如何在python表单中修复此错误？

我试图制作一个全功能的Web应用程序，包括使用WTForms作为数据库资源，我被这个错误难住了

jinja2.exceptions.UndefinedError:“wtforms.fields.core.StringField对象”没有属性“username”

Traceback (most recent call last):
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 2328, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 2314, in wsgi_app
    response = self.handle_exception(e)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 1760, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/_compat.py", line 36, in reraise
    raise value
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 2311, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 1834, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 1737, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/_compat.py", line 36, in reraise
    raise value
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 1832, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/app.py", line 1818, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/lkendon/flaskblog/flaskblog.py", line 36, in register
    return render_template('register.html', title='Register', form=form)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/templating.py", line 135, in render_template
    context, ctx.app)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/flask/templating.py", line 117, in _render
    rv = template.render(context)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/jinja2/asyncsupport.py", line 76, in render
    return original_render(self, *args, **kwargs)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/lkendon/flaskblog/venv/lib/python3.7/site-packages/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "/Users/lkendon/flaskblog/templates/register.html", line 1, in top-level template code
    {% extends "layout.html" %}
  File "/Users/lkendon/flaskblog/templates/layout.html", line 41, in top-level template code
    {% block content %}{% endblock %}
  File "/Users/lkendon/flaskblog/templates/register.html", line 10, in block "content"
    {{form.username.username(class="form-control form-control-lg")}}
jinja2.exceptions.UndefinedError: 'wtforms.fields.core.StringField object' has no attribute 'username'

---

这里到底是什么问题，我在遵循一个教程，我通过提供的片段输入了相同的细节。

查看回溯，您正在调用

{form.username.username（…

，应该是

{{form.username（…