90 lines
3.1 KiB
Markdown
90 lines
3.1 KiB
Markdown
# Day_13 · 文件读写与 JSON：让数据活到下次运行
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> 预计耗时：25~30 分钟
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> 程序一关，变量就没了。想让数据存下来（这就是"持久化"，数据库干的事），先从文件开始。
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> Django 的数据库、缓存的本质，都是"把数据存起来再读回来"。
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## 今日知识点
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- 写文件：`with open("note.txt", "w") as f: f.write(内容)`（`w` = 覆盖写）
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- 读文件：`with open("note.txt", "r") as f: 内容 = f.read()`
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- `with` 会自动关文件，不用手动 close
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- **JSON**：一种"把字典/列表变成文本"的格式，`json.dumps` 变文本、`json.loads` 变回数据
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- 为什么重要：HTTP 接口传输的数据、配置文件，全是 JSON 格式
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## 先学（教程）
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- 菜鸟教程（"文件方法"一节，看 open/read/write 即可）：https://www.runoob.com/python3/python3-file-methods.html
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- 菜鸟教程（"JSON"一节）：https://www.runoob.com/python3/python3-json.html
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- B站黑马教程（看讲"文件操作"的几集）：https://www.bilibili.com/video/BV1qW4y1a7fU/
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## 任务（后做）
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新建 `day13.py`：
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1. 写入再读回：
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```python
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text = input("写一句话存起来：")
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with open("note.txt", "w", encoding="utf-8") as f:
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f.write(text)
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with open("note.txt", "r", encoding="utf-8") as f:
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content = f.read()
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print("读回来的是：", content)
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```
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运行后确认同目录出现了 `note.txt`，且内容一致。（注意 `encoding="utf-8"`，否则中文会乱码。）
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2. 用 JSON 存字典：
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```python
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import json
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user = {"name": "张三", "age": 18, "city": "青岛"}
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with open("user.json", "w", encoding="utf-8") as f:
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json.dump(user, f, ensure_ascii=False) # ensure_ascii=False 让中文可读
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with open("user.json", "r", encoding="utf-8") as f:
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loaded = json.load(f)
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print(loaded["name"], loaded["age"]) # 从文件里还原的字典
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```
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3. **进阶（加分）**：把 Day_5 的"三个用户列表"用 `json.dump` 存进 `users.json`，
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再读回来遍历打印每个人的名字。
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## 完成标准
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- [ ] note.txt 和 user.json 出现在同目录，读回内容一致
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- [ ] 能说出 `json.dump` 和 `json.load` 各是干嘛的
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- [ ] （进阶）用户列表存取成功
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**提交方式**：把 `day13.py` 代码贴在当日工单评论里。
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## 参考解答（先做再看）
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```python
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text = input("写一句话存起来：")
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with open("note.txt", "w", encoding="utf-8") as f:
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f.write(text)
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with open("note.txt", "r", encoding="utf-8") as f:
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print("读回来的是：", f.read())
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import json
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user = {"name": "张三", "age": 18, "city": "青岛"}
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with open("user.json", "w", encoding="utf-8") as f:
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json.dump(user, f, ensure_ascii=False)
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with open("user.json", "r", encoding="utf-8") as f:
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loaded = json.load(f)
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print(loaded["name"], loaded["age"])
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# 进阶
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users = [
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{"name": "张三", "age": 19},
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{"name": "李四", "age": 20},
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{"name": "王五", "age": 18},
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]
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with open("users.json", "w", encoding="utf-8") as f:
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json.dump(users, f, ensure_ascii=False)
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with open("users.json", "r", encoding="utf-8") as f:
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for u in json.load(f):
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print(u["name"])
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```
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