我理解这种结构是如何运作的:
for i in range(10):
print(i)
if i == 9:
print("Too big - I'm giving up!")
break
else:
print("Completed successfully")
但我不明白为什么在这里使用else作为关键字，因为它表明有问题的代码只在for块未完成时运行，这与它所做的相反!无论我怎么想，我的大脑都不能从for语句无缝地过渡到else语句块。对我来说，continue和continuouswith更有意义(我正试着训练自己这样读它)。
我想知道Python程序员是如何在头脑中(或者大声地，如果你喜欢)阅读这个结构的。也许我遗漏了一些可以使这些代码块更容易破译的东西?
这个问题是关于底层设计决策的，也就是说，为什么能够编写这些代码是有用的。另请参阅Python while语句中的Else子句，了解语法含义的具体问题。
当前回答
我只是想让自己重新理解一下。我发现下面的方法很有用!
•将else看作是与循环中的if配对(而不是与for配对)-如果条件满足，则打破循环，否则执行此操作-除非它是与多个if配对的else ! •如果所有的“如果”都不满足，那么就做“其他”。 •多个如果实际上也可以被认为是if-elifs!
其他回答
因为他们不想给这门语言引入一个新的关键字。每一个都窃取一个标识符并导致向后兼容问题，所以通常是最后的手段。
一个循环的else分支只执行一次，不管这个循环是否进入它的循环体，除非循环体已经进入但没有结束。也就是说，在循环中会遇到break或return语句。
my_list = []
for i in my_list:
print(i, end=',')
else:
print('loop did not enter')
##################################
for i in range(1,6,1):
print(i, end=',')
else:
print('loop completed successfully:', i)
##################################
for i in range(1,6,1):
if i == 3:
print('loop did not finish:', i)
break
print(i, end=',')
else:
print('else:', i)
输出:
loop did not enter
1,2,3,4,5,loop completed successfully: 5
1,2,loop did not finish: 3
while-else也是一样的。
import random
random.seed(8)
i = 100
while i < 90:
print(i, end=',')
i = random.randint(0,100)
else:
print('loop did not enter:', i)
##################################
i = 25
while i < 90:
print(i, end=',')
i = random.randint(0,100)
else:
print('loop completed successfully:', i)
##################################
i = 25
while i < 90:
if i % 10 == 0:
print('loop did not finish:', i)
break
print(i, end=',')
i = random.randint(0,100)
else:
print('else:', i)
输出:
loop did not enter: 100
25,29,47,48,16,24,loop completed successfully: 90
25,5,loop did not finish: 10
我只是想让自己重新理解一下。我发现下面的方法很有用!
•将else看作是与循环中的if配对(而不是与for配对)-如果条件满足，则打破循环，否则执行此操作-除非它是与多个if配对的else ! •如果所有的“如果”都不满足，那么就做“其他”。 •多个如果实际上也可以被认为是if-elifs!
我把它读成这样:“当可迭代对象完全耗尽时，在完成for语句后，执行即将继续执行下一个语句，else子句将被执行。”因此，当迭代被break打断时，这将不会执行。
else语句块中的代码将在for循环未被打破时执行。
for x in xrange(1,5):
if x == 5:
print 'find 5'
break
else:
print 'can not find 5!'
#can not find 5!
来自文档:break和continue语句，以及循环中的else子句
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. This is exemplified by the following loop, which searches for prime numbers: >>> for n in range(2, 10): ... for x in range(2, n): ... if n % x == 0: ... print(n, 'equals', x, '*', n//x) ... break ... else: ... # loop fell through without finding a factor ... 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 (Yes, this is the correct code. Look closely: the else clause belongs to the for loop, not the if statement.) When used with a loop, the else clause has more in common with the else clause of a try statement than it does that of if statements: a try statement’s else clause runs when no exception occurs, and a loop’s else clause runs when no break occurs. For more on the try statement and exceptions, see Handling Exceptions. The continue statement, also borrowed from C, continues with the next iteration of the loop: >>> for num in range(2, 10): ... if num % 2 == 0: ... print("Found an even number", num) ... continue ... print("Found a number", num) Found an even number 2 Found a number 3 Found an even number 4 Found a number 5 Found an even number 6 Found a number 7 Found an even number 8 Found a number 9
