运算符与表达式

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一、算术运算符

算术运算符用于执行数学运算。

# 基础算术运算
a = 15
b = 4

print(f"加法：{a} + {b} = {a + b}")
print(f"减法：{a} - {b} = {a - b}")
print(f"乘法：{a} * {b} = {a * b}")
print(f"除法：{a} / {b} = {a / b}")
print(f"整除：{a} // {b} = {a // b}")
print(f"取余：{a} % {b} = {a % b}")
print(f"幂运算：{a} ** {b} = {a ** b}")

实用技巧：整除与取余的应用

# 判断奇偶数
num = 17
if num % 2 == 0:
    print(f"{num} 是偶数")
else:
    print(f"{num} 是奇数")

# 时间转换：秒转时分秒
total_seconds = 3665
hours = total_seconds // 3600
minutes = (total_seconds % 3600) // 60
seconds = total_seconds % 60
print(f"{total_seconds}秒 = {hours}时{minutes}分{seconds}秒")

# 分页计算
item_index = 35
page_size = 10
page_number = (item_index - 1) // page_size + 1
print(f"第{item_index}条数据在第{page_number}页")

二、比较运算符

比较运算符返回布尔值（True 或 False）。

x = 10
y = 20

print(f"x == y: {x == y}")
print(f"x != y: {x != y}")
print(f"x > y: {x > y}")
print(f"x < y: {x < y}")
print(f"x >= 10: {x >= 10}")
print(f"y <= 20: {y <= 20}")

链式比较（Python 特色）

age = 25
is_adult = 18 <= age <= 60
print(f"{age}岁是成年人吗？{is_adult}")

score = 85
if 90 <= score <= 100:
    grade = "A"
elif 80 <= score < 90:
    grade = "B"
elif 70 <= score < 80:
    grade = "C"
else:
    grade = "D"
print(f"成绩等级：{grade}")

三、逻辑运算符

逻辑运算符用于组合多个条件。

print(f"True and False: {True and False}")
print(f"True or False: {True or False}")
print(f"not True: {not True}")

# 用户登录验证
username = "admin"
password = "123456"
is_verified = True
can_login = (username == "admin") and (password == "123456") and is_verified
print(f"可以登录：{can_login}")

# 折扣判断
has_vip = False
has_coupon = True
can_discount = has_vip or has_coupon
print(f"可以享受折扣：{can_discount}")

短路求值

def check():
    print("函数被调用了")
    return True

result = False and check()
print(f"结果：{result}")

# 提供默认值
user_name = ""
display_name = user_name or "匿名用户"
print(f"显示名称：{display_name}")

四、位运算符

位运算符直接操作二进制位。

a = 12
b = 5

print(f"a & b (按位与): {a & b}")
print(f"a | b (按位或): {a | b}")
print(f"a ^ b (按位异或): {a ^ b}")
print(f"~a (按位取反): {~a}")
print(f"a << 1 (左移): {a << 1}")
print(f"a >> 1 (右移): {a >> 1}")

位运算的实用场景

# 判断奇偶
num = 17
is_odd = num & 1
print(f"{num}是奇数吗？{bool(is_odd)}")

# 快速乘除
x = 10
print(f"{x} * 8 = {x << 3}")
print(f"{x} / 4 = {x >> 2}")

# 权限标志位
READ = 1
WRITE = 2
EXECUTE = 4

user_permission = READ | WRITE
print(f"用户权限值：{user_permission}")

has_write = bool(user_permission & WRITE)
print(f"有写权限：{has_write}")

五、赋值运算符

x = 10
x += 5
print(f"x += 5: {x}")

x -= 3
print(f"x -= 3: {x}")

x *= 2
print(f"x *= 2: {x}")

# 多重赋值
a, b, c = 1, 2, 3
print(f"a={a}, b={b}, c={c}")

# 交换变量
a, b = b, a
print(f"交换后：a={a}, b={b}")

六、成员运算符与身份运算符

# 成员运算符
fruits = ["apple", "banana", "orange"]
print(f"'banana' in fruits: {'banana' in fruits}")
print(f"'grape' not in fruits: {'grape' not in fruits}")

# 身份运算符
a = [1, 2, 3]
b = [1, 2, 3]
c = a

print(f"a == b: {a == b}")
print(f"a is b: {a is b}")
print(f"a is c: {a is c}")

# None 比较
value = None
print(f"value is None: {value is None}")

七、运算符优先级

从高到低：括号 > 幂 > 一元 > 乘除 > 加减 > 位移 > 位与 > 位异或 > 位或 > 比较 > not > and > or

result = 2 + 3 * 4
print(f"2 + 3 * 4 = {result}")

result = (2 + 3) * 4
print(f"(2 + 3) * 4 = {result}")

x, y, z = 10, 5, 2
result = x + y * z ** 2
print(f"x + y * z ** 2 = {result}")

八、实战练习：简易计算器

def calculator(a, b, operator):
    operations = {
        '+': lambda x, y: x + y,
        '-': lambda x, y: x - y,
        '*': lambda x, y: x * y,
        '/': lambda x, y: x / y if y != 0 else "错误：除数不能为 0",
        '//': lambda x, y: x // y if y != 0 else "错误",
        '%': lambda x, y: x % y if y != 0 else "错误",
        '**': lambda x, y: x ** y,
    }
    
    if operator in operations:
        return operations[operator](a, b)
    return "错误：不支持的运算符"

if __name__ == "__main__":
    test_cases = [
        (10, 5, '+'), (10, 5, '-'), (10, 5, '*'),
        (10, 5, '/'), (10, 3, '//'), (10, 3, '%'),
        (2, 8, '**'), (10, 0, '/'),
    ]
    
    for a, b, op in test_cases:
        result = calculator(a, b, op)
        print(f"{a} {op} {b} = {result}")

九、今日要点总结

运算符类型符号说明
算术+ - * / // % **基础数学运算
比较== != > < >= <=返回布尔值
逻辑and or not条件组合
位运算&^ ~ << >>二进制操作
赋值= += -= *=变量赋值
成员in not in成员检查
身份is is not对象身份比较

关键记忆点：

  • Python 支持链式比较：18 <= age <= 60
  • 逻辑运算符有短路特性
  • is 比较身份，== 比较值
  • 用括号明确表达式优先级
  • 位运算在权限控制中非常有用
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