Python – Try-except , else & finally statement

測試環境為 CentOS 8 x86_64 (虛擬機)

Try-except statement 格式如下,通常會搭配 raise exception – https://benjr.tw/104314 來使用.

try:
  # 執行以下區塊的程式碼.
except 例外名稱:
  # 如 Try 區塊內的程式碼無法正確執行時則執行 Except 內容(可以指定 Except 錯誤名稱).
except:
  # 如 Try 區塊內的程式碼無法正確執行時則執行 Except 內容.
else:
  # 如 Try 區塊內的程式碼並未發生錯誤時執行 ELSE 區塊內的程式碼.
finally:
  # 不管 Try 區塊內的程式碼是否可以正確執行都要執行 Finally 區塊內的程式碼.

下面範例輸入兩整數,並透過 Try-except 來避免錯誤發生.

[root@localhost ~]# vi try.py
try:
    a=int(input("Input a number:" ))
    b=int(input("Input a number:" ))
    c=a/b
except ZeroDivisionError:
    print('Division by zero')
except Exception as e:
    print(e)
else:
    print(a , "/" , b ,"=" ,c)
finally:
    print("Complete")

• except ZeroDivisionError:
  因為有發生 ZeroDivisionError 所以執行 except ZeroDivisionError 區塊.

  [root@localhost ~]# python3 try.py
  Input a number:1
  Input a number:0
  Division by zero
  Complete
  

  關於 exceptions 例外名稱請參考官方網站 – https://docs.python.org/3/library/exceptions.html , 下面是所有的 exceptions 的階層.

  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
             +-- EncodingWarning
             +-- ResourceWarning
  

• except Exception as e:
  發生非 ZeroDivisionError 錯誤所以執行 except Exception as e 區塊.這邊會建立 名稱為 e 物件 (Object) 的 Exception 類別 (class) .

  [root@localhost ~]# python3 try.py
  Input a number:1
  Input a number:y
  invalid literal for int() with base 10: 'y'
  Complete
  

  可以透過 e 物件 (Object) 得知更多的錯誤訊息.

  print(f"{type(e).__name__} at line {e.__traceback__.tb_lineno} of {__file__}: {e}")
  

  [root@localhost ~]# vi try.py
  try:
      a=int(input("Input a number:" ))
      b=int(input("Input a number:" ))
      c=a/b
  except ZeroDivisionError:
      print('Division by zero')
  except Exception as e:
      print(e)
      print(f"{type(e).__name__} at line {e.__traceback__.tb_lineno} of {__file__}: {e}")
  else:
      print(a , "/" , b ,"=" ,c)
  finally:
      print("Complete")
  

  執行結果.

  [root@localhost ~]# python3 try.py
  Input a number:1
  Input a number:w
  invalid literal for int() with base 10: 'w'
  ValueError at line 3 of try.py: invalid literal for int() with base 10: 'w'
  Complete
  

• else:
  如錯誤發生會執行 else 區塊.

  [root@localhost ~]# python3 try.py
  Input a number:1
  Input a number:2
  1 / 2 = 0.5
  Complete
  

• finally:
  不管前面是否出現錯誤,最後都會執行這一區塊,也就是列印出字串 Complete .

  [root@localhost ~]# python3 try.py
  Input a number:1
  Input a number:2
  1 / 2 = 0.5
  Complete