Python Tip 1:Python循环语句中的else语法(for else, while else)

在Python的学习过程中,for...else..., while...else...语法可能是比较令人困惑的一个Python知识点。先来看一段Python官方的说明:

"Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the list (with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement."

Python循环语句(for, while)有可能带一个else分支,当一个for循环正常执行完毕时或者当一个while循环正常执行完毕(循环条件变为false)时它被触发执行,但是如果这个循环被break语句非正常中止时,则这个else分支不执行。让我们来看几个例子:

1)for循环正常执行:

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>>> for i in range(7):
...     print(i)
... else:
...     print("No abnormal interruption")
... 
0
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6
No abnormal interruption

2) for循环“非正常”break中断:

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>>> for i in range(7):
...     print(i)
...     if i == 3:
...         break
... else:
...     print("No abnormal interruption")
... 
0
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3

3) while循环正常执行:

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>>> i = 0
>>> while i < 7:
...     print(i)
...     i += 1
... else:
...     print("No abnormal interruption")
... 
0
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No abnormal interruption

4) while循环“非正常”break中断:

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>>> i = 0
>>> while i < 7:
...     print(i)
...     i += 1
...     if i == 3:
...         break
... else:
...     print("No abnormal interruption")
... 
0
1
2

总结起来就是:当循环中没有break时,else被执行,当循环中有break并且被触发时,else不被执行,类似这个case,即使循环语句中有break,但是如果没有触发break正常执行完毕,也会执行else:

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>>> for i in range(7):
...     print(i)
...     if i > 7:
...         break
... else:
...     print("No abnormal interruption")
... 
0
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No abnormal interruption

那么,这个有点绕口的Python语法糖有什么好处呢?让我们再看一个Case,这个Case参考自Python官方的一个寻找素数的例子:

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>>> for n in range(2, 20):
...     for x in range(2, n):
...         if n % x == 0:
...            print(n, 'equals', x, '*', n//x)
...            break
...     else:
...         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
10 equals 2 * 5
11 is a prime number
12 equals 2 * 6
13 is a prime number
14 equals 2 * 7
15 equals 3 * 5
16 equals 2 * 8
17 is a prime number
18 equals 2 * 9
19 is a prime number

想一想,如果没有else语句,如何实现?我是这样实现的,加一个flag标志来判断:

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>>> for n in range(2, 20):
...     flag = 0
...     for x in range(2, n):
...         if n % x == 0:
...             print(n, 'equals', x, '*', n//x)
...             flag = 1
...             break
...     if flag == 0:
...         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
10 equals 2 * 5
11 is a prime number
12 equals 2 * 6
13 is a prime number
14 equals 2 * 7
15 equals 3 * 5
16 equals 2 * 8
17 is a prime number
18 equals 2 * 9
19 is a prime number

这样看来,是不是觉得前者更 pythonic 一些?

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