given HTML template, implement render function

U_template_engine.py

string = '''
  <h1>{{customerName}}</h1>
  <ul>
  {% for user in users %}
    <li><a href="{{ user.url }}">{{ user.username }} {{location.CA}}</a></li>
  {% endfor %}
  </ul>
'''

dict_ = {
    'customerName': 'uber',
    'location': {
        'CA': 'SF'
    },
    'users': [
        {
            'url': 'myUrl',
            'username': 'myUsername'
        }
    ]
}

import re


class Engine:
    def __init__(self):
        self.place_holder_pattern = r'(?P<dirty_key>{{\s*(?P<key>[\.\w]+)\s*}})'
        self.loop_pattern_st = r'{%\s+for\s+(?P<item>\w+)\s+in\s+(?P<items>\w+)\s+%}'
        self.loop_pattern_ed = r'{%\s+endfor\s+%}'

    def render(self, string, dict_):
        ret = []
        for_loop = False
        cache = []
        for line in string.splitlines():
            if not for_loop:
                if self.find_for_loop_start(line):
                    for_loop = True
                    cache.append(line)
                else:
                    ret.append(self.fill_in(line, dict_))
            else:
                if self.find_for_loop_end(line):
                    for_loop = False
                    ret.extend(self.fill_in_for_loop(cache, dict_))
                    cache = []
                else:
                    cache.append(line)
        return '\n'.join(ret)

    def find_for_loop_start(self, line):
        return 'for' in line and 'in' in line

    def find_for_loop_end(self, line):
        return 'endfor' in line

    def fill_in_for_loop(self, lines, dict_):
        ret = []
        item_key, key = re.findall(self.loop_pattern_st, lines[0])[0]
        if key in dict_:
            for item_values in dict_[key]:
                curt_lines = []
                for line in lines[1:]:
                    l = self.fill_in(line, dict_, item_key, item_values)
                    curt_lines.append(l)
                ret.extend(curt_lines)
        return ret

    def fill_in(self, line, dict_, item_key='', item_values=''):
        keys = re.findall(self.place_holder_pattern, line)
        for dirty_key, key in keys:
            if '.' not in key:
                if key in dict_:
                    line = line.replace(dirty_key, dict_[key])
            else:
                attrs = key.split('.')
                if attrs[0] == item_key:
                    curt = item_values
                    for attr in attrs[1:]:
                        if attr in curt:
                            curt = curt[attr]
                    else:
                        line = line.replace(dirty_key, curt)
                else:
                    curt = dict_
                    for attr in attrs:
                        if attr in curt:
                            curt = curt[attr]
                    else:
                        line = line.replace(dirty_key, curt)
        return line




if __name__ == '__main__':
    e = Engine()
    print(e.render(string, dict_))