字典中嵌套数据的问题

person = {
    "name": "John",
    "age": 30,
    "address": {
        "street": "123 Main St",
        "city": "New York",
        "country": "USA"
    }
}

students = {
    "John": {
        "age": 15,
        "grade": 9,
        "address": {
            "street": "456 Elm St",
            "city": "New York",
            "country": "USA"
        }
    },
    "Emily": {
        "age": 14,
        "grade": 8,
        "address": {
            "street": "789 Oak St",
            "city": "Los Angeles",
            "country": "USA"
        }
    }
}

age = students["John"]["age"]

students["Emily"]["address"]["city"] = "San Francisco"