# 写入文件
f = open("test.txt", "w", encoding="utf-8")
f.write("第一行\n")
f.write("第二行\n")
f.close() # ⚠️ 必须关闭！否则数据可能没写入
# 读取文件
f = open("test.txt", "r", encoding="utf-8")
content = f.read() # 读取全部内容
print(content)
f.close()
| 模式 | 含义 | 文件不存在时 |
|---|---|---|
"r" | 只读（默认） | 报错 |
"w" | 只写（覆盖） | 创建 |
"a" | 追加 | 创建 |
"r+" | 读写 | 报错 |
"w+" | 读写（覆盖） | 创建 |
"a+" | 读写（追加） | 创建 |
encoding="utf-8" — 否则Windows默认GBK编码，中文容易乱码# 用with语句，代码块结束后自动关闭文件，即使出错也会关闭
with open("test.txt", "w", encoding="utf-8") as f:
f.write("用with语句\n")
f.write("自动关闭\n")
# 离开with块后f自动关闭
# 读取也一样
with open("test.txt", "r", encoding="utf-8") as f:
content = f.read()
# 1. read() — 一次读全部
with open("test.txt", encoding="utf-8") as f:
content = f.read()
# 2. readline() — 每次读一行
with open("test.txt", encoding="utf-8") as f:
line1 = f.readline()
line2 = f.readline()
# 3. readlines() — 读所有行，返回列表
with open("test.txt", encoding="utf-8") as f:
lines = f.readlines() # ['第一行\n', '第二行\n', ...]
# 4. 直接遍历文件对象（最推荐！内存友好）
with open("test.txt", encoding="utf-8") as f:
for line in f: # 一行一行读，不占内存
print(line.strip()) # strip()去掉行尾换行符
# write() — 写入字符串
with open("output.txt", "w", encoding="utf-8") as f:
f.write("Hello\n")
f.write("World\n")
# writelines() — 写入字符串列表（不加换行）
lines = ["第一行\n", "第二行\n", "第三行\n"]
with open("output.txt", "w", encoding="utf-8") as f:
f.writelines(lines)
# print() 直接写文件
with open("output.txt", "w", encoding="utf-8") as f:
print("Hello", file=f)
print("World", file=f)
import os
# 当前工作目录
print(os.getcwd())
# 拼接路径（不要用字符串拼接！）
path = os.path.join("folder", "subfolder", "file.txt")
# Windows: folder\subfolder\file.txt
# Mac/Linux: folder/subfolder/file.txt
# 检查文件是否存在
print(os.path.exists("test.txt")) # True/False
# 检查是文件还是目录
print(os.path.isfile("test.txt")) # True
print(os.path.isdir("folder")) # True/False
# 创建目录
os.makedirs("new_folder/sub", exist_ok=True)
# 列出目录内容
files = os.listdir(".") # 当前目录所有文件
# 推荐用pathlib（更现代）
from pathlib import Path
p = Path("folder") / "file.txt"
p.exists()
p.read_text(encoding="utf-8") # 一行读文件
p.write_text("内容", encoding="utf-8") # 一行写文件
import json
# Python数据 → JSON文件
data = {
"name": "张三",
"age": 20,
"courses": ["数学", "Python"],
"active": True
}
with open("data.json", "w", encoding="utf-8") as f:
json.dump(data, f, ensure_ascii=False, indent=2)
# ensure_ascii=False 让中文正常显示
# indent=2 让格式美观
# JSON文件 → Python数据
with open("data.json", "r", encoding="utf-8") as f:
loaded = json.load(f)
# JSON字符串 ↔ Python字典
json_str = json.dumps(data, ensure_ascii=False) # 字典→字符串
data2 = json.loads(json_str) # 字符串→字典
# 不处理异常 → 程序崩溃
# print(10 / 0) ← ZeroDivisionError，程序崩溃
# 处理异常 → 优雅应对
try:
result = 10 / 0
except ZeroDivisionError:
print("不能除以零！")
# 程序继续运行
try:
# 可能出错的代码
num = int(input("输入数字: "))
result = 100 / num
print(f"结果: {result}")
except ValueError as e:
# 捕获特定异常，e是异常对象
print(f"输入不是数字: {e}")
except ZeroDivisionError:
# 捕获另一个异常
print("不能除以零")
except Exception as e:
# 捕获所有其他异常（兜底）
print(f"未知错误: {e}")
else:
# try块没有异常时执行
print("计算成功！")
finally:
# 不管有没有异常都执行（清理工作）
print("程序结束")
| 关键字 | 作用 | 必须？ |
|---|---|---|
| try | 包裹可能出错的代码 | 是 |
| except | 捕获并处理异常 | 是 |
| else | 没异常时执行 | 否 |
| finally | 无论如何都执行 | 否 |
| 异常 | 触发场景 |
|---|---|
ValueError | 值错误（如int("abc")） |
TypeError | 类型错误（如"abc" + 1） |
