listing the children that will be added by Builder

example1.py

__all__ = ('get_tree_from_widget', 'get_tree_from_root_rule', 'get_tree_from_cls', )

from kivy.lang import Parser, Builder, BuilderBase
from kivy.factory import Factory


def _get_children(rule):
    return [{crule.name: _get_children(crule)} for crule in rule.children]


def get_tree_from_widget(widget):
    return [
        {crule.name: _get_children(crule)}
        for rule in Builder.match(widget)
        for crule in rule.children
    ]


def _match2(widget_cls):
    '''same as BuilderBase.match() except it takes widget-class'''
    cache = BuilderBase._match_cache
    k = (widget_cls, tuple())
    if k in cache:
        return cache[k]
    rules = []
    for selector, rule in Builder.rules:
        if selector.match(widget):
            if rule.avoid_previous_rules:
                del rules[:]
            rules.append(rule)
    cache[k] = rules
    return rules


def get_tree_from_cls(widget_cls):
    return [
        {crule.name: _get_children(crule)}
        for rule in _match2(widget_cls)
        for crule in rule.children
    ]


def get_tree_from_root_rule(kvcode):
    root_rule = Parser(content=kvcode).root
    if not root_rule:
        raise ValueError('There is no root rule.')
    return [
        *get_tree_from_cls(Factory.get(root_rule.name)),
        _get_children(root_rule)
    ]


def _test():
    import textwrap
    import json

    def print_widget_tree(widget_tree):
        print(json.dumps(widget_tree, indent=2))

    class MyWidget(Factory.BoxLayout):
        def apply_class_lang_rules(self, *args, **kwargs):
            # This is the only place that `get_tree_from_widget()` can be used.
            print_widget_tree(get_tree_from_widget(self))
            return super().apply_class_lang_rules(*args, **kwargs)

    KV_CODE = textwrap.dedent('''
        <MyWidget>:
            Label:
                text: '1st'
            Label:
                text: '2nd'
        <MyWidget>:
            BoxLayout:
                Label:
                    text: '3rd'
        ''')
    Builder.load_string(KV_CODE)
    print('test get_tree_from_widget()')
    MyWidget()
    print('\n-----------------------')
    print('test get_tree_from_cls()')
    print_widget_tree(get_tree_from_cls(MyWidget))
    print('\n-----------------------')
    print('test get_tree_from_root_rule()')
    print_widget_tree(get_tree_from_root_rule(textwrap.dedent('''
        MyWidget:
            Button:
        ''')))


if __name__ == "__main__":
    _test()