRange(&range)) {
        printf("Bytecodes from %d (inclusive) to %d (exclusive) ",
               range.start, range.end);
        if (range.line < 0) {
            /* line < 0 means no line number */
            printf("have no line number\n");
        }
        else {
            printf("have line number %d\n", range.line);
        }
    }
}

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.

References

[1]py-spy: Sampling profiler for Python programs (https://github.com/benfred/py-spy)
Source: https://github.com/python/peps/blob/master/pep-0626.rst