form.validate_on_submit()一直是FALSE

Question

从网上看到的解决方法大致是两种：
一是加{{ form.csrf_token }}，method=post
二是在输入框中加入validators=[DataRequired()]；
这两种方法我都试过了，还是不行，但是可以获取界面输入的内容，就是提交不了。希望可以解答~
代码如下：
前端主要代码：

```html
<form method="post" style="font-size:15px">
                                            {{ form.csrf_token }}
                                            {{form.data_source.label}} {{ form.data_source}}&nbsp;&nbsp;&nbsp;&nbsp;
                                            {{form.data_type.label}} {{ form.data_type}}&nbsp;&nbsp;&nbsp;&nbsp;
                                            {{form.username.label}} {{ form.username}}&nbsp;&nbsp;&nbsp;&nbsp;
                                            {{form.start_date.label}} {{ form.start_date}}&nbsp;&nbsp;&nbsp;&nbsp;
                                            {{form.end_date.label}} {{ form.end_date}}&nbsp;&nbsp;&nbsp;&nbsp;
                                            {{form.fan_number.label}} {{ form.fan_number}}&nbsp;&nbsp;&nbsp;&nbsp;
                                            {{form.content_number.label}} {{ form.content_number}}&nbsp;&nbsp;&nbsp;&nbsp;
                                            {{form.page_number.label}} {{ form.page_number}}&nbsp;&nbsp;&nbsp; &nbsp;
                                            {{ form.submit }}
                                       </form>

后端代码：

class SearchForm(FlaskForm):
    data_source = SelectField(u'数据来源', validators=[DataRequired()], choices=[("1", ""), ("2", u"微博"), ("3", u"抖音"), ("4", u"快手")], coerce=str)
    data_type = SelectField(u'数据类型', validators=[DataRequired()],
                              choices=[("1", ""), ("2", u"体育"), ("3", u"财经"), ("4", u"娱乐"), ], coerce=str)
    username = StringField(u"媒体用户", validators=[DataRequired()])
    start_date = DateField(u'开始时间', format='%Y-%m', validators=[DataRequired()])
    end_date = DateField(u'结束时间', format='%Y-%m', validators=[DataRequired()])
    fan_number = SelectField(u'粉丝数', validators=[DataRequired()],
                              choices=[("1", ""), ("10", "top10"), ("20", "top20"), ("50", "top50"), ], coerce=str)
    content_number = SelectField(u'发博数', validators=[DataRequired()],
                             choices=[("1", ""), ("10", "top10"), ("20", "top20"), ("50", "top50"), ],
                             coerce=str)
    page_number = SelectField(u'页数', validators=[DataRequired()],
                              choices=[("0", ""), ("10", u"10条"), ("20", u"20条"), ("50", u"50条"), ], coerce=str)
    submit = SubmitField(u'查询')