How to write a simple Jinja2 extension?

Question

Python

I'm trying to write a simple extension at the level of recoding all content to base64, but it doesn't work. I don't understand where is the error:

class Base64Extension(Extension):
  tags = {'base64'}

  def __init__(self, environment):
    super(Base64Extension, self).__init__(environment)

  def parse(self, parser):
    lineno = next(parser.stream).lineno
    args = [parser.parse_expression()]
    call = self.call_method('_render', args)
    return nodes.Output([call], lineno=lineno)

  def _render(self, text):
    return base64.b64encode(bytes(text, 'utf-8')).decode('utf-8')

env = Environment(loader=FileSystemLoader('project/test'), extensions=[Base64Extension], trim_blocks=True,block_start_string='',variable_start_string='[', variable_end_string=']')


print(env.get_template('data2.txt').render())

The template also looks very simple:

[base64]
Привет текста.
[endbase64]

The output is just text without tags. There are no simple examples of writing your own extensions, maybe someone has examples for example uppercase or the same base64?