如何在Flask中以WTForms呈现optgroup

Question

我想在Flask WTF和WTF表单中呈现这样的HTML

<select name="animal-box">
<optgroup label="mammal">
  <option label="cat" value="cat">cat</option>
  <option label="dog" value="dog">dog</option>
</optgroup>
<optgroup label=reptile">
  <option label="turtle" value="turtle">turtle</option>
  <option label="lizard" value="lizard">lizard</option>
</optgroup>
</select>

如何定义一个表单来呈现上面的HTML

forms.py

from flask_wtf import FlaskForm
from wtforms import  SelectMultipleField
class MyForm(FlaskForm):
    # How can I define optgroup here?
    animal =  SelectMultipleField('animal', choices=[('cat', 'cat'), ('dog', 'dog'), ('turtle', 'turtle'), ('lizard', 'lizard')])

我已经在文档中搜索过了，但假设通过WTForms无法做到这一点。那么，这是实现这一目标的变通方法吗