学习python或者其他有异常控制的编程语言， 大家很有可能说try except finally（try catch finally）的执行很简单，无非就是有异常的话执行except， finally无论是否有异常都会执行， 大致上原则是这样， 但是如果涉及到更加详细的复杂的路径，加上return 语句，就没有那么简单了。
1. 没有return 语句的情况
print 'this is a test of code path in try...except...else...finally'
print '************************************************************'
def exceptTest():
try:
print 'doing some work, and maybe exception will be raised'
raise IndexError('index error')
#print 'after exception raise'
#return 0
except KeyError, e:
print 'in KeyError except'
print e
#return 1
except IndexError, e:
print 'in IndexError except'
print e
#return 2
except ZeroDivisionError, e:
print 'in ZeroDivisionError'
print e
#return 3
else:
print 'no exception'
#return 4
finally:
print 'in finally'
#return 5
resultCode = exceptTest()
print resultCode有异常发生，并且捕获异常，最后在finally进行处理，上面代码的输出：
this is a test of code path in try...except...else...finally ************************************************************ doing some work, and maybe exception will be raised in IndexError except index error in finally None
2. 添加return 语句的情况
print 'this is a test of code path in try...except...else...finally'
print '************************************************************'
def exceptTest():
try:
print 'doing some work, and maybe exception will be raised'
raise IndexError('index error')
print 'after exception raise'
return 0
except KeyError, e:
print 'in KeyError except'
print e
return 1
except IndexError, e:
print 'in IndexError except'
print e
return 2
except ZeroDivisionError, e:
print 'in ZeroDivisionError'
print e
return 3
else:
print 'no exception'
return 4
finally:
print 'in finally'
return 5
resultCode = exceptTest()
print resultCodethis is a test of code path in try...except...else...finally ************************************************************ doing some work, and maybe exception will be raised in IndexError except index error in finally 5
然后，我们在看看把finally 的返回值注释掉，看看返回值是多少？
代码如下：
print 'this is a test of code path in try...except...else...finally'
print '************************************************************'
def exceptTest():
try:
print 'doing some work, and maybe exception will be raised'
raise IndexError('index error')
print 'after exception raise'
return 0
except KeyError, e:
print 'in KeyError except'
print e
return 1
except IndexError, e:
print 'in IndexError except'
print e
return 2
except ZeroDivisionError, e:
print 'in ZeroDivisionError'
print e
return 3
else:
print 'no exception'
return 4
finally:
print 'in finally'
#return 5
resultCode = exceptTest()
print resultCodethis is a test of code path in try...except...else...finally ************************************************************ doing some work, and maybe exception will be raised in IndexError except index error in finally 2
3. 没有异常发生且try语句块没有return
代码如下：
print 'this is a test of code path in try...except...else...finally'
print '************************************************************'
def exceptTest():
try:
print 'doing some work, and maybe exception will be raised'
#raise IndexError('index error')
print 'after exception raise'
#return 0
except KeyError, e:
print 'in KeyError except'
print e
return 1
except IndexError, e:
print 'in IndexError except'
print e
return 2
except ZeroDivisionError, e:
print 'in ZeroDivisionError'
print e
return 3
else:
print 'no exception'
return 4
finally:
print 'in finally'
return 5
resultCode = exceptTest()
print resultCodethis is a test of code path in try...except...else...finally ************************************************************ doing some work, and maybe exception will be raised after exception raise no exception in finally 5
但是，当try语句块里存在return语句是什么情况呢？
4. 没有异常发生且try语句块 存在return语句
print 'this is a test of code path in try...except...else...finally'
print '************************************************************'
def exceptTest():
try:
print 'doing some work, and maybe exception will be raised'
#raise IndexError('index error')
print 'after exception raise'
return 0
except KeyError, e:
print 'in KeyError except'
print e
return 1
except IndexError, e:
print 'in IndexError except'
print e
return 2
except ZeroDivisionError, e:
print 'in ZeroDivisionError'
print e
return 3
else:
print 'no exception'
return 4
finally:
print 'in finally'
return 5
resultCode = exceptTest()
print resultCodethis is a test of code path in try...except...else...finally ************************************************************ doing some work, and maybe exception will be raised after exception raise in finally 5
分析： 这里因为没有发生异常， 所以会执行到try块中的return 语句，但是finally又必须执行，所以执行try中return 之前去执行了finally语句，并且可以认为，finally语句修改了最后返回的值，将try中的返回值修改为5并最终返回，所以else语句并没有得到执行。
5. 有异常发生并且finally 没有return 语句
print 'this is a test of code path in try...except...else...finally'
print '************************************************************'
def exceptTest():
try:
print 'doing some work, and maybe exception will be raised'
raise IndexError('index error')
print 'after exception raise'
return 0
except KeyError, e:
print 'in KeyError except'
print e
return 1
except IndexError, e:
print 'in IndexError except'
print e
return 2
except ZeroDivisionError, e:
print 'in ZeroDivisionError'
print e
return 3
else:
print 'no exception'
return 4
finally:
print 'in finally'
#return 5
resultCode = exceptTest()
print resultCodethis is a test of code path in try...except...else...finally ************************************************************ doing some work, and maybe exception will be raised in IndexError except index error in finally 2
看到这里，我们貌似找到了一些规律
1. 如果没有异常发生， try中有return 语句， 这个时候else块中的代码是没有办法执行到的， 但是finally语句中如果有return 语句会修改最终的返回值， 我个人理解的是try中return 语句先将要返回的值放在某个 CPU寄存器，然后运行finally语句的时候修改了这个寄存器的值，最后在返回到try中的return语句返回修改后的值。
2. 如果没有异常发生， try中没有return语句，那么else块的代码是执行的，但是如果else中有return， 那么也要先执行finally的代码， 返回值的修改与上面一条一致。
3. 如果有异常发生，try中的return语句肯定是执行不到， 在捕获异常的 except语句中，如果存在return语句，那么也要先执行finally的代码，finally里面的代码会修改最终的返回值，然后在从except 块的retrun 语句返回最终修改的返回值， 和第一条一致。
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python try except else finally 执行顺序详细分析
原文地址：http://blog.csdn.net/watkinsong/article/details/27350553
