如何使用flask在表单中添加默认值？

from flask import Flask, render_template, request

app = Flask(__name__)

@app.route('/form', methods=['GET', 'POST'])
def form():
    if request.method == 'POST':
        # 处理表单提交
        # ...
    else:
        # 渲染包含表单的模板
        return render_template('form.html')

<form method="POST" action="/form">
    <input type="text" name="username" value="{{ request.form.get('username', '默认值') }}">
    <input type="submit" value="提交">
</form>