Python 入门教程目录变量与数据类型运算符控制流函数列表元组字典字符串操作文件操作异常处理变量与数据类型

Python中变量不需要声明类型，直接赋值即可。常用数据类型有：整数(int)、浮点数(float)、字符串(str)、布尔值(bool)等。

案例1：基本变量赋值python

# 整数类型age = 25# 浮点数类型height = 1.75# 字符串类型name = "张三"# 布尔类型is_student = Trueprint(type(age)) # <class 'int'>print(type(height)) # <class 'float'>print(type(name)) # <class 'str'>print(type(is_student)) # <class 'bool'>案例2：类型转换python

num_str = "123"num_int = int(num_str) # 将字符串转换为整数num_float = float(num_str) # 将字符串转换为浮点数print(num_int + 10) # 133print(num_float + 10) # 133.0num_int = int(3.14) # 将浮点数转换为整数，结果为3num_str = str(123) # 将整数转换为字符串，结果为"123"案例3：变量交换python

a = 10b = 20# Python特有的变量交换方式a, b = b, aprint(a) # 20print(b) # 10案例4：多变量赋值python

# 同时给多个变量赋相同的值x = y = z = 0# 同时给多个变量赋不同的值name, age, country = "李四", 30, "中国"print(x, y, z) # 0 0 0print(name, age, country) # 李四 30 中国案例5：删除变量python

score = 90print(score) # 90del score # 删除变量scoretry: print(score)except NameError: print("变量score已被删除") # 输出：变量score已被删除

运算符

Python支持多种运算符，包括算术运算符、比较运算符、逻辑运算符等。

案例1：算术运算符python

a = 10b = 3print(a + b) # 加法，输出13print(a - b) # 减法，输出7print(a * b) # 乘法，输出30print(a / b) # 除法，输出3.3333333333333335print(a // b) # 整除，输出3print(a % b) # 取模，输出1print(a ** b) # 幂运算，输出1000案例2：比较运算符python

x = 5y = 8print(x == y) # 等于，输出Falseprint(x != y) # 不等于，输出Trueprint(x > y) # 大于，输出Falseprint(x < y) # 小于，输出Trueprint(x >= y) # 大于等于，输出Falseprint(x <= y) # 小于等于，输出True案例3：逻辑运算符python

p = Trueq = Falseprint(p and q) # 逻辑与，输出Falseprint(p or q) # 逻辑或，输出Trueprint(not p) # 逻辑非，输出False案例4：复合赋值运算符python

num = 10num += 5 # 等价于 num = num + 5，结果为15num -= 3 # 等价于 num = num - 3，结果为12num *= 2 # 等价于 num = num * 2，结果为24num /= 4 # 等价于 num = num / 4，结果为6.0num %= 5 # 等价于 num = num % 5，结果为1.0案例5：运算符优先级python

result = (3 + 5) * 2 ** 2 / 4 - 1# 运算顺序：括号 -> 幂运算 -> 乘法 -> 除法 -> 减法# 计算过程：# (8) * 4 /4 -1 = 32/4 -1 = 8 -1 =7print(result) # 输出7.0控制流

控制流语句包括条件判断(if-elif-else)和循环(for、while)。

案例1：if-elif-else语句python

score = 85if score >= 90: grade = 'A'elif score >= 80: grade = 'B'elif score >= 70: grade = 'C'elif score >= 60: grade = 'D'else: grade = 'F'print(f"分数{score}对应的等级是：{grade}") # 分数85对应的等级是：B案例2：for循环python

fruits = ["apple", "banana", "cherry"]# 遍历列表元素for fruit in fruits: print(fruit)# 输出：# apple# banana# cherry# 使用range函数生成序列for i in range(5): print(i, end=' ') # 输出：0 1 2 3 4 print() # 换行案例3：while循环python

count = 0while count < 5: print(count) count += 1 # 输出：0 1 2 3 4# 无限循环# while True:# print("这是一个无限循环")# 带条件的无限循环退出方式# while True:# user_input = input("输入'quit'退出循环：")# if user_input == "quit":# break案例4：循环控制语句python

numbers = [1, 3, 5, 7, 9, 11, 13]for num in numbers: if num == 7: continue # 跳过当前循环的剩余语句，继续下一次循环 if num == 11: break # 当num等于11时退出循环 print(num)# 输出：1 3 5案例5：嵌套循环与标签python

