python中 try、except、finally执行顺序

我们虽然经常用到try...except 作为异常补货，但是其实很少去研究try源码和机制，也许点进去看过，也是看不出个所以然来

class Exception(BaseException):
    """ Common base class for all non-exit exceptions. """
    def __init__(self, *args, **kwargs): # real signature unknown
        pass

继承了BaseException

class BaseException(object):
    """ Common base class for all exceptions """
    def with_traceback(self, tb): # real signature unknown; restored from __doc__
        """
        Exception.with_traceback(tb) --
            set self.__traceback__ to tb and return self.
        """
        pass

    def __delattr__(self, *args, **kwargs): # real signature unknown
        """ Implement delattr(self, name). """
        pass

    def __getattribute__(self, *args, **kwargs): # real signature unknown
        """ Return getattr(self, name). """
        pass

    def __init__(self, *args, **kwargs): # real signature unknown
        pass
..................

 是不是仍然一脸懵逼，同感，哈哈，那还是直接上代码，事实大于雄辩

代码举例转自https://blog.csdn.net/u010159842/article/details/54924940 我觉得这个作者写的很清楚

一：在 try 中 raise一个异常，就立刻转入 except 中执行，在except 中遇到 return 时，就强制转到 finally 中执行， 在 finally 中遇到 return 时就返回，最终函数返回值是finally 返回的

ef test1():
   try:
      print('to do stuff')
      raise Exception('hehe')
      print('to return in try')
      return 'try'
 except Exception:
      print('process except'）return 'except'
 finally:
      print('to return in finally')
      return 'finally'

test1Return = test1()
print('test1Return : ' + test1Return)
#输出：

#to do stuff
#process except
#to return in finally
#test1Return : finally

 

二：这里在 try 中没有抛出异常，因此不会转到 except 中，但是在try 中遇到return时，也会立即强制转到finally中执行，并在finally中返回

def test2():
   try:
      print('to do stuff')
      print('to return in try')
      return 'try'
 except Exception:
      print('process except')
      print('to return in except')
      return 'except'
 finally:
      print('to return in finally')
      return 'finally'

test2Return = test2()
print('test1Return : ' + test2Return)
#输出：
#to do stuff
#to return in try
#to return in finally
#test2Return : finally

 

test1和test2得到的结论：

无论是在try还是在except中，遇到return时，只要设定了finally语句，就会中断当前的return语句，跳转到finally中执行，如果finally中遇到return语句，就直接返回，不再跳转回try/excpet中被中断的return语句

三

def test3():
   i = 0
 try:
      i += 1
 print('i in try : %s'%i)
      raise Exception('hehe')
 except Exception:
      i += 1
 print('i in except : %s'%i)
      return i
      finally:
      i += 1
 print ('i in finally : %s'%i )

print('test3Return : %s'% test3())
#输出：

#i in try : 1
#i in except : 2
#i in finally : 3
#test3Return : 2

 

 

def test4():
   i = 0
 try:
      i += 1
 return i
   finally:
      i += 1
 print ('i in finally : %s'%i )
print('test4Return : %s' % test4())
#输出
#i in finally : 2
#test4Return : 1

 

test3和test4得到的结论：

在except和try中遇到return时，会锁定return的值，然后跳转到finally中，如果finally中没有return语句，则finally执行完毕之后仍返回原return点，将之前锁定的值返回(即finally中的动作不影响返回值)，如果finally中有return语句，则执行finally中的return语句。

 

四：test5得到的结论：在一个循环中，最终要跳出循环之前，会先转到finally执行，执行完毕之后才开始下一轮循环

def test5():
   for i in range(5):
      try:
         print('do stuff %s'%i)
         raise Exception(i)
      except Exception:
         print('exception %s'%i)
         continue
 finally:
         print('do finally %s'%i)
test5()
输出

do stuff 0
exception 0
do finally 0
do stuff 1
exception 1
do finally 1
do stuff 2
exception 2
do finally 2
do stuff 3
exception 3
do finally 3
do stuff 4
exception 4
do finally 4

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