ZeroDivisionError | 除以零 |
IndexError | 列表索引越界 |
KeyError | 字典键不存在 |
FileNotFoundError | 文件不存在 |
AttributeError | 属性/方法不存在 |
NameError | 变量名未定义 |
ImportError | 导入模块失败 |
def set_age(age):
if age < 0 or age > 150:
raise ValueError(f"年龄必须在0-150之间，你输入了{age}")
return age
try:
set_age(-5)
except ValueError as e:
print(e) # 年龄必须在0-150之间，你输入了-5
# 自定义异常类（继承Exception）
class InsufficientFunds(Exception):
def __init__(self, balance, amount):
self.balance = balance
self.amount = amount
super().__init__(f"余额{balance}不足，无法取{amount}")
def withdraw(balance, amount):
if amount > balance:
raise InsufficientFunds(balance, amount)
return balance - amount
try:
withdraw(100, 200)
except InsufficientFunds as e:
print(e) # 余额100不足，无法取200
print(e.balance) # 100
# 方式1：import 模块名
import math
print(math.pi) # 3.14159...
print(math.sqrt(16)) # 4.0
# 方式2：import 模块名 as 别名
import numpy as np # 常用别名
import pandas as pd
# 方式3：from 模块名 import 具体内容
from math import pi, sqrt
print(pi) # 不用math.前缀
print(sqrt(16))
# 方式4：from 模块名 import *（不推荐！污染命名空间）
# from math import *
# 方式5：导入自定义模块
# 假设有 utils.py 文件，里面有函数 helper()
# from utils import helper
# 在 utils.py 中
def helper():
print("工具函数")
# 这行的作用：只有直接运行此文件时才执行
# 被import时不执行
if __name__ == "__main__":
print("直接运行utils.py")
helper()
| 模块 | 用途 | 常用功能 |
|---|---|---|
os | 操作系统接口 | 文件/目录操作、环境变量 |
sys | 系统相关 | sys.argv命令行参数、sys.exit() |
json | JSON处理 | json.load/dump/loads/dumps |
datetime | 日期时间 | date.today()、datetime.now() |
random | 随机数 | random.randint/choice/shuffle |
math | 数学函数 | math.pi/sqrt/ceil/floor |
re | 正则表达式 | re.match/search/findall/sub |
collections | 扩展数据结构 | Counter/defaultdict/deque |
pathlib | 路径处理(现代) | Path对象 |
itertools | 迭代工具 | chain/product/combinations |
# === datetime ===
from datetime import datetime, date, timedelta
now = datetime.now()
print(now) # 2026-08-02 14:30:00.123456
print(now.strftime("%Y-%m-%d %H:%M")) # 2026-08-02 14:30
today = date.today()
tomorrow = today + timedelta(days=1)
print(tomorrow)
# 字符串 → 日期
dt = datetime.strptime("2026-01-15", "%Y-%m-%d")
# === random ===
import random
print(random.randint(1, 100)) # 1-100随机整数
print(random.choice(["A", "B", "C"])) # 随机选一个
nums = [1, 2, 3, 4, 5]
random.shuffle(nums) # 打乱顺序
print(random.sample(range(1, 50), 6)) # 随机选6个不重复
# === collections.Counter ===
from collections import Counter
text = "the cat sat on the mat"
word_count = Counter(text.split())
print(word_count) # Counter({'the': 2, 'cat': 1, ...})
print(word_count.most_common(2)) # [('the', 2), ('cat', 1)]
# === collections.defaultdict ===
from collections import defaultdict
# 自动初始化默认值
word_group = defaultdict(list)
words = [("fruit", "apple"), ("fruit", "banana"), ("veg", "carrot")]
for category, word in words:
word_group[category].append(word)
print(dict(word_group)) # {'fruit': ['apple', 'banana'], 'veg': ['carrot']}
# === re 正则表达式 ===
import re
# 查找所有邮箱
text = "联系: alice@qq.com 或 bob@gmail.com"
emails = re.findall(rr'\S+@\S+\.\S+', text)
print(emails) # ['alice@qq.com', 'bob@gmail.com']
# 替换
result = re.sub(rr'\d+', "#", "电话13800001234,邮编100000")
print(result) # 电话#,邮编#
import json
import os
CONTACTS_FILE = "contacts.json"
def load_contacts():
"""从文件加载通讯录"""
if not os.path.exists(CONTACTS_FILE):
return {}
try:
