如何确保在flask WTForms中，表单字段只有在填写后才会进行有效性测试？

from flask_wtf import FlaskForm
from wtforms import StringField, SubmitField
from wtforms.validators import DataRequired

class MyForm(FlaskForm):
    my_field = StringField('My Field', validators=[DataRequired()])
    submit = SubmitField('Submit')

@app.route('/my_form', methods=['GET', 'POST'])
def my_form():
    form = MyForm()
    if form.validate_on_submit():
        # 在此处进行有效性测试
        # 如果表单字段通过测试，执行相应的操作
        # 如果未通过测试，返回错误信息给用户
        pass
    return render_template('my_form.html', form=form)