Python - Access to a protected member _ of a class

Question

Given a class with some protected members and a public interface to modify them, when is it generally accepted to access the protected members directly? I have some specific examples in mind:

1. Unit testing
2. Internal private methods such as __add__ or __cmp__ accessing other's protected attributes
3. Recursive data structures (e.g. accessing next._data in a linked list)

I don't want to make these attributes public as I don't want them touched publicly. My syntax IDE syntax highlighting keeps saying that I'm wrong with accessing protected members - who is right here?

EDIT - adding a simple example below:

class Complex:
    def __init__(self, imaginary, base):
        self._imaginary = imaginary
        self._base = base

    def __str__(self):
        return "%fi + %f" % self._base, self._imaginary

    def __add__(self, other):
        return Complex(self._imaginary + other._imaginary, self._base + other._base)

Pycharm highlights other._imaginary and other._base with the following:

Access to a protected member _imaginary of a class

Answer 1

Solved - the problem was actually to do with lack of type-hinting. The below now works:

class Complex:
    def __init__(self, imaginary, base):
        self._imaginary = imaginary
        self._base = base

    def __str__(self):
        return "%fi + %f" % self._base, self._imaginary

    def __add__(self, other):
        """
        :type other: Complex
        :rtype Complex:
        """
        return Complex(self._imaginary + other._imaginary, self._base + other._base)