with open(CONTACTS_FILE, "r", encoding="utf-8") as f:
return json.load(f)
except json.JSONDecodeError:
print("通讯录文件损坏，重新创建")
return {}
def save_contacts(contacts):
"""保存通讯录到文件"""
with open(CONTACTS_FILE, "w", encoding="utf-8") as f:
json.dump(contacts, f, ensure_ascii=False, indent=2)
def main():
contacts = load_contacts()
while True:
print("\n===== 通讯录 =====")
print("1.添加 2.查找 3.删除 4.全部 5.退出")
choice = input("选择: ").strip()
if choice == "1":
name = input("姓名: ").strip()
if not name:
print("姓名不能为空")
continue
phone = input("电话: ").strip()
contacts[name] = phone
save_contacts(contacts)
print(f"已添加 {name}")
elif choice == "2":
name = input("查找姓名: ").strip()
if name in contacts:
print(f"{name}: {contacts[name]}")
else:
print("未找到")
elif choice == "3":
name = input("删除姓名: ").strip()
if name in contacts:
del contacts[name]
save_contacts(contacts)
print("已删除")
else:
print("未找到")
elif choice == "4":
if not contacts:
print("通讯录为空")
else:
for name, phone in sorted(contacts.items()):
print(f" {name}: {phone}")
elif choice == "5":
print("再见！")
break
else:
print("无效选择")
if __name__ == "__main__":
main()
写一个函数 logger(message, level="INFO")，把消息写入 log.txt，格式为 [时间] [级别] 消息。用datetime获取时间，用with语句写文件（追加模式）。
from datetime import datetime
def logger(message, level="INFO"):
timestamp = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
line = f"[{timestamp}] [{level}] {message}\n"
with open("log.txt", "a", encoding="utf-8") as f:
f.write(line)
print(line.strip())
logger("程序启动")
logger("文件未找到", "WARNING")
logger("数据库连接失败", "ERROR")用try-except写一个计算器函数，处理：除以零、输入非数字、未知错误。每种错误给出友好的中文提示。
def safe_calculate(a_str, op, b_str):
try:
a = float(a_str)
b = float(b_str)
if op == "+": return a + b
elif op == "-": return a - b
elif op == "*": return a * b
elif op == "/": return a / b
else: return f"不支持运算符: {op}"
except ValueError:
return "错误：请输入有效数字"
except ZeroDivisionError:
return "错误：不能除以零"
except Exception as e:
return f"未知错误：{e}"
print(safe_calculate("10", "+", "5")) # 15.0
print(safe_calculate("10", "/", "0")) # 错误：不能除以零
print(safe_calculate("abc", "+", "5")) # 错误：请输入有效数字写程序：①输入5个学生姓名和成绩，保存到JSON文件；②读取JSON文件，输出平均分和最高分学生。
import json
# 写入
students = []
for i in range(5):
name = input(f"学生{i+1}姓名: ")
score = float(input(f"{name}成绩: "))
students.append({"name": name, "score": score})
with open("grades.json", "w", encoding="utf-8") as f:
json.dump(students, f, ensure_ascii=False, indent=2)
# 读取
with open("grades.json", "r", encoding="utf-8") as f:
data = json.load(f)
avg = sum(s["score"] for s in data) / len(data)
best = max(data, key=lambda s: s["score"])
print(f"平均分: {avg:.1f}")
print(f"最高分: {best['name']} ({best['score']})")读取一个文本文件，统计：总行数、总单词数、总字符数、出现最多的5个单词。用Counter。
from collections import Counter
def analyze_file(filepath):
try:
with open(filepath, "r", encoding="utf-8") as f:
content = f.read()
lines = content.splitlines()
words = content.split()
chars = len(content)
word_freq = Counter(word.lower().strip(".,!?") for word in words)
print(f"总行数: {len(lines)}")
print(f"总单词数: {len(words)}")
print(f"总字符数: {chars}")
print(f"最常见5个单词:")
for word, count in word_freq.most_common(5):
print(f" {word}: {count}次")
except FileNotFoundError:
print(f"文件 {filepath} 不存在")
analyze_file("test.txt")