I expected Background init() to be called. To pass "a, b" to Fields init(), Field to assign a and b

So far, so good.

then to assign a list with three 0's in it to field.

Ah. This is where we get the error.

    self.field = self.buildField()

Even though this line occurs within Field.__init__, self is an instance of Background. so self.buildField finds Background's buildField method, not Field's.

Since Background.buildField expects 2 arguments instead of 1,

self.field = self.buildField()

raises an error.

So how do we tell Python to call Field's buildField method instead of Background's?

The purpose of name mangling (naming an attribute with double underscores) is to solve this exact problem.

class Field(object):
    def __init__(self, a, b):
        self.a = a
        self.b = b
        self.field = self.__buildField()

    def __buildField(self):
        field = [0,0,0]
        return field

class Background(Field):
    def __init__(self, a, b, c):
        super(Background, self).__init__(a, b)
        self.field = self.__buildField(c)

    def __buildField(self, c):
        field = [c]
        return field

a, b, c = 0, 1, 2
background = Background(a, b, c)

The method name __buildField is "mangled" to _Field__buildField inside Field so inside Field.__init__,

    self.field = self.__buildField()

calls self._Field__buildField(), which is Field's __buildField method. While similarly,

    self.field = self.__buildField(c)

inside Background.__init__ calls Background's __buildField method.