異常

異常的繼承

基類：Exception
先處理子類異常

class B(Exception):
    pass
class C(B):
    pass
class D(C):
    pass

for cls in [D,C,B]:
    try:
        raise cls()
    except D:       #先處理子類異常
        print("D")
    except C:
        print("C")
    except B:
        print("B")

輸出：
D
C
B

一次處理多個異常

except (RuntimeError, TypeError, NameError):
    pass

引發異常

raise  NameError("hello")

輸出：
Traceback (most recent call last):
File “11.py”, line 68, in
raise NameError(“hello”)
NameError: hello

raise  NameError

Traceback (most recent call last):
File “11.py”, line 69, in
raise NameError
NameError

不處理異常，重新引發異常

try:
    raise NameError("hello")
except NameError:
    print("An exception occured")
    raise

輸出：
An exception occured
Traceback (most recent call last):
File “11.py”, line 71, in
raise NameError(“hello”)
NameError: hello

自定義異常

直接或間接繼承Exception類

finally

try:
    raise NameError
finally:      #有無異常，必定執行
    print('Goodbye')

完整示例

def divide(x, y):
    try:
        result = x / y
    except ZeroDivisionError:
        print("division by zero!")
    else:       #無異常執行
        print("result is", result)
    finally:    #有無異常，必定執行;定義 Clean-up
        print("executing finally clause")

# divide(2,1)
# divide(2,0)
# divide("hi",1)

divide(2,1)

輸出：
result is 2.0
executing finally clause

divide(2,0)

輸出：
division by zero!
executing finally clause

divide(“hi”,1)

輸出：
executing finally clause
Traceback (most recent call last):
File “11.py”, line 91, in
divide(“hi”,1)
File “11.py”, line 80, in divide
result = x / y
TypeError: unsupported operand type(s) for /: ‘str’ and ‘int’

其他實例

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(s.strip())
except OSError as err:    #文件不存在，捕捉此異常
    print(sys.exc_info())
    print(err)
    print("OS error: {0}".format(err))
except ValueError:
    print("Could not convert data to an integer.")
except:
    print("Unexpected error:", sys.exc_info()[0])   #打印異常類，如：<class 'FileNotFoundError'>
    raise

輸出：
(<class ‘FileNotFoundError’>, FileNotFoundError(2, ‘No such file or directory’), <traceback object at 0x000001EAD831D588>)
[Errno 2] No such file or directory: ‘myfile.txt’
OS error: [Errno 2] No such file or directory: ‘myfile.txt’

for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except OSError:
        print('cannot open', arg)
    else:   #放在else語句，防止非代碼錯誤被隱藏
        print(arg, 'has', len(f.readlines()), 'lines')
        f.close()

try:
    raise Exception('spam', 'eggs')
except Exception as inst:
    print(type(inst))    # the exception instance
    print(inst.args)     # arguments stored in .args
    print(inst)          # __str__ allows args to be printed directly,
    # but may be overridden in exception subclasses
    x, y = inst.args     # unpack args
    print('x =', x)
    print('y =', y)

輸出：
<class ‘Exception’>
(‘spam’, ‘eggs’)
(‘spam’, ‘eggs’)
x = spam
y = eggs