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python --version  # Windows
2
python3 --version  # macOS/Linux

1
print("Hello, World!")

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python hello.py  # Windows
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python3 hello.py  # macOS/Linux

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Hello, World!

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# 变量赋值
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name = "John"
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age = 30
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height = 1.75
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is_student = False
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# 打印变量
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print(name)
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print(age)
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print(height)
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print(is_student)

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# 算术运算符
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print(1 + 2)  # 加法
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print(5 - 3)  # 减法
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print(2 * 3)  # 乘法
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print(6 / 2)  # 除法（返回浮点数）
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print(7 // 3)  # 整除
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print(7 % 3)  # 取模
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print(2 ** 3)  # 幂运算

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# 比较运算符
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print(1 == 2)  # 等于
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print(1 != 2)  # 不等于
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print(1 < 2)  # 小于
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print(1 > 2)  # 大于
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print(1 <= 2)  # 小于等于
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print(1 >= 2)  # 大于等于

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# 逻辑运算符
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print(True and False)  # 与
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print(True or False)  # 或
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print(not True)  # 非

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# 输出
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print("Hello, World!")
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print("Name:", "John")
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print("Age:", 30)
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print("Height:", 1.75)
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# 格式化输出
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name = "John"
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age = 30
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print(f"My name is {name} and I am {age} years old.")  # f-string
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print("My name is {} and I am {} years old.".format(name, age))  # format方法
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print("My name is %s and I am %d years old." % (name, age))  # 百分号格式化

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# 输入
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name = input("Enter your name: ")
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age = input("Enter your age: ")
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print(f"Hello, {name}! You are {age} years old.")
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# 输入转换
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age = int(input("Enter your age: "))
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print(f"You will be {age + 1} years old next year.")

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# if语句
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age = 18
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if age >= 18:
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    print("You are an adult.")
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else:
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    print("You are a minor.")
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# if-elif-else语句
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score = 85
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if score >= 90:
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    print("A")
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elif score >= 80:
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    print("B")
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elif score >= 70:
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    print("C")
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elif score >= 60:
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    print("D")
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else:
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    print("F")

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# for循环
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fruits = ["apple", "banana", "cherry"]
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for fruit in fruits:
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    print(fruit)
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# 遍历数字
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for i in range(5):
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    print(i)
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# 遍历字典
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person = {"name": "John", "age": 30, "city": "New York"}
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for key, value in person.items():
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    print(key, ":", value)

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# while循环
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count = 0
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while count < 5:
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    print(count)
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    count += 1
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# 无限循环（谨慎使用）
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# while True:
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#     print("Hello")

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# break语句
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for i in range(10):
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    if i == 5:
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        break
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    print(i)
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# continue语句
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for i in range(10):
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    if i % 2 == 0:
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        continue
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    print(i)
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# pass语句
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for i in range(10):
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    if i == 5:
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        pass  # 占位符，什么都不做
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    print(i)

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# 创建列表
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fruits = ["apple", "banana", "cherry"]
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numbers = [1, 2, 3, 4, 5]
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mixed = ["apple", 1, True, 3.14]
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# 访问列表元素
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print(fruits[0])  # 第一个元素
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print(fruits[-1])  # 最后一个元素
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# 修改列表元素
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fruits[0] = "orange"
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print(fruits)
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# 添加元素
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fruits.append("grape")  # 添加到末尾
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fruits.insert(1, "pear")  # 插入到指定位置
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print(fruits)
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# 删除元素
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fruits.remove("banana")  # 删除指定值
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fruits.pop()  # 删除最后一个元素
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print(fruits)
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# 列表操作
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print(len(fruits))  # 长度
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print("apple" in fruits)  # 检查元素是否存在
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print(fruits + ["kiwi", "mango"])  # 列表拼接
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print(fruits * 2)  # 列表重复
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# 列表方法
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numbers = [3, 1, 4, 1, 5, 9, 2, 6]
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numbers.sort()  # 排序
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print(numbers)
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numbers.reverse()  # 反转
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print(numbers)
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print(numbers.count(1))  # 计数
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print(numbers.index(5))  # 查找索引

