在学习flask web开发这本书的时候，在第四章，遇到了一个问题。

按照书上，hello.py代码如下：

from flask import Flask, render_template
from flask_bootstrap import Bootstrap
from flask_moment import Moment
from datetime import datetime
from flask_wtf import Form
from wtforms import StringField, SubmitField
from wtforms.validators import Required


app = Flask(__name__)
app.config['SECRET_KEY'] = 'hard to guess string'
bootstrap = Bootstrap(app)
moment = Moment(app)

class NameForm(Form):
    name = StringField('What is your name?',validators=[Required])
    submit = SubmitField('Submit')

@app.route('/', methods=['GET', 'POST'])
def index():
    name = None
    form = NameForm()
    if form.validate_on_submit():
        name = form.name.data
        form.name.data = ''
    return render_template('index.html',name=name,form=form)

index.html代码如下：

{% extends "base.html" %}
{% import "bootstrap/wtf.html" as wtf %}

{% block title %}Flasky{% endblock %}

{% block page_content %}

   
{% if name %}Hello,{{ name }}{% else %}Hello,Stranger{% endif %}!

{{ wtf.quick_form(form) }}
{% endblock %}

之后在浏览器表单里输入内容提交后，会报错：

Traceback (most recent call last):
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/lovehxy/Desktop/flask study/hello.py", line 24, in index
    if form.validate_on_submit():
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/flask_wtf/form.py", line 156, in validate_on_submit
    return self.is_submitted() and self.validate()
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/wtforms/form.py", line 271, in validate
    return super(Form, self).validate(extra)
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/wtforms/form.py", line 130, in validate
    if not field.validate(self, extra):
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/wtforms/fields/core.py", line 175, in validate
    stop_validation = self._run_validation_chain(form, chain)
  File "/Users/lovehxy/.virtualenvs/flask/lib/python3.4/site-packages/wtforms/fields/core.py", line 195, in _run_validation_chain
    validator(form, self)
TypeError: __init__() takes from 1 to 2 positional arguments but 3 were given

求教是什么原因？