在執行程式時，可能會遇到一些問題導致程式執行失敗，這時候 python 就會丟出 exception，並且印出 stack trace，可以看是程式執行到哪邊發生錯誤．

>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

如果執行的是一個檔案，會說是檔案哪一行錯了．

> python3 calc.py
5
6
8
Traceback (most recent call last):
  File "calc.py", line 4, in <module>
    print(1/0)
ZeroDivisionError: division by zero

寫程式時通常會有兩種方式來處理程式異常，一種就是先把會出現錯誤的情況避免掉也稱作 LBYL (look before you leap)，另一種是就等發生了再來處理也稱作 EAFP (easier to ask for forgiveness than permission)．

使用 LBYL 的話可以將上面的程式改成，用條件控制先判斷 num2 不為 0 才除，

>>> def division(num1,num2):
...  if num2 != 0:
...   print(num1 / num2)
...  else:
...   print(0)
...
>>> division(1,0)
0
>>> division(1,2)
0.5

可是假設將 None 丟進去，又會出錯了．所以使用 LBYL 的方式，需要考慮到很多情況來避免出錯．

>>> division(1,None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in division
TypeError: unsupported operand type(s) for /: 'int' and 'NoneType'

使用 EAFP 的方式，將可能會出錯的地方放在 try 區塊裡，

>>> def division(num1,num2):
...  try:
...   print(num1 / num2)
...  except ZeroDivisionError as e:
...   print(0)
...
>>> division(1,0)
0
>>> division(1,2)
0.5
>>> division(1,None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in division
TypeError: unsupported operand type(s) for /: 'int' and 'NoneType'

由於上面只有捕捉 ZeroDivisionError 的錯誤，所以如果不屬於該類型的錯誤，程式還是會執行錯誤，所以可以在下面再多一個比 ZeroDivisionError 層級更高的錯誤，就可以捕捉到．

>>> def division(num1,num2):
...  try:
...   print(num1 / num2)
...  except ZeroDivisionError as e:
...   print(0)
...  except Exception as e:
...   print('wrong number')
...
>>> division(1,None)
wrong number

raise & finally

如果補到例外後，想用不同的例外丟給呼叫該 function 的使用者，可以使用 raise 這關鍵字，像下面的範例捕捉到了 TypeError 會再把 RuntimeError 也拋出來給使用者．而 finally 區塊是不管有沒有發生例外，最後都會被執行的區塊．

>>> def division(num1,num2):
...  try:
...   print(num1 / num2)
...  except ZeroDivisionError as e:
...   print(0)
...  except Exception as e:
...   raise RuntimeError('wrong number')
...  finally:
...   print('do something...')
...
>>> division(1,2)
0.5
do something...
>>> division(1,0)
0
do something...
>>> division(1,None)
do something...
Traceback (most recent call last):
  File "<stdin>", line 3, in division
TypeError: unsupported operand type(s) for /: 'int' and 'NoneType'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in division
RuntimeError: wrong number

自訂 exception

如果要定義自己的 exception，只需要將 class 繼承 Exception 即可．

>>> class CustomError(Exception):
...  pass
...

就可以使用了．

>>> def division(num1,num2):
...  try:
...   print(num1 / num2)
...  except ZeroDivisionError as e:
...   print(0)
...  except Exception as e:
...   raise CustomError('Custom wrong error')
...
>>> division(1,None)
Traceback (most recent call last):
  File "<stdin>", line 3, in division
TypeError: unsupported operand type(s) for /: 'int' and 'NoneType'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in division
__main__.CustomError: Custom wrong error