请在此代码中解释"default()"的使用

username = request.args.get('username', default='Guest')

CREATE TABLE users (
    id INT PRIMARY KEY,
    username VARCHAR(50) NOT NULL DEFAULT 'Guest'
);

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
      - name: my-container
        image: my-image
        ports:
        - containerPort: 80
        resources:
          limits:
            cpu: default(2)
            memory: default(1Gi)
          requests:
            cpu: default(1)
            memory: default(512Mi)