我注意到下面的代码在Python中是合法的。我的问题是为什么?有什么特别的原因吗?
n = 5
while n != 0:
print n
n -= 1
else:
print "what the..."
许多初学者在尝试在while或for循环中放入if/else块时无意中发现了这种语法，并且没有正确地缩进else。解决方案是确保else块与if块对齐，假设您打算对它们进行配对。这个问题解释了为什么它没有导致语法错误，以及产生的代码意味着什么。我还得到了一个IndentationError。我该怎么解决呢?，用于报告语法错误的情况。
当前回答
当while-condition的值为false时，else子句将被执行。
从文档中可以看到:
The while statement is used for repeated execution as long as an expression is true: while_stmt ::= "while" expression ":" suite ["else" ":" suite] This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the else clause, if present, is executed and the loop terminates. A break statement executed in the first suite terminates the loop without executing the else clause’s suite. A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression.
其他回答
假设要在单链表中搜索元素x
def search(self, x):
position = 1
p =self.start
while p is not None:
if p.info == x:
print(x, " is at position ", position)
return True
position += 1
p = p.link
else:
print(x, "not found in list")
return False
因此，如果条件失败，其他将执行，希望它有助于!
else子句只在while条件变为false时执行。如果跳出循环，或者引发异常，则不会执行该循环。
一种考虑它的方法是作为一个关于条件的if/else结构:
if condition:
handle_true()
else:
handle_false()
类似于循环结构:
while condition:
handle_true()
else:
# condition is false now, handle and go on with the rest of the program
handle_false()
一个例子可能是:
while value < threshold:
if not process_acceptable_value(value):
# something went wrong, exit the loop; don't pass go, don't collect 200
break
value = update(value)
else:
# value >= threshold; pass go, collect 200
handle_threshold_reached()
thing = 'hay'
while thing:
if thing == 'needle':
print('I found it!!') # wrap up for break
break
thing = haystack.next()
else:
print('I did not find it.') # wrap up for no-break
可能不幸命名为else-子句的地方是您从循环耗尽中不间断地结束的地方。
你可以没有它，如果
你中断与return或raise→调用或尝试后的整个代码是你的不可中断的地方 你在while之前设置了默认值(例如found = False) 但它可能会隐藏else子句知道要避免的错误
如果您使用带有非平凡结束符的multi-break，那么您应该在break之前使用一个简单的赋值，对no-break使用一个else子句赋值，并使用If -elif-else或match-case来避免重复非平凡的break处理代码。
注:同样适用于thing in haystack:
当while-condition的值为false时，else子句将被执行。
从文档中可以看到:
The while statement is used for repeated execution as long as an expression is true: while_stmt ::= "while" expression ":" suite ["else" ":" suite] This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the else clause, if present, is executed and the loop terminates. A break statement executed in the first suite terminates the loop without executing the else clause’s suite. A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression.
我知道这是个老问题，但是…
正如Raymond Hettinger所说，应该调用while/no_break而不是while/else。 如果你看一下这个片段，我发现它很容易理解。
n = 5
while n > 0:
print n
n -= 1
if n == 2:
break
if n == 0:
print n
现在，我们不用在while循环后检查condition，而是用else替换它，然后去掉那个检查。
n = 5
while n > 0:
print n
n -= 1
if n == 2:
break
else: # read it as "no_break"
print n
我总是把它读成while/no_break来理解代码，这种语法对我来说更有意义。
