使用Bootstrap渲染SelectFields的WTF FieldList

我想用渲染一个flask_wtf表格bootstrap/wtf.html。该表单包含一个常规字段SelectField和一个FieldListSelectFields。使用函数可以渲染单个SelectField wtf.form_field。但是，将相同的函数应用于FieldList的每个SelectField会引发错误：

  File "/usr/local/lib/python3.5/dist-packages/flask_bootstrap/templates/bootstrap/wtf.html", line 119, in template
    {{field.label(class="control-label")|safe}}
TypeError: 'str' object is not callable

我对错误的解释是，字符串“ field.label”的调用就像使用括号的函数一样。另一方面，对于单个SelectField似乎也是如此。

这是form.py：

from flask_wtf import FlaskForm
from wtforms import SelectField, FieldList, FormField

class FormEntry(FlaskForm):
    selectfield = SelectField('Name', coerce=int)

class MyForm(FlaskForm):
    selectfield = SelectField('Name', coerce=int, choices=[(2, "choice 2"), (1, "choice 1")])
    form_entries = FieldList(FormField(FormEntry))

这是render.html：

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

 {{ form.hidden_tag() }}
 {{ wtf.form_field(form.selectfield) }}
 {% for entry in form.form_entries %}
     {{ wtf.form_field(entry.selectfield) }}
 {% endfor %}