4. 控制流#
4.1. if语句#
x = int(input("请输入一个数字:"))
---------------------------------------------------------------------------
StdinNotImplementedError Traceback (most recent call last)
Cell In[1], line 1
----> 1 x = int(input("请输入一个数字:"))
File ~/myenv/lib/python3.10/site-packages/ipykernel/kernelbase.py:1281, in Kernel.raw_input(self, prompt)
1279 if not self._allow_stdin:
1280 msg = "raw_input was called, but this frontend does not support input requests."
-> 1281 raise StdinNotImplementedError(msg)
1282 return self._input_request(
1283 str(prompt),
1284 self._parent_ident["shell"],
1285 self.get_parent("shell"),
1286 password=False,
1287 )
StdinNotImplementedError: raw_input was called, but this frontend does not support input requests.
if x< 0:
x = 0
print("小于零")
elif x == 0:
print("等于0")
else:
print("大于零")
大于零
4.2. for语句#
## 迭代列表
animals = ['cat', 'dog', 'lion']
for a in animals:
print(a, len(a))
cat 3
dog 3
lion 4
## 迭代字典
users = {'Hans': 'active', 'Éléonore': 'inactive', '景太郎': 'active'}
for user, status in users.items():
print(user, status)
Hans active
Éléonore inactive
景太郎 active
## 通过索引迭代列表
a = ['Mary', 'had', 'a', 'little', 'lamb']
for i in range(len(a)):
print(i, a[i])
0 Mary
1 had
2 a
3 little
4 lamb
4.3. break 和 continue 语句#
break 语句将跳出最近的一层 for 或 while 循环:
for n in range(2, 10):
for x in range(2, n):
if n % x == 0:
print(f"{n} equals {x} * {n//x}")
break
4 equals 2 * 2
6 equals 2 * 3
8 equals 2 * 4
9 equals 3 * 3
continue 语句将继续执行循环的下一次迭代:
for num in range(2, 10):
if num % 2 == 0:
print(f"Found an even number {num}")
continue
print(f"Found an odd number {num}")
Found an even number 2
Found an odd number 3
Found an even number 4
Found an odd number 5
Found an even number 6
Found an odd number 7
Found an even number 8
Found an odd number 9
4.4. 循环的 else 子句#
在 for 或 while 循环中 break 语句可能对应一个 else 子句。 如果循环在未执行 break 的情况下结束，else 子句将会执行。
在 for 循环中，else 子句会在循环结束其他最后一次迭代之后，即未执行 break 的情况下被执行。
在 while 循环中，它会在循环条件变为假值后执行。
for n in range(2, 10):
for x in range(2, n):
if n % x == 0:
print(n, 'equals', x, '*', n//x)
break
else:
# 循环到底未找到一个因数
print(n, 'is a prime number')
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3
4.5. pass语句#
pass 语句不执行任何动作。语法上需要一个语句，但程序毋需执行任何动作时，可以使用该语句。
# 创建一个最小的类
class MyEmptyClass:
pass
# pass 还可用作函数或条件语句体的占位符
def initlog(*args):
pass # 记得实现这个！
4.6. match语句#
match 语句接受一个表达式并把它的值与一个或多个 case 块给出的一系列模式进行比较。
def http_error(status):
match status:
case 400:
return "Bad request"
case 404:
return "Not found"
case 418:
return "I'm a teapot"
case 401 | 403 | 404: # 可以使用 | （“ or ”）在一个模式中组合几个字面值
return "Not allowed"
case _: # 变量名” _ 被作为 通配符 并必定会匹配成功
return "Something's wrong with the internet"
print(http_error(401))
Not allowed
形如解包赋值的模式可被用于绑定变量：
def point_fn(point):
# point 是一个 (x, y) 元组
match point:
case (0, 0):
print("Origin")
case (0, y):
print(f"Y={y}")
case (x, 0):
print(f"X={x}")
case (x, y):
print(f"X={x}, Y={y}")
case _:
raise ValueError("Not a point")
point_fn((0,0))
point_fn((0, 1))
point_fn((100, 0))
point_fn((1, 2))
Origin
Y=1
X=100
X=1, Y=2
将对象属性捕获到变量里面：
# 定义类
class Point:
def __init__(self, x, y):
self.x = x
self.y = y
def where_is(point):
match point:
case Point(x=0, y=0):
print("Origin")
case Point(x=0, y=y):
print(f"Y={y}")
case Point(x=x, y=0):
print(f"X={x}")
# case Point(x, y) if x == y: # 添加if字句作为约束项
# print(f"Y=X at {x}")
case Point():
print("Somewhere else")
case _:
print("Not a point")
where_is(Point(0,0))
where_is(Point(0, 1))
where_is(Point(1, 0))
where_is(Point(1, 1))
where_is(100)
Origin
Y=1
X=1
Somewhere else
Not a point
模式可以使用具名常量。它们必须作为带点号的名称出现，以防止它们被解释为用于捕获的变量:
from enum import Enum
class Color(Enum):
RED = 'red'
GREEN = 'green'
BLUE = 'blue'
color = Color(input("Enter your choice of 'red', 'blue' or 'green': "))
match color:
case Color.RED:
print("I see red!")
case Color.GREEN:
print("Grass is green")
case Color.BLUE:
print("I'm feeling the blues :(")
I see red!