📖 第3堂課：字典、字串與檔案

第一堂：變數、if/else、while → 猜數字遊戲
第二堂：list 清單、for 迴圈、def 函式 → 記帳機

這堂課要學兩個超實用工具：字典和檔案操作。

字典（dict）用鍵（key）→ 值（value）的對應來存資料：

student = {"name": "丁丁","age": 25,"city": "台北"}
print(student["name"])  # 丁丁
print(student.get("phone", "無資料"))  # 安全取值

for key, value in student.items():
    print(f"{key}: {value}")

text = "  Hello, Python!  "
print(text.strip())          # "Hello, Python!"
print(text.replace("Python", "程式"))
print(text.split(","))       # ['  Hello', ' Python!  ']
print(",".join(["A","B"]))  # "A,B"
name = "丁丁"; score = 95
print(f"{name} 的成績是 {score} 分")  # f-string!

with open("diary.txt", "w", encoding="utf-8") as f:
    f.write("今天天氣很好！\n")

with open("diary.txt", "r", encoding="utf-8") as f:
    content = f.read()

with open("diary.txt", "a", encoding="utf-8") as f:
    f.write("追加一行！\n")  # 不覆蓋

import json

def load_contacts():
    try:
        with open("contacts.json", "r", encoding="utf-8") as f:
            return json.load(f)
    except FileNotFoundError:
        return []

def save_contacts(contacts):
    with open("contacts.json", "w", encoding="utf-8") as f:
        json.dump(contacts, f, ensure_ascii=False, indent=2)

contacts = load_contacts()
contacts.append({"name": "丁丁", "phone": "0912-345-678"})
save_contacts(contacts)