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
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()
其他回答
如果while循环没有中断，则执行Else。
我有点喜欢用“跑步者”来比喻。
“else”就像越过终点线，与你是从赛道的起点还是终点出发无关。"else"只有当你在两者之间的某个地方中断时才不会执行。
runner_at = 0 # or 10 makes no difference, if unlucky_sector is not 0-10
unlucky_sector = 6
while runner_at < 10:
print("Runner at: ", runner_at)
if runner_at == unlucky_sector:
print("Runner fell and broke his foot. Will not reach finish.")
break
runner_at += 1
else:
print("Runner has finished the race!") # Not executed if runner broke his foot.
主要的用例是使用这种打破嵌套循环，或者如果你想只在循环没有在某处中断时运行某些语句(认为中断是一种不寻常的情况)。
例如，下面是一个关于如何在不使用变量或try/catch的情况下跳出内部循环的机制:
for i in [1,2,3]:
for j in ['a', 'unlucky', 'c']:
print(i, j)
if j == 'unlucky':
break
else:
continue # Only executed if inner loop didn't break.
break # This is only reached if inner loop 'breaked' out since continue didn't run.
print("Finished")
# 1 a
# 1 b
# Finished
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()
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!
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。
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.
