流程控制
每种编程语言都会有自己的流程控制方式，也就是程序的执行流程，无非下面这几种： 1. 顺序执行 2. 执行分支 3. 循环执行
顺序执行，比较好理解，就是一行行的执行代码。
0x01. 条件语句
执行分支，Python提供了以下几种方式： 1. if...elif....elif....else 2. match语句（Python 3.10+）
if...elif...else
# 基本 if 语句
age = 18
if age >= 18:
print('你是成年人')
# if-else
age = 15
if age >= 18:
print('你是成年人')
else:
print('你是未成年人')
# if-elif-else
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'你的成绩等级是: {grade}') # B
# 嵌套 if
age = 25
has_license = True
if age >= 18:
if has_license:
print('你可以开车')
else:
print('你需要先考驾照')
else:
print('你还不能开车')
# 条件表达式（三元运算符）
age = 20
status = '成年' if age >= 18 else '未成年'
print(status) # 成年
# 多条件判断
x = 15
if x > 0 and x < 10:
print('x 在 0 到 10 之间')
elif x >= 10 and x < 20:
print('x 在 10 到 20 之间')
# 使用 in 判断
fruit = 'apple'
if fruit in ['apple', 'banana', 'orange']:
print(f'{fruit} 是常见水果')
# 真值测试
# 以下值在布尔上下文中为 False：False, None, 0, 0.0, '', [], {}, set()
items = []
if items:
print('列表不为空')
else:
print('列表为空')
# 链式比较
x = 15
if 10 < x < 20:
print('x 在 10 到 20 之间')
match 语句（Python 3.10+）
match 语句类似于其他语言的 switch 语句，但功能更强大。
# 基本 match 语句
status = 404
match status:
case 200:
print('OK')
case 404:
print('Not Found')
case 500:
print('Internal Server Error')
case _:
print('Unknown status')
# 匹配多个值
day = 'Monday'
match day:
case 'Monday' | 'Tuesday' | 'Wednesday' | 'Thursday' | 'Friday':
print('工作日')
case 'Saturday' | 'Sunday':
print('周末')
# 使用 if 守卫
point = (1, 2)
match point:
case (0, 0):
print('原点')
case (x, 0):
print(f'x 轴上的点: {x}')
case (0, y):
print(f'y 轴上的点: {y}')
case (x, y) if x == y:
print(f'对角线上的点: ({x}, {y})')
case (x, y):
print(f'普通点: ({x}, {y})')
# 匹配类实例
class Point:
def __init__(self, x, y):
self.x = x
self.y = y
def locate(point):
match point:
case Point(x=0, y=0):
print('原点')
case Point(x=x, y=0):
print(f'x 轴: {x}')
case Point(x=0, y=y):
print(f'y 轴: {y}')
case Point(x=x, y=y):
print(f'点: ({x}, {y})')
locate(Point(0, 0)) # 原点
locate(Point(5, 0)) # x 轴: 5
locate(Point(0, 3)) # y 轴: 3
locate(Point(2, 3)) # 点: (2, 3)
# 匹配字典
user = {'name': 'Alice', 'age': 25}
match user:
case {'name': str(name), 'age': int(age)} if age >= 18:
print(f'{name} 是成年人')
case {'name': str(name), 'age': int(age)}:
print(f'{name} 是未成年人')
case _:
print('无效用户数据')
0x02. 循环语句
Python 提供了两种循环语句： 1. while 循环 2. for 循环
while 循环
# 基本 while 循环
count = 0
while count < 5:
print(count)
count += 1
# 输出: 0 1 2 3 4
# 使用 break 跳出循环
n = 0
while True:
if n >= 5:
break
print(n)
n += 1
# 输出: 0 1 2 3 4
# 使用 continue 跳过本次循环
n = 0
while n < 10:
n += 1
if n % 2 == 0:
continue
print(n)
# 输出: 1 3 5 7 9
# while-else：循环正常结束时执行 else
count = 0
while count < 5:
print(count)
count += 1
else:
print('循环正常结束')
# 输出: 0 1 2 3 4 循环正常结束
# 如果循环被 break 中断，else 不会执行
count = 0
while count < 5:
if count == 3:
break
print(count)
count += 1
else:
print('这行不会执行')
# 输出: 0 1 2
for 循环
# 遍历列表
fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
print(fruit)
# 输出: apple banana cherry
# 遍历字典
person = {'name': 'Alice', 'age': 25, 'city': 'Beijing'}
for key in person:
print(f'{key}: {person[key]}')
# 使用 items() 方法
for key, value in person.items():
print(f'{key}: {value}')
# 遍历字符串
for char in 'Hello':
print(char)
# 输出: H e l l o
# 使用 range() 函数
for i in range(5):
print(i)
# 输出: 0 1 2 3 4
# range(start, stop, step)
for i in range(0, 10, 2):
print(i)
# 输出: 0 2 4 6 8
# 反向遍历
for i in range(5, 0, -1):
print(i)
# 输出: 5 4 3 2 1
# 使用 enumerate() 获取索引
fruits = ['apple', 'banana', 'cherry']
for index, fruit in enumerate(fruits):
print(f'{index}: {fruit}')
# 输出: 0: apple 1: banana 2: cherry
# 指定起始索引
for index, fruit in enumerate(fruits, start=1):
print(f'{index}: {fruit}')
# 输出: 1: apple 2: banana 3: cherry
# 并行遍历多个序列
names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]
for name, age in zip(names, ages):
print(f'{name} is {age} years old')
# for-else
for i in range(5):
if i == 10: # 不会发生
break
print(i)
else:
print('循环正常结束')
# 输出: 0 1 2 3 4 循环正常结束
0x03. 循环控制语句
break
