2026/2/1大约 4 分钟
流程控制
Python 提供了多种流程控制语句，包括条件语句、循环语句和跳转语句。
条件语句
if-elif-else
# 基本语法
score = 85
if score >= 90:
grade = "A"
elif score >= 80:
grade = "B"
elif score >= 70:
grade = "C"
elif score >= 60:
grade = "D"
else:
grade = "F"
print(f"成绩等级: {grade}")
# 单行 if（三元运算符）
age = 20
status = "成年" if age >= 18 else "未成年"
# 多条件
x, y = 5, 10
if x > 0 and y > 0:
print("两者都为正数")
if x > 0 or y > 0:
print("至少有一个为正数")
if not (x < 0):
print("x 不是负数")嵌套条件
# 嵌套 if 语句
age = 25
has_ticket = True
if age >= 18:
if has_ticket:
print("可以入场")
else:
print("需要购票")
else:
print("未成年不能入场")
# 使用逻辑运算符简化
if age >= 18 and has_ticket:
print("可以入场")
elif age >= 18 and not has_ticket:
print("需要购票")
else:
print("未成年不能入场")match 语句（Python 3.10+）
# match-case 语句
status = 404
match status:
case 200:
message = "OK"
case 404:
message = "Not Found"
case 500:
message = "Server Error"
case _:
message = "Unknown Status"
print(message)
# 模式匹配
def describe_point(point):
match point:
case (0, 0):
return "原点"
case (0, y):
return f"Y轴上的点 ({y})"
case (x, 0):
return f"X轴上的点 ({x})"
case (x, y) if x == y:
return f"对角线上的点 ({x}, {y})"
case (x, y):
return f"普通点 ({x}, {y})"
print(describe_point((0, 0))) # 原点
print(describe_point((0, 5))) # Y轴上的点 (5)循环语句
for 循环
# 遍历序列
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)
# 遍历字典
person = {"name": "Alice", "age": 25}
for key in person:
print(f"{key}: {person[key]}")
# 遍历键值对
for key, value in person.items():
print(f"{key}: {value}")
# 使用 range()
for i in range(5):
print(i) # 0, 1, 2, 3, 4
for i in range(2, 10, 2):
print(i) # 2, 4, 6, 8
# 遍历字符串
for char in "Python":
print(char)
# 使用 enumerate() 获取索引
for index, fruit in enumerate(fruits):
print(f"{index}: {fruit}")
# 使用 zip() 同时遍历多个序列
names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
for name, age in zip(names, ages):
print(f"{name} is {age} years old")while 循环
# 基本 while 循环
count = 0
while count < 5:
print(count)
count += 1
# 无限循环（需要 break）
while True:
response = input("继续吗? (y/n): ")
if response.lower() == 'n':
break
# while-else
count = 0
while count < 5:
print(count)
count += 1
else:
print("循环正常结束")循环控制
# break：跳出循环
for i in range(10):
if i == 5:
break
print(i) # 只打印 0-4
# continue：跳过本次迭代
for i in range(5):
if i == 2:
continue
print(i) # 打印 0, 1, 3, 4
# 循环中的 else
for i in range(5):
if i == 10:
break
else:
print("没有找到 10") # 会执行
for i in range(5):
if i == 3:
break
else:
print("不会执行") # 不会执行列表推导式
# 基本列表推导式
squares = [x**2 for x in range(10)]
evens = [x for x in range(20) if x % 2 == 0]
# 嵌套列表推导式
matrix = [[i*j for j in range(3)] for i in range(3)]
# [[0, 0, 0], [0, 1, 2], [0, 2, 4]]
# 条件表达式
result = [x if x % 2 == 0 else "odd" for x in range(5)]
# ['odd', 0, 'odd', 2, 'odd']
# 字典推导式
squares_dict = {x: x**2 for x in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
# 集合推导式
even_squares = {x**2 for x in range(10) if x % 2 == 0}
# {0, 4, 16, 36, 64}
# 生成器表达式
squares_gen = (x**2 for x in range(10))
print(sum(squares_gen)) # 285pass 语句
# pass 是空操作，用作占位符
# 空函数
def my_function():
pass
# 空类
class MyClass:
pass
# 条件占位
if condition:
pass # TODO: 待实现
else:
do_something()循环示例
# 九九乘法表
for i in range(1, 10):
for j in range(1, i + 1):
print(f"{j}x{i}={i*j}", end="\t")
print()
# 查找素数
def is_prime(n):
if n < 2:
return False
for i in range(2, int(n**0.5) + 1):
if n % i == 0:
return False
return True
primes = [x for x in range(2, 100) if is_prime(x)]
print(f"100以内的素数: {primes}")
# 斐波那契数列
def fibonacci(n):
a, b = 0, 1
result = []
for _ in range(n):
result.append(a)
a, b = b, a + b
return result
print(fibonacci(10)) # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]流程控制图
最佳实践
使用 for 循环而非 while
# ❌ 不推荐
i = 0
while i < len(items):
print(items[i])
i += 1
# ✅ 推荐
for item in items:
print(item)使用 enumerate 获取索引
# ❌ 不推荐
for i in range(len(items)):
print(i, items[i])
# ✅ 推荐
for i, item in enumerate(items):
print(i, item)使用 dict.items() 遍历字典
# ❌ 不推荐
for key in dict:
value = dict[key]
# ✅ 推荐
for key, value in dict.items():
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