Functions starting with underscore are not grouped with function variables

[jasongrout]

Over in ipython/ipykernel#1072, we noticed that sometimes getting the variables in a frame lists a function starting with a underscore as a normal variable, instead of grouping it with the function variables.

Here's a self-contained example illustrating the issue if run in ipython.

1. Install ipython and debugpy: pip install ipython debugpy
2. Create the repro.py file below:

repro.py

# This is example reproduces code from https://github.com/ipython/ipykernel/blob/main/ipykernel/debugger.py

# This import is required to have the next ones working...
from debugpy.server import api  # noqa

from _pydevd_bundle import pydevd_frame_utils  # isort: skip
from _pydevd_bundle.pydevd_suspended_frames import (  # isort: skip
    SuspendedFramesManager,
    _FramesTracker,
)

class _FakeCode:
    def __init__(self, co_filename, co_name):
        self.co_filename = co_filename
        self.co_name = co_name


class _FakeFrame:
    def __init__(self, f_code, f_globals, f_locals):
        self.f_code = f_code
        self.f_globals = f_globals
        self.f_locals = f_locals
        self.f_back = None


class _DummyPyDB:
    def __init__(self):
        from _pydevd_bundle.pydevd_api import PyDevdAPI

        self.variable_presentation = PyDevdAPI.VariablePresentation()


class VariableExplorer:
    def __init__(self):
        self.suspended_frame_manager = SuspendedFramesManager()
        self.py_db = _DummyPyDB()
        self.tracker = _FramesTracker(self.suspended_frame_manager, self.py_db)
        self.frame = None

    def track(self):
        var = get_ipython().user_ns
        self.frame = _FakeFrame(_FakeCode("<module>", 'filename'), var, var)
        self.tracker.track("thread1", pydevd_frame_utils.create_frames_list_from_frame(self.frame))

    def untrack_all(self):
        self.tracker.untrack_all()

    def get_children_variables(self, variable_ref=None):
        var_ref = variable_ref
        if not var_ref:
            var_ref = id(self.frame)
        variables = self.suspended_frame_manager.get_variable(var_ref)
        return [x.get_var_data() for x in variables.get_children_variables()]


# Create a variable explorer
v = VariableExplorer()
v.track()


# Define two functions, one starting with an underscore
def abc():
    ...

def _abc():
    ...

# Note that `abc` does not appear in the results, but `_abc` does appear in the results.
import pprint
pprint.pprint([v for v in v.get_children_variables() if 'abc' in v["name"]])

3. Run ipython repro.py. Notice that _abc is listed in the output, but abc is not listed in the output.

My confusion is: it appears that abc is grouped with function variables, which is why it doesn't appear in the output. Why is _abc not grouped with the function variables, i.e., why does it appear in the output?