Can't change instance attribute of my class?

Hope this helps You have a few problems:
1) Function inherts from MainApp - don't do that - its weird!!

class MainApp(App):

    link_buttons = 0

    def build(self):
        layout = GridLayout(cols=1, padding=1, spacing=10,
                    size_hint=(None, None), width=10)
        layout.bind(minimum_height=layout.setter('height'))
        sv = ScrollView(size_hint=(None, None), size=(200, 400),
             pos_hint={'center_x': .5, 'center_y': .5}, do_scroll_x=False)
        sv.add_widget(layout)
        self.layout = layout
        root = RootWidget()
        c = CustomLayout()
        s = Scrollbox()
        root.add_widget(c)
        root.add_widget(s)
        s.add_widget(sv)



        get_buttons = Button(
            text='Get links',
            size_hint=(1, 0),
            pos=(20, 20))
        s.add_widget(get_buttons)

        get_buttons.bind(on_press=self.buttons)
        return root



    def buttons(self, btn):


        layout = self.layout  
        self.link_buttons += 1

        for buttn in range(20):
            btn = Button(text='test', size=(200, 50),
                     size_hint=(None, None))

            self.layout.add_widget(btn)

Any of those help It's tempting to say that __dict__ has to be a descriptor because implementing it as a __dict__ entry would require you to find the __dict__ before you can find the __dict__, but Python already bypasses normal attribute lookup to find __dict__ when looking up other attributes, so that's not quite as compelling as it initially sounds. If the descriptors were replaced with a '__dict__' key in every __dict__, __dict__ would still be findable.
There's some space savings by not having a key for '__dict__' in every __dict__, but that's not the big reason. There's also time saved by not having to set a '__dict__' key, and time and space saved by not creating a circular reference, and these benefits are all really nice, but they're still probably smaller than the next thing.

will be helpful for those in need Because details is created from the original values. After the concatenation occurs, it's entirely independent of the other values. If you want a dynamically constructed string using the current values of name/author, use a property to compute details dynamically on access (without storing it as an attribute at all):

class Book(object):
    def __init__(self, name, author):
        self.name = name
        self.author = author
        # Don't make a `details` attribute at all

    @property
    def details(self):
        # This method is called whenever you refer to `somebook.details`
        return self.name + ' written by ' + self.author