sys.monitoring in 3.13+ does not trigger LINE event when there is no line break before

[Oberon00]

Bug report

Bug description:

In sys.monitoring there seems to be a regression from 3.13+ on where the LINE event does not fire for the first line of a code object if it is on the same line as the def /lambda statement. In Pyhton 3.12 it works. The bug disappears if on all versions if the PY_START event is added to the event mask.

Reproduction code:

import sys

print(f"Python {sys.version}")

TOOL = 1
sys.monitoring.use_tool_id(TOOL, "test")
hits = []
sys.monitoring.register_callback(TOOL, sys.monitoring.events.LINE,
                                 lambda _code, line: hits.append(line))

# A: LINE event never fires in single-line function/lambda on 3.13+
def f(x): return x + 1  #<<<<<<<<<< LINE 12
#f = lambda x: x + 1 # same issue

sys.monitoring.set_local_events(TOOL, f.__code__, sys.monitoring.events.LINE)
f(0)
print(f"Example A: {hits}")

hits.clear()

# B: Adding PY_START makes the LINE fire (even though we have no callback registred for PY_START)
sys.monitoring.set_local_events(TOOL, f.__code__, sys.monitoring.events.LINE | sys.monitoring.events.PY_START)
f(0)
print(f"Example B: {hits}")

hits.clear()

# C: LINE fires but never for first line
def g(x): x = 0; x += 1; \
    x += 2; return x  #<<<<<<<<<< LNE 30
sys.monitoring.set_local_events(TOOL, g.__code__, sys.monitoring.events.LINE)
g(0)
print(f"Example C: {hits}")

The expected output would be as in Python 3.12, we get line events for line 12 and 29 even if PY_START (example B) is not set:

$ python3.12 cpython_issue_monitoring_line_event.py # Only one that fully works
Python 3.12.3 (main, Jun 19 2026, 12:46:00) [GCC 13.3.0]
Example A: [12]
Example B: [12]
Example C: [29, 30]

But the actual output from 3.13 on is missing the first lines:

$ python3.13 cpython_issue_monitoring_line_event.py 
Python 3.13.15 (main, Aug  6 2026, 11:06:22) [GCC 13.3.0]
Example A: []
Example B: [12]
Example C: [30]

I.e., same with:

• Python 3.14.6 (main, Jun 11 2026, 12:32:48) [GCC 13.3.0]
• Python 3.15.0rc1 (main, Aug 5 2026, 11:04:42) [GCC 13.3.0]

CPython versions tested on:

3.12, 3.13, 3.14, 3.15

Operating systems tested on:

Linux

Linked PRs

• gh-155992: Fix missing LINE event for first line of a code object #155996

[PrathamGhaywat]

Hey @Oberon00,
I was able to reproduce the problem myself on Windows and got the same results you described above. I've started working on a fix.

[sergey-miryanov]

@PrathamGhaywat It's kind to ask the author of the issue before jumping in to implement it.

[PrathamGhaywat]

@sergey-miryanov I am really sorry for not having asked properly. I am still new to contributing to OSS projects, so please pardon my lack of knowledge.

@Oberon00 sorry as well!