# break 立即终止循环
for i in range(10):
if i == 5:
break
print(i)
# 输出: 0 1 2 3 4
# 在嵌套循环中，break 只跳出最内层循环
for i in range(3):
for j in range(3):
if j == 1:
break
print(f'({i}, {j})')
# 输出: (0, 0) (1, 0) (2, 0)
continue
# continue 跳过本次循环的剩余语句
for i in range(10):
if i % 2 == 0:
continue
print(i)
# 输出: 1 3 5 7 9
pass
# pass 是空语句，用于占位
for i in range(5):
pass # TODO: 实现循环体
# 在定义空函数或类时使用
def function_to_implement():
pass
class EmptyClass:
pass
0x04. 列表推导式
列表推导式提供了一种简洁的创建列表的方式。
# 基本语法：[expression for item in iterable]
squares = [x**2 for x in range(10)]
print(squares) # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
# 带条件的推导式
even_squares = [x**2 for x in range(10) if x % 2 == 0]
print(even_squares) # [0, 4, 16, 36, 64]
# 嵌套推导式
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [num for row in matrix for num in row]
print(flat) # [1, 2, 3, 4, 5, 6, 7, 8, 9]
# 带条件的嵌套推导式
even_flat = [num for row in matrix for num in row if num % 2 == 0]
print(even_flat) # [2, 4, 6, 8]
# 多变量推导式
pairs = [(x, y) for x in range(3) for y in range(3)]
print(pairs) # [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]
# 条件表达式
labels = ['even' if x % 2 == 0 else 'odd' for x in range(5)]
print(labels) # ['even', 'odd', 'even', 'odd', 'even']
字典推导式
# 基本语法：{key_expr: value_expr for item in iterable}
squares = {x: x**2 for x in range(5)}
print(squares) # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
# 带条件的推导式
even_squares = {x: x**2 for x in range(10) if x % 2 == 0}
print(even_squares) # {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}
# 从两个列表创建字典
keys = ['a', 'b', 'c']
values = [1, 2, 3]
d = {k: v for k, v in zip(keys, values)}
print(d) # {'a': 1, 'b': 2, 'c': 3}
集合推导式
# 基本语法：{expression for item in iterable}
squares = {x**2 for x in range(10)}
print(squares) # {0, 1, 4, 9, 16, 25, 36, 49, 64, 81}
# 带条件
even_squares = {x**2 for x in range(10) if x % 2 == 0}
print(even_squares) # {0, 4, 16, 36, 64}
0x05. 常搭配的函数
range()
# range(stop)
print(list(range(5))) # [0, 1, 2, 3, 4]
# range(start, stop)
print(list(range(2, 7))) # [2, 3, 4, 5, 6]
# range(start, stop, step)
print(list(range(0, 10, 2))) # [0, 2, 4, 6, 8]
print(list(range(10, 0, -2))) # [10, 8, 6, 4, 2]
len()
# 获取长度
print(len([1, 2, 3])) # 3
print(len('hello')) # 5
print(len({'a': 1, 'b': 2})) # 2
enumerate()
# 获取索引和值
fruits = ['apple', 'banana', 'cherry']
for i, fruit in enumerate(fruits):
print(f'{i}: {fruit}')
# 指定起始索引
for i, fruit in enumerate(fruits, 1):
print(f'{i}: {fruit}')
zip()
# 并行遍历多个序列
names = ['Alice', 'Bob', 'Charlie']
ages = [25, 30, 35]
for name, age in zip(names, ages):
print(f'{name}: {age}')
# 不等长序列
a = [1, 2, 3]
b = ['a', 'b']
print(list(zip(a, b))) # [(1, 'a'), (2, 'b')]
# 使用 zip_longest 处理不等长
from itertools import zip_longest
print(list(zip_longest(a, b, fillvalue='?'))) # [(1, 'a'), (2, 'b'), (3, '?')]
map()
# 对序列中的每个元素应用函数
numbers = [1, 2, 3, 4, 5]
squares = list(map(lambda x: x**2, numbers))
print(squares) # [1, 4, 9, 16, 25]
# 多个序列
a = [1, 2, 3]
b = [4, 5, 6]
sums = list(map(lambda x, y: x + y, a, b))
print(sums) # [5, 7, 9]
filter()
# 过滤序列
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = list(filter(lambda x: x % 2 == 0, numbers))
print(evens) # [2, 4, 6, 8, 10]
# 使用 None 过滤假值
values = [0, 1, False, True, '', 'hello', None]
truthy = list(filter(None, values))
print(truthy) # [1, True, 'hello']
sorted()
# 排序
numbers = [3, 1, 4, 1, 5, 9, 2, 6]
print(sorted(numbers)) # [1, 1, 2, 3, 4, 5, 6, 9]
# 反向排序
print(sorted(numbers, reverse=True)) # [9, 6, 5, 4, 3, 2, 1, 1]
# 自定义排序
words = ['banana', 'Apple', 'cherry']
print(sorted(words)) # ['Apple', 'banana', 'cherry'] (按 ASCII)
print(sorted(words, key=str.lower)) # ['Apple', 'banana', 'cherry']
# 按字典值排序
students = [
{'name': 'Alice', 'grade': 88},
{'name': 'Bob', 'grade': 95},
{'name': 'Charlie', 'grade': 82}
]
sorted_students = sorted(students, key=lambda x: x['grade'], reverse=True)
print(sorted_students)
参考
目录