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# 创建元组
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tuple1 = ("apple", "banana", "cherry")
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tuple2 = (1, 2, 3, 4, 5)
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tuple3 = ("apple", 1, True)
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# 访问元组元素
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print(tuple1[0])
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print(tuple1[-1])
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# 元组是不可变的
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# tuple1[0] = "orange"  # 会报错
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# 元组操作
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print(len(tuple1))
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print("apple" in tuple1)
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print(tuple1 + ("grape", "pear"))
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print(tuple1 * 2)
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# 元组方法
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print(tuple1.count("apple"))
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print(tuple1.index("banana"))
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# 单元素元组
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single_tuple = ("apple",)  # 注意逗号
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print(type(single_tuple))

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# 创建字典
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person = {
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    "name": "John",
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    "age": 30,
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    "city": "New York"
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}
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# 访问字典值
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print(person["name"])
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print(person.get("age"))
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print(person.get("gender", "Unknown"))  # 不存在的键返回默认值
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# 修改字典
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person["age"] = 31
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person["gender"] = "Male"
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print(person)
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# 删除字典元素
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del person["gender"]
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print(person)
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# 字典操作
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print(len(person))
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print("name" in person)  # 检查键是否存在
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print(person.keys())  # 获取所有键
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print(person.values())  # 获取所有值
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print(person.items())  # 获取所有键值对
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# 字典方法
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person.update({"city": "Boston", "job": "Engineer"})
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print(person)
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copy_person = person.copy()
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print(copy_person)
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print(person.pop("job"))
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print(person)

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# 创建集合
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fruits = {"apple", "banana", "cherry"}
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numbers = {1, 2, 3, 4, 5}
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# 添加元素
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fruits.add("grape")
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print(fruits)
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# 删除元素
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fruits.remove("banana")  # 元素不存在会报错
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fruits.discard("orange")  # 元素不存在不会报错
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print(fruits)
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# 集合操作
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print(len(fruits))
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print("apple" in fruits)
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# 集合运算
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set1 = {1, 2, 3, 4, 5}
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set2 = {4, 5, 6, 7, 8}
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print(set1.union(set2))  # 并集
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print(set1.intersection(set2))  # 交集
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print(set1.difference(set2))  # 差集
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print(set1.symmetric_difference(set2))  # 对称差集

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# 定义函数
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def greet():
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    print("Hello, World!")
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# 调用函数
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greet()
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# 带参数的函数
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def greet(name):
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    print(f"Hello, {name}!")
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greet("John")
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# 带默认参数的函数
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def greet(name, greeting="Hello"):
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    print(f"{greeting}, {name}!")
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greet("John")
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greet("John", "Hi")
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# 带返回值的函数
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def add(a, b):
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    return a + b
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result = add(3, 5)
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print(result)
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# 带多个返回值的函数
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def get_person():
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    return "John", 30, "New York"
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name, age, city = get_person()
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print(name, age, city)

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def describe_pet(animal_type, pet_name):
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    print(f"I have a {animal_type}.")
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    print(f"My {animal_type}'s name is {pet_name}.")
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describe_pet("dog", "Buddy")

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describe_pet(animal_type="cat", pet_name="Whiskers")
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describe_pet(pet_name="Charlie", animal_type="bird")

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# *args：接收任意数量的位置参数
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def sum_numbers(*args):
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    return sum(args)
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print(sum_numbers(1, 2, 3))
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print(sum_numbers(1, 2, 3, 4, 5))
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# **kwargs：接收任意数量的关键字参数
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def describe_person(**kwargs):
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    for key, value in kwargs.items():
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        print(f"{key}: {value}")
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describe_person(name="John", age=30, city="New York")

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# 全局变量
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x = 10
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def print_global():
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    print(x)
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print_global()
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# 局部变量
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def print_local():
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    y = 20
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    print(y)
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print_local()
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# print(y)  # 会报错，局部变量在函数外不可访问
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# 修改全局变量
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def modify_global():
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    global x
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    x = 30
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    print(x)
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modify_global()
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print(x)

