Incorrect code collapse while prompting where the error is.

[PC-Killer]

Bug report

Bug description:

Read the following code:

# incorrect_collapse.py
def fib(number: int) -> int:
    assert number > 0
    if number == 1:
        return 1

    return fib(number - 1) + fib(number - 2)  # Notice here


print(fib(2))

There is a mistake in the code above, so if you try to run it, you will get the following traceback:

PC-Killer@ArchLinuxPC ~/p/bugs> python incorrect_collapse.py
Traceback (most recent call last):
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 9, in <module>
    print(fib(2))
          ~~~^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
                             ~~~^^^^^^^^^^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 2, in fib
    assert number > 0
           ^^^^^^^^^^
AssertionError

Notice that the error happened while running the code fib(number - 2).
However, if the number is 5, the duplicate code will be automatically collapsed:

# incorrect_collapse.py
def fib(number: int) -> int:
    assert number > 0
    if number == 1:
        return 1

    return fib(number - 1) + fib(number - 2)  # Notice here


print(fib(5))

PC-Killer@ArchLinuxPC ~/p/bugs [1]> python incorrect_collapse.py
Traceback (most recent call last):
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 9, in <module>
    print(fib(5))
          ~~~^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
           ~~~^^^^^^^^^^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
           ~~~^^^^^^^^^^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
           ~~~^^^^^^^^^^^^
  [Previous line repeated 1 more time]
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 2, in fib
    assert number > 0
           ^^^^^^^^^^
AssertionError

The error happened in fib(number - 2). However, in the traceback above, I can only suppose the error happened while running fib(number - 1), which increases the time to find where the real bug is.

CPython versions tested on:

3.13, 3.14

Operating systems tested on:

Linux

[picnixz]

This the the expected behaviour. Because we are running fib(4) before fib(3), we'll see fib(4) raising. If you write fib(2) you execute fib(1) and fib(0). but it's fib(0) that is having an issue, not fib(1) (it immediately returns).

If you unroll the calls you have:

fib(2) -> fib(1) + fib(0)
fib(1) -> 1
fib(0) -> assertion error

So it's normal that fib(n-2) is the error location. For fib(5), if you unroll the call you have:

fib(5) -> fib(4) + fib(3)
----
fib(4) -> fib(3) + fib(2)
fib(3) -> fib(2) + fib(1) 
fib(2) -> fib(1) + fib(0)  # f(0) will error
----
fib(3) -> fib(2) + fib(1)
fib(2) -> fib(1) + fib(0)  # f(0) will error

So the error happened in the n-2 inner branch of the top n-1 branch. It would also fail on the n-2 inner branch of the top n-2 branch but this n-2 branch wouldn't be even executed yet.

This can be verified as follows:

def fib(number: int, s='') -> int:
    print("called with:", number, s)
    assert number > 0, (number, s)
    if number == 1: return 1
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")

fib(5)

This wil print:

called with: 5
called with: 4 l
called with: 3 ll
called with: 2 lll
called with: 1 llll
called with: 0 lllr
Traceback (most recent call last):
  File "test.py", line 8, in <module>
    fib(5)
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  [Previous line repeated 1 more time]
  File "test.py", line 3, in fib
    assert number > 0, (number, s)
           ^^^^^^^^^^
AssertionError: (0, 'lllr')

[PC-Killer]

Thanks for reading my issue! I am sorry that I may have not explained myself clearly.
Please notice your output above:

-- skip --
called with: 1 llll
called with: 0 lllr
Traceback (most recent call last):
  File "test.py", line 8, in <module>
    fib(5)
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  [Previous line repeated 1 more time]
  File "test.py", line 3, in fib
    assert number > 0, (number, s)
           ^^^^^^^^^^
AssertionError: (0, 'lllr')

That is to say, while calculating fib(2), the former, fib(number - 1) has been calculated successfully, and the mistake happened while calculating fib(number - 2), which refers to fib(0)
Therefore, in my opinion, the traceback should be like the following:

Traceback (most recent call last):
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 9, in <module>
    print(fib(5))
          ~~~^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
           ~~~^^^^^^^^^^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
           ~~~^^^^^^^^^^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
           ~~~^^^^^^^^^^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
                             ~~~^^^^^^^^^^^^
  File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 2, in fib
    assert number > 0
           ^^^^^^^^^^
AssertionError

[picnixz]

File "/home/PC-Killer/python_work/bugs/incorrect_collapse.py", line 6, in fib
    return fib(number - 1) + fib(number - 2)  # Notice here
                             ~~~^^^^^^^^^^^^

Ah I see what you mean. I think the issue is that we are considering the traceback to be identical because we haven't yet added the markers (^). I'm not on my dev session so it's a bit hard to investigate, but we should decide if the rendered lines are the same by also checking where the carrets would be rendered before hiding them.

For instance, fib(4) shows:

called with: 4
called with: 3 l
called with: 2 ll
called with: 1 lll
called with: 0 llr
Traceback (most recent call last):
  File "test.py", line 8, in <module>
    fib(3)
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "test.py", line 5, in fib
    return fib(number - 1, s + 'l') + fib(number - 2, s + "r")
                                      ^^^^^^^^^^^^^^^^^^^^^^^^
  File "test.py", line 3, in fib
    assert number > 0, (number, s)
           ^^^^^^^^^^
AssertionError: (0, 'llr')

cc @pablogsal