我是这样读的:

如果还在运行循环的条件下，做点别的，否则做点别的。

这里的else关键字可能会让人困惑，正如许多人指出的那样，像nobreak, notbreak这样的关键字更合适。

为了理解……其他的……从逻辑上讲，将它与try进行比较……Else，不是if…Else…，大多数python程序员都熟悉以下代码:

try:
    do_something()
except:
    print("Error happened.") # The try block threw an exception
else:
    print("Everything is find.") # The try block does things just find.

类似地，可以将break视为一种特殊的异常:

for x in iterable:
    do_something(x)
except break:
    pass # Implied by Python's loop semantics
else:
    print('no break encountered')  # No break statement was encountered

不同之处在于python暗示除了break，你不能把它写出来，所以它变成:

for x in iterable:
    do_something(x)
else:
    print('no break encountered')  # No break statement was encountered

是的，我知道这种比较可能会很困难和令人厌倦，但它确实澄清了困惑。

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

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。

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

else子句在循环正常完成后执行。这意味着The:==> 只有当循环没有被break语句终止时，才会在for/while语句之后执行else块

for item in lista:
if(obj == item ):
    print("if True then break will run and else not run")
    break;
else:
print("in  else => obj not fount ")