Looping through empty list yields a None object

[jwezel]

Bug report

Bug description:

I've stumbled over a seemingly impossible situation where a for loop over an empty list yields a None object in its body. The code starts on line 136 of module listconfig.py of OmegaConf. See the attached log of the terminal session demonstrating the effect.
situation.txt
situation-nocolor.txt

CPython versions tested on:

3.12

Operating systems tested on:

Linux

[sobolevn]

Can you please try to create a smaller reproducer? Right now it is not clear from the issue: how can one try to reproduce the problem.

[jwezel]

Sure, here is a zip. Create a venv, install requirements.txt and run t2.py through python 3.12.

reproduction.zip

[nineteendo]

Here's a smaller reproducer, but this doesn't seem a problem in CPython:

from omegaconf import OmegaConf

Config = OmegaConf.create()
Config["exposed"] = []
Config["exposed"].append(Config["exposed"])

Traceback (most recent call last):
  File "/Users/wannes/Downloads/reproduction/t2.py", line 5, in <module>
    Config["exposed"].append(Config["exposed"])
  File "/usr/local/lib/python3.12/site-packages/omegaconf/listconfig.py", line 301, in append
    self._format_and_raise(key=index, value=item, cause=e)
  File "/usr/local/lib/python3.12/site-packages/omegaconf/base.py", line 231, in _format_and_raise
    format_and_raise(
  File "/usr/local/lib/python3.12/site-packages/omegaconf/_utils.py", line 899, in format_and_raise
    _raise(ex, cause)
  File "/usr/local/lib/python3.12/site-packages/omegaconf/_utils.py", line 797, in _raise
    raise ex.with_traceback(sys.exc_info()[2])  # set env var OC_CAUSE=1 for full trace
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/omegaconf/listconfig.py", line 298, in append
    self._set_item_impl(index, item)
  File "/usr/local/lib/python3.12/site-packages/omegaconf/basecontainer.py", line 539, in _set_item_impl
    value = copy.deepcopy(value)
            ^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/Cellar/python@3.12/3.12.6/Frameworks/Python.framework/Versions/3.12/lib/python3.12/copy.py", line 143, in deepcopy
    y = copier(memo)
        ^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/omegaconf/listconfig.py", line 136, in __deepcopy__
    old_parent = v.__dict__["_parent"]
                 ^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute '__dict__'
    full_key: exposed[0]
    object_type=list

[zware]

This is not a bug in Python. omegaconf's ListConfig.append method is doing its best to hide self-incriminating evidence here:

https://github.com/omry/omegaconf/blob/v2.3.0/omegaconf/listconfig.py#L296-L301