Python支持一种独特的语法特性：条件表达式可以连写。这种链式比较（Chained Comparisons）可以让代码更加简洁和易读。本文将详细介绍Python中条件表达式连写的用法。
一、链式比较的基本用法
1. 数学风格比较
1
2
3
4
5
6
7
8
9
|
x = 5
if x > 0 and x < 10:
print("x在0到10之间")
x = 5
if 0 < x < 10:
print("x在0到10之间")
|
2. 更多示例
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
age = 25
if 18 <= age <= 65:
print("工作年龄")
x = 0.5
if 0 < x < 1:
print("x是0到1之间的分数")
a, b, c = 3, 5, 7
if a < b < c:
print("a < b < c 成立")
|
二、链式比较的原理
1. 等价的展开形式
1
2
3
4
5
6
7
|
if 0 < x < 10:
print("x在0到10之间")
if x > 0 and x < 10:
print("x在0到10之间")
|
2. 多个比较链
1
2
3
4
5
6
7
|
if a < b < c < d:
print("a < b < c < d")
if a < b and b < c and c < d:
print("a < b < c < d")
|
3. 不同比较运算符
1
2
3
4
5
6
7
8
9
| x = 5
y = 10
if 0 < x <= 10:
print("0 < x <= 10 成立")
if 0 < x < y > 3:
print("0 < x < y > 3 成立")
|
三、链式比较的实际应用
1. 范围检查
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
|
def validate_score(score):
if 0 <= score <= 100:
return True
return False
print(validate_score(85))
print(validate_score(-5))
print(validate_score(105))
def is_lowercase_letter(c):
if 'a' <= c <= 'z':
return True
return False
print(is_lowercase_letter('g'))
print(is_lowercase_letter('A'))
|
2. 边界条件检查
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
|
def classify_age(age):
if 0 <= age < 18:
return "未成年"
elif 18 <= age < 65:
return "工作年龄"
elif 65 <= age <= 120:
return "退休年龄"
else:
return "无效年龄"
ages = [-5, 0, 17, 18, 64, 65, 120, 150]
for age in ages:
print(f"age={age}: {classify_age(age)}")
|
3. 坐标范围检查
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
|
def is_in_rectangle(x, y, x1, y1, x2, y2):
"""检查点(x,y)是否在矩形(x1,y1)到(x2,y2)内"""
if x1 <= x <= x2 and y1 <= y <= y2:
return True
return False
def is_in_rectangle_chained(x, y, x1, y1, x2, y2):
"""使用链式比较的版本"""
if x1 <= x <= x2 and y1 <= y <= y2:
return True
return False
print(is_in_rectangle_chained(5, 5, 0, 0, 10, 10))
print(is_in_rectangle_chained(15, 5, 0, 0, 10, 10))
|
四、链式比较与逻辑运算符的对比
1. 代码简洁性
1
2
3
4
5
6
7
8
9
10
11
12
|
x = 5
if x > 0 and x < 10 and x != 7:
print("通过")
if 0 < x < 10 != 7:
print("通过")
if 0 < x < 10 and x != 7:
print("通过")
|
2. 可读性
1
2
3
4
5
6
7
8
9
10
11
12
13
14
|
a, b, c = 1, 2, 3
if a > 0 and b > 0 and c > 0:
print("都大于0")
if a > 0 and b > 0 and c > 0:
print("都大于0")
if min(a, b, c) > 0:
print("都大于0")
|
五、综合示例
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
|
"""
链式比较综合示例
"""
def check_triangle(a, b, c):
"""判断是否能构成三角形"""
if a > 0 and b > 0 and c > 0:
if a + b > c and a + c > b and b + c > a:
return "能构成三角形"
return "不能构成三角形"
def check_leap_year(year):
"""判断是否闰年"""
if year % 4 == 0 and (year % 100 != 0 or year % 400 == 0):
return "闰年"
return "平年"
def check_bmi_category(bmi):
"""根据BMI判断体重类别"""
if bmi < 18.5:
return "体重过轻"
elif 18.5 <= bmi < 24:
return "正常"
elif 24 <= bmi < 28:
return "超重"
else:
return "肥胖"
def validate_ip_octet(octet):
"""验证IP段是否合法（0-255）"""
if 0 <= octet <= 255:
return True
return False
def demo():
"""演示"""
print("=== 三角形检查 ===")
triangles = [(3, 4, 5), (1, 2, 3), (5, 5, 5)]
for a, b, c in triangles:
print(f"({a}, {b}, {c}): {check_triangle(a, b, c)}")
print("\n=== 闰年检查 ===")
years = [2000, 1900, 2024, 2100]
for year in years:
print(f"{year}: {check_leap_year(year)}")
print("\n=== BMI检查 ===")
bmis = [17, 22, 25, 30]
for bmi in bmis:
print(f"BMI={bmi}: {check_bmi_category(bmi)}")
print("\n=== IP段检查 ===")
octets = [0, 128, 255, 256]
for octet in octets:
print(f"{octet}: {validate_ip_octet(octet)}")
if __name__ == "__main__":
demo()
|
六、注意事项
1. 链式比较的顺序
1
2
3
4
5
6
7
|
x = 5
if 10 > x > 0:
print("正确")
|
2. 不要混淆逻辑运算符
1
2
3
4
5
6
7
8
9
10
11
|
x = 5
if 0 < x < 10:
print("正确")
|
3. 性能考虑
1
2
3
4
5
6
7
8
|
def check(n):
if n > 0 and n < 10 and 100 // n > 5:
return True
return False
|