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# 导入整个模块
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import mymodule
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print(mymodule.greet("John"))
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print(mymodule.add(3, 5))
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print(mymodule.PI)
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# 导入模块中的特定函数或变量
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from mymodule import greet, add
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print(greet("John"))
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print(add(3, 5))
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# 导入模块中的所有内容
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from mymodule import *
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print(greet("John"))
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print(add(3, 5))
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print(PI)
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# 使用别名
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import mymodule as mm
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print(mm.greet("John"))
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from mymodule import greet as g
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print(g("John"))

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# 数学模块
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import math
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print(math.pi)
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print(math.sqrt(16))
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print(math.pow(2, 3))
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# 随机模块
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import random
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print(random.randint(1, 10))
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print(random.choice(["apple", "banana", "cherry"]))
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# 日期时间模块
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import datetime
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now = datetime.datetime.now()
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print(now)
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print(now.year)
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print(now.month)
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print(now.day)
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# 时间模块
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import time
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print(time.time())
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time.sleep(1)  # 暂停1秒
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print(time.time())
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# 操作系统模块
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import os
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print(os.getcwd())  # 获取当前工作目录
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print(os.listdir("."))  # 列出当前目录中的文件和文件夹

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mypackage/
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├── __init__.py
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├── module1.py
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└── module2.py

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# 导入包
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import mypackage
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print(mypackage.__version__)
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# 导入包中的模块
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import mypackage.module1
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print(mypackage.module1.function1())
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# 导入模块中的函数
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from mypackage.module2 import function2
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print(function2())

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# 写入文件
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with open("example.txt", "w") as f:
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    f.write("Hello, World!\n")
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    f.write("This is a test file.\n")
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    f.write("Python is awesome!\n")
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# 追加文件
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with open("example.txt", "a") as f:
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    f.write("Adding more content.\n")

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# 读取整个文件
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with open("example.txt", "r") as f:
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    content = f.read()
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    print(content)
5

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# 逐行读取
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with open("example.txt", "r") as f:
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    for line in f:
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        print(line.strip())
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# 读取所有行
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with open("example.txt", "r") as f:
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    lines = f.readlines()
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    print(lines)

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# 异常处理
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try:
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    with open("non_existent_file.txt", "r") as f:
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        content = f.read()
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        print(content)
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except FileNotFoundError:
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    print("File not found.")
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except Exception as e:
9
    print(f"An error occurred: {e}")
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finally:
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    print("This block always executes.")

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# 定义类
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class Person:
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    # 类变量
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    species = "Homo sapiens"
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    # 初始化方法
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    def __init__(self, name, age):
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        # 实例变量
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        self.name = name
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        self.age = age
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    # 实例方法
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    def greet(self):
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        return f"Hello, my name is {self.name}."
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    def get_age(self):
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        return self.age
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# 创建对象
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person1 = Person("John", 30)
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person2 = Person("Jane", 25)
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# 访问实例变量
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print(person1.name)
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print(person2.age)
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# 调用实例方法
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print(person1.greet())
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print(person2.get_age())
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# 访问类变量
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print(Person.species)
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print(person1.species)

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# 父类
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class Animal:
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    def __init__(self, name):
4
        self.name = name
5

6
    def speak(self):
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        pass
8

9
# 子类
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class Dog(Animal):
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    def speak(self):
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        return "Woof!"
13

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class Cat(Animal):
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    def speak(self):
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        return "Meow!"
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# 创建对象
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dog = Dog("Buddy")
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cat = Cat("Whiskers")
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print(dog.name)
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print(dog.speak())
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print(cat.name)
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print(cat.speak())

1
def animal_speak(animal):
2
    print(animal.speak())
3

4
dog = Dog("Buddy")
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cat = Cat("Whiskers")
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animal_speak(dog)  # 调用Dog的speak方法
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animal_speak(cat)  # 调用Cat的speak方法