例外狀況

Python 物件導向程式設計入門

George Boorman

Curriculum Manager, DataCamp

a = 1
a / 0

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

a = [1,2,3]
a[5]

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    a[5]
IndexError: list index out of range

a = 1
a + "Hello"

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
    a + "Hello"
TypeError: unsupported operand type(s) for +: /
'int' and 'str'

a = 1
a + b

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    a + b
NameError: name 'b' is not defined

例外處理

當拋出例外時，避免程式終止
try - except - finally：

try:
    print(5 + "a")

except TypeError:
    print("You can't add an integer to a string, but you can multiply them!")

# 可有多個 except 區塊
except AnotherExceptionHere:
    # 若發生 AnotherExceptionHere，執行此程式碼

# 選用的 finally 區塊
finally:
    print(5 * "a")

例外處理輸出

You can't add an integer to a string, but you can multiply them!
aaaaa

拋出例外

def make_list_of_ones(length):
    if length <= 0:
        # 發生 ValueError 時的自訂訊息
        # 將停止程式並拋出錯誤
       raise ValueError("Invalid length!")
    return [1]*length


make_list_of_ones(-1)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    make_list_of_ones(-1)
  File "<stdin>", line 3, in make_list_of_ones
    raise ValueError("Invalid length!")
ValueError: Invalid length!

例外是類別

例外從 BaseException 或 Exception 繼承

BaseException
+-- Exception
    +-- ArithmeticError                     
    |    +-- FloatingPointError
    |    +-- OverflowError
    |    +-- ZeroDivisionError              
    +-- TypeError
    +-- ValueError
    |    +-- UnicodeError
    |         +-- UnicodeDecodeError
    |         +-- UnicodeEncodeError
    |         +-- UnicodeTranslateError
    +-- RuntimeError
   ...
+-- SystemExit
...

¹ https://docs.python.org/3/library/exceptions.html

自訂例外

從 Exception 或其子類別繼承
通常是空類別

class BalanceError(Exception): 
    pass


class Customer:
    def __init__(self, name, balance):
        if balance < 0 :
            raise BalanceError("Balance has to be non-negative!")
        else:
            self.name = name
            self.balance = balance

建構子中的例外

cust = Customer("Larry Torres", -100)

Traceback (most recent call last):
  File "script.py", line 11, in <module>
    cust = Customer("Larry Torres", -100)
  File "script.py", line 6, in __init__
    raise BalanceError("Balance has to be non-negative!")
BalanceError: Balance has to be non-negative!

例外會終止程式

例外中斷了建構子 → 物件未建立

cust

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    cust
NameError: name 'cust' is not defined

捕捉自訂例外

try:
    cust = Customer("Larry Torres", -100)
except BalanceError:
    cust = Customer("Larry Torres", 0)

一起來練習吧！

Python 物件導向程式設計入門