python - flask-wtf validate_on_submit()-PHP中文网问答

验证用户名和密码，validate_on_validate()返回值因为True，但是下面的代码却是False，求解答：

class LoginForm(Form):
    username = StringField('Username')
    password = PasswordField('Password')
    submit = SubmitField('Login')
    def validate_username(self, field):
        if field.data != 'you':
            raise ValidationError('Invalid username')
    def validate_password(self, field):
        if field.data != 'flask':
            raise ValidationError('Invalid password')

@app.route('/', methods=['POST', 'GET'])
def login_view():
    form = LoginForm()
    if form.validate_on_submit():
            flash('Username and password are correct')
            return redirect('/')
    return render_template('form.html', form = form)

如上form.validate_on_submit()返回值总是为False
我只能把代码改为：

@app.route('/', methods=['POST', 'GET'])
def login_view():
    form = LoginForm()
    if request.method == 'POST':
        if not form.validate_on_submit():
            flash('Username and password are correct')
            return redirect('/')
    else:
        return render_template('form.html', form = form)

这样才会有用，求解答