Python 基础：常用内置库

7.1 os - 操作系统接口

import os

os.getcwd()                    # 当前工作目录
os.chdir("/path/to/dir")     # 切换目录
os.listdir(".")               # 列出目录内容
os.mkdir("new_dir")          # 创建目录
os.makedirs("a/b/c")         # 递归创建目录
os.remove("file.txt")        # 删除文件
os.rmdir("empty_dir")        # 删除空目录
os.path.exists("file.txt")    # 文件是否存在
os.path.join("dir", "file")  # 拼接路径

7.2 sys - 系统相关

import sys

sys.argv                      # 命令行参数列表
sys.version                   # Python 版本
sys.path                      # 模块搜索路径
sys.exit(0)                  # 退出程序

7.3 datetime - 日期时间

from datetime import datetime, timedelta

now = datetime.now()
print(now)                   # 2024-01-15 10:30:45.123456

# 格式化
print(now.strftime("%Y-%m-%d %H:%M:%S"))  # 2024-01-15 10:30:45
print(now.strftime("%B %d, %Y"))           # January 15, 2024

# 解析
dt = datetime.strptime("2024-01-15", "%Y-%m-%d")

# 时间计算
tomorrow = now + timedelta(days=1)
yesterday = now - timedelta(days=1)

7.4 json - JSON 处理

import json

# Python → JSON 字符串
data = {"name": "Alice", "age": 25}
json_str = json.dumps(data, ensure_ascii=False, indent=2)

# JSON 字符串 → Python
data2 = json.loads(json_str)

7.5 re - 正则表达式

import re

text = "我的手机号是 138-1234-5678"
pattern = r"1[3-9]\d{1}-\d{4}-\d{4}"

match = re.search(pattern, text)
if match:
    print(match.group())  # 138-1234-5678

# 常用方法
re.findall(r"\d+", text)     # 找到所有数字
re.sub(r"\d+", "XXX", text)   # 替换数字为XXX
re.split(r"\D+", text)        # 按非数字分割

7.6 collections - 容器类型

from collections import Counter, defaultdict, OrderedDict

# Counter 计数器
words = ["apple", "banana", "apple", "cherry", "banana", "apple"]
counter = Counter(words)
print(counter.most_common(2))  # [('apple', 3), ('banana', 2)]

# defaultdict 默认字典
d = defaultdict(list)
d["fruits"].append("apple")
print(d["fruits"])  # ["apple"]

# OrderedDict 有序字典（Python 3.7+ 普通 dict 已有序）

7.7 itertools - 迭代工具

import itertools

# count 无限计数器
for i in zip(range(5), itertools.count(10)):
    print(i)  # (0,10), (1,11), (2,12), (3,13), (4,14)

# cycle 循环
cycler = itertools.cycle(["A", "B", "C"])
for _ in range(5):
    print(next(cycler))  # A, B, C, A, B

# chain 连接多个迭代器
combined = list(itertools.chain([1, 2], [3, 4]))
print(combined)  # [1, 2, 3, 4]

# permutations 排列
print(list(itertools.permutations([1, 2, 3], 2)))
# [(1,2), (1,3), (2,1), (2,3), (3,1), (3,2)]

# combinations 组合
print(list(itertools.combinations([1, 2, 3], 2)))
# [(1,2), (1,3), (2,3)]

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