Exception(異常）

當遇到異常的條件時，Python使用exception物件來描述。當遭遇一個錯誤的時候，程式就丟擲一個異常，如果沒有來處理此類異常的策略的話，程式將會以一個traceback的方式終止，程式碼如下：

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

每個異常都是一個類的例項（上例中是ZeroDivisionError），有許多方法來丟擲和捕捉異常，最好要處理這些異常，而不是讓程式崩潰。

丟擲異常

>>> raise Exception 
Traceback (most recent call last): 
File "<stdin>", line 1, in ? 
Exception 
>>> raise Exception('hyperdrive overload') 
Traceback (most recent call last): 
File "<stdin>", line 1, in ? 
Exception: hyperdrive overload

列出所有的Python自帶的異常型別

>>> import exceptions 
>>> dir(exceptions) 
['ArithmeticError', 'AssertionError', 'AttributeError', ...]

這些異常都可以用raise語句丟擲

>>> raise ArithmeticError 
Traceback (most recent call last): 
File "<stdin>", line 1, in ? 
ArithmeticError

定義自己的異常類

class SomeCustomException(Exception): pass

捕獲異常

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

在函式中丟擲的異常，可以傳遞到呼叫它的地方之外，如果函式不能捕獲異常，那麼這個異常則會像冒泡一樣，一直傳遞到程式的頂端，這意味著，try/catch語句可以處理其他函式的異常。

如果一個異常，無法處理，可以利用raise語句繼續將它丟擲交給其他的地方來捕獲。如下程式碼：

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

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

try: 
    x = input('Enter the first number: ') 
    y = input('Enter the second number: ') 
    print x/y 
except ZeroDivisionError: 
    print "The second number can't be zero!" 
except TypeError: 
    print "That wasn't a number, was it?"

try: 
    x = input('Enter the first number: ') 
    y = input('Enter the second number: ') 
    print x/y 
except (ZeroDivisionError, TypeError, NameError): 
    print 'Your numbers were bogus...'

try: 
    x = input('Enter the first number: ') 
    y = input('Enter the second number: ') 
    print x/y 
except (ZeroDivisionError, TypeError), e: 
    print e

except  (ZeroDivisionError, TypeError) as e.

ry: 
    x = input('Enter the first number: ') 
    y = input('Enter the second number: ') 
    print x/y 
except: 
    print 'Something wrong happened...'

except Exception, e 

然後對e進行某些檢測。

某些情況下，如下程式碼可以讓程式一直正確執行，直到碰到異常

try: 
    print 'A simple task' 
except: 
    print 'What? Something went wrong?' 
else: 
    print 'Ah... It went as planned.'

while True: 
    try: 
        x = input('Enter the first number: ') 
        y = input('Enter the second number: ') 
        value = x/y 
        print 'x/y is', value 
    except Exception, e: 
        print 'Invalid input:', e 
        print 'Please try again' 
    else: 
        break

x = None 
try: 
     x = 1/0 
finally: 
    print 'Cleaning up...' 
    del x

finally語句中的東西在程式崩潰前執行

你也可以將這四個東西放在同一個語句快中

try: 
    1/0 
except NameError: 
    print "Unknown variable" 
else: 
    print "That went well!" 
finally: 
    print "Cleaning up."