The test expects that calling the callback raises an exception. Currently, it raises a TypeError since [] (list) is not callable. I suggest making the exception more explicit:

                def callback(wr):
                    raise Exception("error")

                def f():
                    ref(lambda: 0, callback)
                    f()

Passing ord() is more surprising to me.

If I correctly understand the purpose of this test, we cannot do this. This would insert a Python frame, this would give a non-NULL source_line. This is why the test used such indirect way of raising an exception.

I am not completely sure in my reconstruction, this is why I have not added an explaining comment, which would be helpful here.

The test was added by commit c836231.

At first ref(lambda: 0, []) calls, display_source_line() returns rc=0, no exception set, source_line is not NULL. But when we are getting closer to the recursion limit, display_source_line() can fail with RecursionError on the call:

    fob = _PyObject_CallMethod(io, &_Py_ID(TextIOWrapper),
                               "Os", binary, encoding);

In this case, display_source_line() clears the exception, returns 0 and source_line is NULL. Calling ignore_source_errors() here just sets err=0 (no exception is set), but err is already equal to 0. So I don't understand well what we are testing here.

I am not confident in my explanation, and adding incorrect comment may be worse than missing comment.