Debugger out-of-sync with code with blank line

[paul-rogers]

Type: Bug

The debugger seems to be thrown off by a blank line at the start of a code cell, resulting in an "off by one" of breakpoints and the yellow "current statement" indicator.

To see this, create a new notebook and choose the Python kernel. Create two cells.

Cell 1. Important: there is a blank line before the def statement.

def fib(n):
    if n < 0:
        raise Exception("Invalid input")
    if n < 2:
        return n
    return fib(n-1) + fib(n-2)

Cell 2:

fib(5)

• Set breakpoints on the first three lines of fib(.).
• Choose Debug Cell from the side menu for Cell 2.
• The debugger stops at the raise statement without stopping at the if statement. Note that the argument is 5, so we're not actually on the raise line, we are actually on the if n < 0 line.
• Click Step Over (or hit F10 if that is your binding)
• The debugger is now in the return n line, without hitting the second if statement. However, actually, the intperter is on the if statement: the UI is wrong.
• Click Step Over again. We again hit the breakpoint on raise. What actually happened is that the debugger is executing the return... line, which calls the method recursively. However, the visual indicator did not show this was happening.

Screen shot when hitting the first breakpoint:

As you can see, when the UI indicator is out of sync with the code, it is very hard to follow what is going on.

The workaround is to remove the blank line:

• Remove the blank line before def in cell 1 and execute the cell.
• Again Choose Debug Cell for cell 2.
• Now the debugger yellow arrow indicator is in sync with the actual code.

A simple explanation is that, in the first case, the code is out of sync with the interpreter: that we forgot to excute the cell to save our changes. This is easy to verify: just add the blank line back and choose Debug Cell. The debugger does not stop because the byte codes are out of sync with the source.

With the blank line added back, save the cell. Debug again. The debugger is again out of sync. This shows that the problem is not user error, but rather an actual repeatable bug.

Finally, note that the problem does not occur if the blank line is replaced by a comment:

# Example
def fib(n):
    ...

• Make the above edit and execute the function declaration.
• Ensure that the top three lines within the function have breakpoints as before.
• Choose Debug Cell from the second cell.
• Step through the code: the debugger is now in sync with the source.

This problem may be new: I don't recall seeing it in previous versions over the last year, but it could be that I never had a blank line above my code previously.

Extension version: 2024.11.0
VS Code version: Code 1.96.4 (cd4ee3b1c348a13bafd8f9ad8060705f6d4b9cba, 2025-01-16T00:16:19.038Z)
OS version: Linux x64 6.8.0-51-generic
Modes:

System Info

Item	Value
CPUs	AMD Ryzen 7 9700X 8-Core Processor (16 x 5536)
GPU Status	2d_canvas: enabled canvas_oop_rasterization: enabled_on direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off video_decode: enabled video_encode: disabled_software vulkan: disabled_off webgl: enabled webgl2: enabled webgpu: disabled_off webnn: disabled_off
Load (avg)	1, 1, 1
Memory (System)	60.44GB (35.39GB free)
Process Argv	--crash-reporter-id 5e232e7b-355a-468c-ac4a-02c300f898bd
Screen Reader	no
VM	0%
DESKTOP_SESSION	cinnamon
XDG_CURRENT_DESKTOP	X-Cinnamon
XDG_SESSION_DESKTOP	cinnamon
XDG_SESSION_TYPE	x11

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