while_else, for_else 以及try_exception_else_finally

Posted 2021-01-27 zmiao

     在python中的while, for和try是有else分支的. 关于while的else分支描述如下

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.

总结一句话就是当while或者for正常结束循环，那么else分支就会被执行.即当expression条件不再满足，

如果在执行循环过程中由于break, return等退出，那么else分支不被执行.

对于try... exception...else 当try分支执行后没有产生exception，那么执行else分支.如果有finally分支，则

无论有没有exception，最后都会执行finally分支.但是前提是finally得有except X/except处理了exception，finally

才会被执行. 例子如下: file_bname是一个bytes类型的文件名字, file_name是字符串形式的

（1）当没有出现except时，else和finally被执行
>>> try:
...     file_bname.decode(‘gb2312‘)
... except TypeError:
...     print(‘hello‘)
... else:
...     print(‘else‘)
... finally:
...     print(‘finally‘)
...
‘迈瑞医疗(300760)_利润表.xls‘
else
finally

（2）当有except的时候，else没有被执行, finally被执行
>>> file_name.decode(‘gb2312‘)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: ‘str‘ object has no attribute ‘decode‘
>>> try:
...     file_name.decode(‘gb2312‘)
... except AttributeError:
...     print(‘Error‘)
... else:
...     print(‘else‘)
... finally:
...     print(‘finally‘)
...
Error
finally

(3)else前面必须跟着except X或者except
>>> try:
...     file_name.decode(‘gb2312‘)
... else:
  File "<stdin>", line 3
    else:
       ^
SyntaxError: invalid syntax

(4) 如果except没有得到处理，即使有finally，也没有被执行
>>> try:
...     file_name.decode(‘gb2312‘)
... finally:
...     print(‘finally‘)
...
finally
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AttributeError: ‘str‘ object has no attribute ‘decode‘
>>>

 

 

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 elseclause’s suite. A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression.