How to call when using double underscore (Private attribute) for class function

I specified a double underscore for the function in the class and tried it as a Private attribute, but it was recorded when calling the function. <Addition (see pep8): In general, the method of adding two underscores at the beginning of the name is to avoid conflicting attributes of classes designed to be subclassed. Should only be used. >

    class HogeHoge:
        def __init__(self):
            pass

        def __FugaFuga(self):
            iam = "king"
            return iam

I want to print the return value "king" of __FugaFuga at this time,

    iam = _HogeHoge__FugaFuga()
    print(iam)
    #    iam = _HogeHoge__FugaFuga()
    # NameError: name '_HogeHoge__FugaFuga' is not defined

If _HogeHoge__FugaFuga is not found in NameError ...

    HogeHoge = HogeHoge()
    iam = HogeHoge.__FugaFuga()
    #    iam = HogeHoge.__FugaFuga()
    # AttributeError: 'HogeHoge' object has no attribute '__FugaFuga'

Then, when the class and the function are separated, the function __FugaFuga cannot be found ... Success story:

    class HogeHoge:
        def __init__(self):
            pass

        def __FugaFuga(self):
            iam = "common people"
            return iam
    HogeHoge = HogeHoge()
    iam = HogeHoge._HogeHoge__FugaFuga()
    print(iam)

Execution result:

    C:\Users\***\test>python main.py
    common people

It seems that the function of class HogeHoge is _HogeHoge__FugaFuga.

I referred to the article on the following site. Let's master Python underscore (_)! About python encapsulation and mangling PEP8: Python Code Style Guide