# 嵌套循环for i in range(3): for j in range(2): print(f"i={i}, j={j}")# 输出：# i=0, j=0# i=0, j=1# i=1, j=0# i=1, j=1# i=2, j=0# i=2, j=1# 使用标签跳出多层循环outer_loop: for i in range(3): for j in range(3): if i == 1 and j == 1: break outer_loop # 跳出标记为outer_loop的循环 print(f"i={i}, j={j}")# 输出：# i=0, j=0# i=0, j=1# i=0, j=2# i=1, j=0函数

函数是一段可重复使用的代码块，使用def关键字定义。

案例1：基本函数定义与调用python

def greet(name): """这是一个问候函数""" print(f"Hello, {name}!")greet("Alice") # 输出：Hello, Alice!案例2：带返回值的函数python

def add(a, b): return a + bresult = add(5, 3)print(result) # 输出：8案例3：默认参数python

def describe_pet(animal_type='dog', pet_name='Rover'): print(f"I have a {animal_type} named {pet_name}.")describe_pet() # 使用默认参数，输出：I have a dog named Roverdescribe_pet(pet_name='Buddy') # 修改第二个参数，输出：I have a dog named Buddydescribe_pet(animal_type='cat', pet_name='Whiskers') # 修改两个参数，输出：I have a cat named Whiskers案例4：可变参数(*args)python

def sum_numbers(*args): total = 0 for num in args: total += num return totalprint(sum_numbers(1, 2, 3)) # 输出：6print(sum_numbers(1, 2, 3, 4, 5)) # 输出：15案例5：关键字参数(**kwargs)python

def print_person_info(**kwargs): for key, value in kwargs.items(): print(f"{key}: {value}")print_person_info(name="张三", age=30, city="北京")# 输出：# name: 张三# age: 30# city: 北京列表

列表是Python中最常用的数据结构，可以存储不同类型的元素。

案例1：创建与访问列表python

fruits = ["apple", "banana", "cherry"]numbers = [1, 2, 3, 4, 5]mixed = ["apple", 1, True]print(fruits[0]) # 访问第一个元素：appleprint(numbers[-1]) # 访问最后一个元素：5print(mixed[2]) # 访问第三个元素：Truefruits.append("orange") # 在列表末尾添加元素print(fruits) # ['apple', 'banana', 'cherry', 'orange']案例2：列表切片与修改python

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]print(numbers[2:5]) # 切片操作，取索引2到4的元素：[2, 3, 4]print(numbers[:3]) # 取前三个元素：[0, 1, 2]print(numbers[5:]) # 从索引5开始到末尾：[5, 6, 7, 8, 9]numbers[0] = 10 # 修改列表元素print(numbers) # [10, 1, 2, 3, 4, 5, 6, 7, 8, 9]案例3：列表常用方法python

fruits = ["apple", "banana", "cherry"]fruits.append("orange") # 添加元素到末尾fruits.insert(1, "grape") # 在指定位置插入元素fruits.remove("banana") # 删除指定元素fruits.pop() # 删除并返回最后一个元素fruits.sort() # 对列表进行排序fruits.reverse() # 反转列表顺序length = len(fruits) # 获取列表长度print(fruits) # ['apple', 'grape', 'cherry', 'orange']的逆序案例4：列表推导式python

squares = [x**2 for x in range(10)] # 创建一个包含0-9平方的列表even_numbers = [num for num in range(10) if num % 2 == 0] # 筛选偶数print(squares) # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]print(even_numbers) # [0, 2, 4, 6, 8]案例5：多维列表python

matrix = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]]print(matrix[0][1]) # 访问第一行第二列元素：2print(matrix[2][2]) # 访问第三行第三列元素：9# 遍历二维列表for row in matrix: for element in row: print(element, end=' ') print()元组

