无法为Flask中的表单数据设置默认值

from flask import Flask, render_template
from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, SubmitField

class LoginForm(FlaskForm):
    username = StringField('Username', default='admin')
    password = PasswordField('Password')
    submit = SubmitField('Submit')

@app.route('/login', methods=['GET', 'POST'])
def login():
    form = LoginForm()
    if form.validate_on_submit():
        # 处理表单提交的数据
        username = form.username.data
        password = form.password.data
        # 其他逻辑处理
    return render_template('login.html', form=form)