Python入門：異常和異常處理頭條網

2021-01-26 21:56:35 佚名

異常

程序執行過程中遇到問題無法繼續，這個問題就是異常。這些異常可能是程序員在編寫程序時的疏忽或者考慮不周所致，也可由程序員根據需要自定義、拋出異常。通過閱讀異常信息，我們可以改進程序，排除自己不想要的結果，也可提示信息，提高用戶體驗。

內置異常及層次結構：

<code>BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StopAsyncIteration
      +-- ArithmeticError
      |    +-- FloatingPointError
      |    +-- OverflowError
      |    +-- ZeroDivisionError
      +-- AssertionError
      +-- AttributeError
      +-- BufferError
      +-- EOFError
      +-- ImportError
      |    +-- ModuleNotFoundError
      +-- LookupError
      |    +-- IndexError
      |    +-- KeyError
      +-- MemoryError
      +-- NameError
      |    +-- UnboundLocalError
      +-- OSError
      |    +-- BlockingIOError
      |    +-- ChildProcessError
      |    +-- ConnectionError
      |    |    +-- BrokenPipeError
      |    |    +-- ConnectionAbortedError
      |    |    +-- ConnectionRefusedError
      |    |    +-- ConnectionResetError
      |    +-- FileExistsError
      |    +-- FileNotFoundError
      |    +-- InterruptedError
      |    +-- IsADirectoryError
      |    +-- NotADirectoryError
      |    +-- PermissionError
      |    +-- ProcessLookupError
      |    +-- TimeoutError
      +-- ReferenceError
      +-- RuntimeError
      |    +-- NotImplementedError
      |    +-- RecursionError
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning
           +-- ResourceWarning/<code>

源自：https://docs.python.org/3/library/exceptions.html#base-classes

異常處理

異常必須進行處理，否則程序無法繼續運行。

簡單來說，異常處理過程分3步：產生（拋出）異常-->檢測並捕獲異常-->處理異常

基本應用(try...except...)

<code># 檢測異常
try:
    pass
# 捕獲異常並處理異常
except:
    pass/<code>

例：

<code># 除數為0的錯誤
a = 1
b = 0
c = a / b/<code>

報錯：ZeroDivisionError: division by zero

<code># 發生異常時捕獲異常並提示
# 程序不終止執行
try:
    a = 1
    b = 0
    c = a / b
except:
    print('Some exception occurs!')/<code>

獲取異常信息(try...except Exception as e...)

<code>try:
    a = 1
    b = 0
    c = a / b
except Exception as e:
    print('Some exception occurs!-->', e)/<code>

沒有異常時執行某操作(try...except...else...)

<code>def op(a, b):
    r = a / b
    return r

c = None
try:
    a = 1
    b = 2
    c = op(a, b)
except Exception as e:
    print('Some exception occurs!-->', e)
else:
    print('Result is: ', c)/<code>

不管有沒有異常都要執行某操作(try...except..finally...)

<code>def op(a, b):
    r = a / b
    return r

c = None
try:
    a = 1
    b = 0
    c = op(a, b)
except Exception as e:
    print('Some exception occurs!-->', e)
else:
    print('Result is: ', c)
finally:
    print('Operation done.')/<code>

注意如果其中有return時的返回值情況：

<code>try:
    return 1
except:
    return 2
finally:
    return 3/<code>

結果：無論有沒有Exception都返回3

自定義觸發異常

使用raise語句自定義觸發異常

<code>def func(i):
    if i < 0:
        raise Exception('數量不能為負。')

try:
    func(-1)
except Exception as e:
    print(e)/<code>

自定義異常

通過繼承的方式創建自己的異常類，使用方式與內置異常類相同。

<code>class UserExcption(Exception):
    def __init__(self, arg):
        self.args = arg

try:
    raise UserExcption('User defined exception.')
except UserExcption as e:
    print(e)/<code>

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