異常

Python使用異常物件來表示異常狀態，並在遇到錯誤時引發異常。異常物件未被處理（或捕獲）時，程式將終止並顯示一條錯誤訊息。

異常捕獲示例：

>>> try:
...    x=int(input('Enter the first number:'))
...    y=int(input('Enter the second number:'))
...    print(x/y)
... except ZeroDivisionError:
...    print("The second number can't be zero!")
...
Enter the first number:1
Enter the second number:0
The second number can't be zero!
 

捕獲異常後，如果要重新引發它（即繼續向上傳播），可呼叫raise且不提供任何引數（也可以顯式地提供捕獲的異常）
發生除零行為時，如果啟用了“抑制”功能，方法calc將（隱式地）返回None。換而言之，如果啟用了“抑制”功能，就不應依賴返回值。

>>> class MuffledCalculator:
...    muffled=False
...    def calc(self,expr):
...       try:
...          return eval(expr)
...       except ZeroDivisionError:
...          if self.muffled:
...             print('Division by zero is illegal')
...          else:
...              raise
...
>>> calculator=MuffledCalculator()
>>> calculator.calc('10/2')
5.0
>>> calculator.calc('10/0')     #關閉了抑制功能
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in calc
  File "<string>", line 1, in <module>
ZeroDivisionError: division by zero
>>> calculator.muffled=True
>>> calculator.calc('10/0')
Division by zero is illegal

可以給try/except語句新增一個else子句，當代碼段沒有引發異常時，才會跳出迴圈。

>>> while True:
...    try:
...       x=int(input('Enter the first number:'))
...       y=int(input('Enter the second number:'))
...       value=x/y
...       print('x/y is',value)
...    except:
...       print('Invalid input.Please try again.')
...    else:
...       break
...
Enter the first number:1
Enter the second number:0
Invalid input.Please try again.
Enter the first number:'foo'
Invalid input.Please try again.
Enter the first number:baz
Invalid input.Please try again.
Enter the first number:10
Enter the second number:2
x/y is 5.0

當try/except中用到finally是為了發生異常時執行清理工作。不管try子句中發生什麼異常，都將執行finally子句。

>>> x=None
>>> try:
...     x=1/0
... finally:
...     print('Cleaning up...')
...     del x
...
Cleaning up...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
ZeroDivisionError: division by zero

如果只想發出警告，可使用模組warning中的函式warn
如果其他程式碼在使用你的模組，可使用模組warning時 中的函式filterwarnings來抑制你發出的警告

>>> from warnings import filterwarnings
>>> filterwarnings("error")
>>> warn("This function is really old...",DeprecationWarning)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
DeprecationWarning: This function is really old...
>>> filterwarnings("ignore",category=DeprecationWarning)
>>> warn("Another deprecation warning.",DeprecationWarning)
>>> warn("Something else.")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UserWarning: Something else.