元组与列表类似，但是创建后不可修改，使用圆括号()`定义。

案例1：创建与访问元组python

coordinates = (3, 5)person = ("Alice", 30, "Female")print(coordinates[0]) # 输出：3print(person[1:]) # 输出：(30, 'Female')# 创建单元素元组single_element = (42,) # 注意后面的逗号案例2：元组解包python

coordinates = (3, 5)x, y = coordinates # 元组解包print(x, y) # 输出：3 5data = ("Alice", 30, "Female")name, age, gender = dataprint(name, age, gender) # Alice 30 Female案例3：元组与列表转换python

numbers = [1, 2, 3, 4, 5]numbers_tuple = tuple(numbers) # 列表转元组print(numbers_tuple) # (1, 2, 3, 4, 5)new_list = list(numbers_tuple) # 元组转列表print(new_list) # [1, 2, 3, 4, 5]案例4：元组常用操作python

colors = ("red", "green", "blue")print(colors.count("red")) # 统计元素出现次数：1print(len(colors)) # 获取元组长度：3# 注意：元组元素不可修改，以下操作会报错# colors[0] = "yellow" # TypeError: 'tuple' object does not support item assignment案例5：元组嵌套python

person = ("Bob", 25, ("New York", "USA"))city, country = person[2] # 解包嵌套元组print(person) # ('Bob', 25, ('New York', 'USA'))print(city) # New Yorkprint(country) # USA字典

字典是一种无序的键值对(key-value)集合，使用花括号{}定义。

案例1：创建与访问字典python

student = { "name": "张三", "age": 20, "major": "计算机科学"}print(student["name"]) # 访问键"name"对应的值：张三print(student.get("gender")) # 使用get方法访问不存在的键，返回Nonestudent["gender"] = "male" # 添加新键值对student["age"] = 21 # 修改现有键的值del student["major"] # 删除键值对print(student) # {'name': '张三', 'age': 21, 'gender': 'male'}案例2：字典遍历python

fruits = { "apple": 3, "banana": 2, "orange": 5}# 遍历键for key in fruits: print(key)# 遍历值for value in fruits.values(): print(value)# 遍历键值对for key, value in fruits.items(): print(f"{key}: {value}")案例3：字典常用方法python

person = { "name": "李四", "age": 30, "city": "北京"}keys = person.keys() # 获取所有键values = person.values() # 获取所有值items = person.items() # 获取所有键值对length = len(person) # 获取字典长度print(keys) # dict_keys(['name', 'age', 'city'])print(values) # dict_values(['李四', 30, '北京'])print(items) # dict_items([('name', '李四'), ('age', 30), ('city', '北京')])print(length) # 3person.update({"age": 31, "job": "engineer"}) # 更新字典案例4：嵌套字典python

company = { "name": "Tech Corp", "employees": { "Alice": {"age": 30, "position": "Engineer"}, "Bob": {"age": 25, "position": "Designer"} }}# 访问嵌套字典的值print(company["employees"]["Alice"]["position"]) # Engineer# 修改嵌套字典的值company["employees"]["Bob"]["age"] = 26案例5：默认字典python

from collections import defaultdict# 创建默认值为列表的字典dd = defaultdict(list)# 可以直接追加元素，无需先判断键是否存在dd["fruits"].append("apple")dd["fruits"].append("banana")dd["vegetables"].append("carrot")print(dd) # defaultdict(<class 'list'>, {'fruits': ['apple', 'banana'], 'vegetables': ['carrot']})字符串操作

字符串是Python中最常用的数据类型之一，支持多种操作和方法。

案例1：基本字符串操作python

text = "Hello, World!"print(text[0]) # 访问第一个字符：Hprint(text[7:]) # 截取子串：World!print(len(text)) # 获取字符串长度：13print(text.upper()) # 转换为大写：HELLO, WORLD!print(text.lower()) # 转换为小写：hello, world!print(text.replace("World", "Python")) # 替换子串：Hello, Python!案例2：字符串格式化python

name = "Alice"age = 30height = 1.68# 使用%格式化print("Name: %s, Age: %d, Height: %.2f" % (name, age, height))# 使用format方法print("Name: {}, Age: {}, Height: {:.2f}".format(name, age, height))# 使用f-string（Python 3.6+）print(f"Name: {name}, Age: {age}, Height: {height:.2f}")案例3：字符串分割与连接python

csv_data = "apple,banana,orange,grape"# 分割字符串fruits = csv_data.split(",")print(fruits) # ['apple', 'banana', 'orange', 'grape']# 连接列表元素成字符串joined = "-".join(fruits)print(joined) # apple-banana-orange-grape# 处理空格text = " Hello, World! "print(text.strip()) # 去除首尾空格："Hello, World!"案例4：字符串查找与判断python

