python Flask WTForms form.errors不起作用

Question

实际上，当验证者不满意时，我们应该从 form.errors 中得到错误信息。但即使验证者不满意，我也会遇到任何错误。

app.py

@app.route('/home')
def home():
    return render_template('home.html')

@app.route('/register',methods=['GET','POST'])
def register():
    form = RegistrationForm()
    if form.validate_on_submit():
        return redirect(url_for('home'))
    return render_template('register.html',title='Register',form=form)

forms.py

    from flask_wtf import FlaskForm
    from wtforms import StringField, PasswordField, SubmitField, BooleanField,SelectField
    from wtforms.validators import InputRequired, Length, Email, EqualTo,ValidationError

    class RegistrationForm(FlaskForm):
        username = StringField('Username',validators= 
                   [DataRequired(),Length(min=2,max=20)])
        email = StringField('Email',validators=[DataRequired(),Email()])
        branches = SelectField('',[validators.input_required("Please choose your branch")],choices=[('-1','-- select --'),('0','cse'),('1','ece'),('2','mech'),('3','chem'),('4','mme'),('5','civil')])
        password = PasswordField('Password',validators=[DataRequired()])
        confirm_password = PasswordField('Confirm Password',validators=[DataRequired(),EqualTo('password')])
        submit = SubmitField('Sign Up')

register.html

   <form method="POST" action="/register">
    {{ form.hidden_tag() }}
    {{ form.csrf_token }}
    {{ form.username.errors }}
    {{ form.username(class="form-control form-control-lg") }}
    {{ form.email(class="form-control form-control-lg") }}
    {{ form.password(class="form-control form-control-lg") }}
    {{ form.confirm_password(class="form-control form-control-lg") }}
    </form>

即使没有提供任何输入也不会产生错误。