# [Python] 例外執行順序
###### tags: `python`
:::info
* break、return、raise都會使程序跳出try/except/finally對應的區塊
* finally的區塊總是在整個try/except之後執行
* return的結果可以被覆蓋
* 函數的返回值在finally之後才"返回"
* Exception是BaseException的子類別，可以捕捉除了系統例外以外的所有例外，也就可以藉由KeyboardInterrupt中斷迴圈。
:::
try finally break return raise

def test():
    try:
        int("TEST")
        return "try"
    except Exception as error:
        print("Before raise")
        return "except"
        raise ValueError from error
        print("After raise")
    finally:
        print("Finally")
        return "finally"
    
print("return: ", test())

===output===
Before raise
Finally
return:  finally

finally即使是在return的情況下依然會執行，甚至可以覆蓋return的結果!

def test():
    try:
        int("TEST")
        return "try"
    except Exception as error:
        print("Before raise")        
        raise ValueError from error
        print("After raise")
        return "except"
    finally:
        print("Finally")
        return "finally"
    
print("return: ", test())

===output===
Before raise
Finally
return:  finally

不管使用raise或return都會立即跳出該區塊! 而raise拋出的順序會在finally執行之後，但是遇到return就不執行。

註解 # return "finally"
===output===
Before raise
Finally
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-21-34cfccebe447> in test()
      2     try:
----> 3         int("TEST")
      4         return "try"

ValueError: invalid literal for int() with base 10: 'TEST'

The above exception was the direct cause of the following exception:

ValueError                                Traceback (most recent call last)
<ipython-input-21-34cfccebe447> in <module>()
     12 #         return "finally"
     13 
---> 14 print("return: ", test())

<ipython-input-21-34cfccebe447> in test()
      5     except Exception as error:
      6         print("Before raise")
----> 7         raise ValueError from error
      8         print("After raise")
      9         return "except"

ValueError: 

i = 0
while True:
    try:
        if i == 0:
            print("break")
            break
    except Exception:
        print("except")
    finally:
        print("finally")
        
===output===
break
finally

def test_return(i):
    while True:
        try:
            if i == 0:
                print("return")
                return i
        except Exception:
            print("except")
        finally:
            print("finally")
    
print("return: ", test_return(0))

===output===
return
finally
return:  0

import sys

try:
    raise EOFError
except:
    type_, message, traceback = sys.exc_info()
    print(type_)
    print(message)
    print(traceback)
    
===output===
<class 'EOFError'>

<traceback object at 0x000001E526C17048>

try:
    try:
        raise EOFError('EOFError')
    except EOFError as e:
        print(e.args)
        raise IndexError('IndexError') from e
except IndexError as e:
    print(e.args)
    print(e.__cause__.args)
    print(e.__context__.args)
        
===output===
('EOFError',)
('IndexError',)
('EOFError',)
('EOFError',)

raise from語法，會將from後接上的例外實例，設定給被raise的例外實例之__cause__。
實際上，如果你在except中raise某個例外，則原except所比對到的例外，無論有無使用raise from，都會自動設定給__context__。
## 參考
[實作執行情況(1)](https://www.jianshu.com/p/343fe0942b80)
[實作執行情況(2)](https://openhome.cc/Gossip/Python/TryRaise.html)