text = "Python programming is fun!"print(text.find("programming")) # 查找子串位置：7print(text.index("is")) # 查找子串位置：14print(text.count("n")) # 统计字符出现次数：3print(text.startswith("Python")) # 是否以指定字符串开头：Trueprint(text.endswith("!")) # 是否以指定字符串结尾：Trueprint(text.isalpha()) # 是否全为字母：False案例5：字符串编码与解码python

text = "你好，世界！"# 编码为bytes对象encoded = text.encode("utf-8")print(encoded) # b'\xe4\xbd\xa0\xe5\xa5\xbd\xef\xbc\x8c\xe4\xb8\x96\xe7\x95\x8c\xef\xbc\x81'# 解码为字符串decoded = encoded.decode("utf-8")print(decoded) # 你好，世界！# 处理文件路径中的特殊字符file_path = "C:/我的文件夹/file.txt"encoded_path = file_path.encode("unicode_escape")print(encoded_path) # b'C:/\\u6211/e6/b3/a4e5/ad/97e6/96/b9/e5/a4/b4/file.txt'文件操作

Python提供了简单易用的文件操作接口，可以方便地读写文件。

案例1：基本文件写入python

# 写入文本文件with open("example.txt", "w", encoding="utf-8") as f: f.write("Hello, World!") f.write("这是一个文件操作的例子。")# 写入多行lines = ["第一行", "第二行", "第三行"]with open("lines.txt", "w", encoding="utf-8") as f: f.writelines(lines)案例2：基本文件读取python

# 读取整个文件with open("example.txt", "r", encoding="utf-8") as f: content = f.read() print(content)# 逐行读取with open("example.txt", "r", encoding="utf-8") as f: for line in f: print(line.strip())# 读取所有行到列表with open("example.txt", "r", encoding="utf-8") as f: lines = f.readlines() print(lines) # ['Hello, World!', '这是一个文件操作的例子。']案例3：文件指针操作python

with open("example.txt", "r+", encoding="utf-8") as f: # 移动文件指针到第7个字符处 f.seek(7) # 读取剩余内容 print(f.read()) # 'World!这是一个文件操作的例子。' # 在当前位置写入内容 f.write("这是追加的内容")案例4：处理CSV文件python

import csv# 写入CSV文件data = [ ["姓名", "年龄", "城市"], ["张三", 25, "北京"], ["李四", 30, "上海"]]with open("people.csv", "w", newline="", encoding="utf-8") as f: writer = csv.writer(f) writer.writerows(data)# 读取CSV文件with open("people.csv", "r", encoding="utf-8") as f: reader = csv.reader(f) for row in reader: print(row)案例5：处理JSON文件python

import jsondata = { "name": "Alice", "age": 30, "hobbies": ["reading", "traveling"]}# 写入JSON文件with open("data.json", "w", encoding="utf-8") as f: json.dump(data, f, ensure_ascii=False, indent=4)# 读取JSON文件with open("data.json", "r", encoding="utf-8") as f: loaded_data = json.load(f) print(loaded_data)异常处理

异常处理可以帮助我们优雅地处理程序运行中出现的错误。

案例1：基本try-except结构python

try: num1 = int(input("请输入第一个数字：")) num2 = int(input("请输入第二个数字：")) result = num1 / num2 print(f"结果：{result}")except ValueError: print("输入的不是有效数字")except ZeroDivisionError: print("除数不能为零")except Exception as e: print(f"发生未知错误：{e}")else: print("计算成功")finally: print("程序结束")案例2：raise抛出异常python

def divide(a, b): if b == 0: raise ValueError("除数不能为零") return a / btry: result = divide(10, 0)except ValueError as e: print(e) # 输出：除数不能为零案例3：自定义异常类python

class NegativeNumberError(Exception): """负数错误异常""" def __init__(self, value): self.value = value super().__init__(f"不允许输入负数：{value}")def check_positive(num): if num < 0: raise NegativeNumberError(num)try: check_positive(-5)except NegativeNumberError as e: print(e) # 输出：不允许输入负数：-5案例4：使用finally清理资源python

file = Nonetry: file = open("test.txt", "w", encoding="utf-8") file.write("Hello, World!")except IOError: print("文件操作失败")finally: if file is not None: file.close() print("文件已关闭")案例5：assert断言语句python

def calculate_age(birth_year): assert birth_year <= 2023, "出生年份不能晚于2023年" return 2023 - birth_yeartry: age = calculate_age(2025)except AssertionError as e: print(e) # 输出：出生年份不能晚于2023年