gh-136057: Allow step and next to step over for loops #136160

gaogaotiantian · 2025-07-01T04:55:57Z

Due to historical reasons, sys.settrace issues a line event for for loops even if the jump target is on the same line:

# 10 line events on this line
for _ in range(10): pass

We don't want to touch this behavior, because it's been like this for like forever.

However, it's not desirable to pdb. When the user does next or step command, they want to step over this line and reach the next line.

Our documentation also clearly states such behavior:

s(tep)
Execute the current line, stop at the first possible occasion (either in a function that is called or on the next line in the current function).

n(ext)
Continue execution until the next line in the current function is reached or it returns. (The difference between next and step is that step stops inside a called function, while next executes called functions at (nearly) full speed, only stopping at the next line in the current function.)

So I consider the current behavior as a bug, which we should fix.

A thought might be - why don't we check this in stop_here? The reason is because dispatch_exception also uses stop_here and we should trigger user_exception even if it's on the same line.

An extra check in dispatch_line is the cleanest way I can think of to make this happen - only line events should be dealt with. We don't need to worry about the extra reference to a frame because the eventual continue (or anything other than step or next) will clear this reference.

Let me know if anyone has a better idea of how to implement this.

Issue: pdb: 'n' and 'until' do not skip list comprehension after setting a breakpoint #136057

iritkatriel · 2025-07-01T17:30:50Z

Lib/bdb.py

-        if self.stop_here(frame) or self.break_here(frame):
+        # GH-136057
+        # For line events, besides whether we should stop at the frame, we
+        # also need to check if it's the same line as we issue the command.


I don't think this is very clear: "it's the same line as we issue the command". I think you mean "the same frame and line at which the break command was issued by the user".

It's not the break command, it's either next or step. But yes I'm open to all suggestions to make this clearer.

iritkatriel · 2025-07-01T17:47:47Z

Lib/bdb.py

        self.stoplineno = stoplineno
+        # startframe/startlineno is the frame/line number when the user does
+        # step or next. We don't want to stop at the same line for those commands.
+        self.startframe = startframe


Maybe we could pick a more informative name, something like "command frame", because it's ambiguous here what exactly started at that frame.

Yeah we can change it, but there's actually a caveat here - it's not really the command frame. You can do up to an upper frame and do next/step - the behavior is different. next will next to the line at the upper frame, so the frame we need is the "command frame". step however, will stop at the innermost frame, no matter which frame you are inspecting - that's just how step works.

So it's more like a "do not stop here again if it's a line event" frame/lineno. Do you think there's better name to describe it?

gaogaotiantian · 2025-07-05T18:16:49Z

@iritkatriel do you think the current name and comments are better?

Lib/bdb.py

iritkatriel · 2025-07-05T20:16:33Z

Lib/bdb.py

-        if self.stop_here(frame) or self.break_here(frame):
+        # GH-136057
+        # For line events, we don't want to stop at the same line where
+        # we issue the previous next/step command.


If we stop, it's not necessarily due to a next/step, right? Could it also be a breakpoint or until, etc?

So maybe the comment can say:

If we are stopping due to a next/step command, we don't want to stop on the same line on which
the command was issued.

So I'm thinking. If we are currently at line 10, and the user does a until 10, should we stop at the current line? And if we stopped by a breakpoint, then we do continue, should we stop at the same line? Those are kind of undefined behaviors, unlike step and next which explicitly said that the debugger should stop at the "next" line.

Lib/bdb.py

Co-authored-by: Irit Katriel <[email protected]>

gaogaotiantian · 2025-11-14T06:02:51Z

Hi @iritkatriel , want to circle back to this. I made a small change to initialize the attributes - just merged a PR for pdb so I think might as well do it for at least the newly added attributes for bdb.

iritkatriel · 2025-11-14T06:08:45Z

Lib/test/test_pdb.py


+def test_pdb_issue_gh_136057():
+    """See GH-136057
+    "step" and "next" commands should be able to get over list comprehensions


Shouldn't it be that next steps over the list comp but step executes every iteration?

Not according to our documentation

Execute the current line, stop at the first possible occasion (either in a function that is called or on the next line in the current function).

I mean we can have different interpretation of the phrase "first possible occasion". Technically we can do this at bytecode level - but that's probably not we really want. I think we should stick to the explanation in the parenthesis.

miss-islington-app · 2025-11-16T21:57:11Z

Thanks @gaogaotiantian for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

…-136160) (cherry picked from commit 8be3b2f) Co-authored-by: Tian Gao <[email protected]>

miss-islington-app · 2025-11-16T21:57:18Z

Sorry, @gaogaotiantian, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 8be3b2f479431f670f2e81e41b52e698c0806289 3.13

bedevere-app · 2025-11-16T21:57:19Z

GH-141640 is a backport of this pull request to the 3.14 branch.

bedevere-app · 2025-11-16T22:01:10Z

GH-141641 is a backport of this pull request to the 3.13 branch.

…) (#141640) gh-136057: Allow step and next to step over for loops (GH-136160) (cherry picked from commit 8be3b2f) Co-authored-by: Tian Gao <[email protected]>

…) (#141641) (cherry picked from commit 8be3b2f)

Allow step and next to step over for loops

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gaogaotiantian added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Jul 1, 2025

bedevere-app bot added the awaiting core review label Jul 1, 2025

bedevere-app bot mentioned this pull request Jul 1, 2025

pdb: 'n' and 'until' do not skip list comprehension after setting a breakpoint #136057

Closed

📜🤖 Added by blurb_it.

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gaogaotiantian requested review from iritkatriel and markshannon July 1, 2025 04:58

gaogaotiantian added 2 commits June 30, 2025 22:08

Update 2025-07-01-04-57-57.gh-issue-136057.4-t596.rst

e47731e

Deal with enterframe == None in set_step

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iritkatriel reviewed Jul 1, 2025

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Rename the variable and update the comments

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Fix lint

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iritkatriel reviewed Jul 5, 2025

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gaogaotiantian and others added 3 commits July 5, 2025 13:57

Apply suggestions from code review

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Co-authored-by: Irit Katriel <[email protected]>

Initialize new attributes

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Merge branch 'main' into pdb-next-for-loop

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iritkatriel reviewed Nov 14, 2025

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iritkatriel approved these changes Nov 14, 2025

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bedevere-app bot added awaiting merge and removed awaiting core review labels Nov 14, 2025

Merge branch 'main' into pdb-next-for-loop

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gaogaotiantian merged commit 8be3b2f into python:main Nov 16, 2025
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gaogaotiantian deleted the pdb-next-for-loop branch November 16, 2025 21:57

bedevere-app bot removed the awaiting merge label Nov 16, 2025

miss-islington pushed a commit to miss-islington/cpython that referenced this pull request Nov 16, 2025

pythongh-136057: Allow step and next to step over for loops (pythonGH…

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…-136160) (cherry picked from commit 8be3b2f) Co-authored-by: Tian Gao <[email protected]>

miss-islington-app bot assigned gaogaotiantian Nov 16, 2025

bedevere-app bot removed the needs backport to 3.14 bugs and security fixes label Nov 16, 2025

bedevere-app bot removed the needs backport to 3.13 bugs and security fixes label Nov 16, 2025

gaogaotiantian added a commit that referenced this pull request Nov 16, 2025

[3.13] gh-136057: Allow step and next to step over for loops (GH-136160…

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…) (#141641) (cherry picked from commit 8be3b2f)

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