Python中for循環搭配else的陷阱

假設有如下代碼：

for i in range(10):
    if i == 5:
        print 'found it! i = %s' % i
else:
    print 'not found it ...'

你期望的結果是，當找到5時打印出：

found it! i = 5

實際上打印出來的結果為：

found it! i = 5
not found it ...

顯然這不是我們期望的結果。

根據官方文檔說法：

>When the items are exhausted (which is immediately when the sequence is empty), the suite in 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 continues with the next item, or with the else clause if there was no next item.

https://docs.python.org/2/reference/compound_stmts.html#the-for-statement

大意是說當迭代的對象迭代完並為空時，位於else的子句將執行，而如果在for循環中含有break時則直接終止循環，並不會執行else子句。

所以正確的寫法應該為：

for i in range(10):
    if i == 5:
        print 'found it! i = %s' % i
        break
else:
    print 'not found it ...'

當使用pylint檢測代碼時會提示 Else clause on loop without a break statement (useless-else-on-loop)

所以養成使用pylint檢測代碼的習慣還是很有必要的，像這種邏輯錯誤不注意點還是很難發現的。

唔~