第 9 章 文件与数据格式

# 写入（w = 覆盖；a = 追加）
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("第一行\n")
    f.writelines(["第二行\n", "第三行\n"])

# 读取
with open("notes.txt", encoding="utf-8") as f:
    content = f.read()          # 整个读入一个字符串

with open("notes.txt", encoding="utf-8") as f:
    for line in f:              # 逐行读（大文件友好，内存 O(1)）
        print(line.rstrip())    # 行尾含 \n，通常 rstrip 处理

from pathlib import Path

p = Path("notes.txt")
p.write_text("hello\n", encoding="utf-8")   # 小文件一步写入
p.read_text(encoding="utf-8")               # 一步读出
p.read_bytes()                              # 二进制

with open("huge.log", encoding="utf-8") as f:
    errors = [line for line in f if "ERROR" in line]

import json

data = {"name": "Alice", "age": 25, "tags": ["admin", "dev"]}

# Python 对象 → JSON 字符串
s = json.dumps(data, ensure_ascii=False, indent=2)

# JSON 字符串 → Python 对象
obj = json.loads(s)

# 直接读写文件
with open("data.json", "w", encoding="utf-8") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

with open("data.json", encoding="utf-8") as f:
    data2 = json.load(f)

import csv

# 写
with open("users.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "age"])
    writer.writerow(["Alice", 25])
    writer.writerows([["Bob", 30], ["Carol", 28]])

# 读
with open("users.csv", newline="", encoding="utf-8") as f:
    for row in csv.reader(f):
        print(row)              # ['Alice', '25'] —— 全是字符串

# DictReader：按列名取值，更可读
with open("users.csv", newline="", encoding="utf-8") as f:
    for row in csv.DictReader(f):
        print(row["name"], row["age"])

# configparser —— INI 配置文件
import configparser
cfg = configparser.ConfigParser()
cfg.read("app.ini", encoding="utf-8")
cfg["db"]["host"]

# pickle —— Python 专属序列化（能存任意对象）
import pickle
with open("obj.pkl", "wb") as f:
    pickle.dump(my_object, f)
with open("obj.pkl", "rb") as f:
    obj = pickle.load(f)
# ⚠️ 永远不要 pickle.load 不受信任的文件 —— 反序列化可执行任意代码

# sqlite3 —— 内置零配置数据库
import sqlite3
conn = sqlite3.connect("app.db")
conn.execute("CREATE TABLE IF NOT EXISTS users (name TEXT, age INT)")
conn.execute("INSERT INTO users VALUES (?, ?)", ("Alice", 25))   # ? 占位符防注入
conn.commit()
rows = conn.execute("SELECT * FROM users WHERE age > ?", (18,)).fetchall()
conn.close()

from pathlib import Path
import shutil

Path("a/b/c").mkdir(parents=True, exist_ok=True)   # 建多级目录（已存在不报错）
p = Path("old.txt")
p.rename("new.txt")               # 重命名/移动
p.unlink(missing_ok=True)         # 删除文件（3.8+ 可容忍不存在）
shutil.copy("a.txt", "b.txt")     # 复制文件
shutil.copytree("src", "dst")     # 复制目录
shutil.rmtree("tmp_dir")          # 删除整棵目录树（危险，慎用！）

Path(".").glob("*.py")            # 通配匹配
Path(".").rglob("*.py")           # 递归匹配所有子目录
[x for x in Path(".").iterdir() if x.is_file()]
p.suffix, p.stem, p.parent        # '.txt', 'new', Path('.')
p.exists(), p.is_file(), p.is_dir()

import tempfile
with tempfile.TemporaryDirectory() as tmpdir:   # 块结束自动删除
    work = tempfile.Path(tmpdir) if False else None
    # 在 tmpdir 里随便造文件，退出即清理

import os
os.getenv("HOME")                     # 读环境变量（Windows 下如 'USERPROFILE'）
os.environ["MY_FLAG"] = "1"