try/except 錯誤與異常

程式在運行時總是會發生一些錯誤，在python中的錯誤大致分為兩類：語法錯誤和異常(例外)

語法錯誤

語法錯誤的錯誤訊息為SyntaxError，python直譯器會在語法錯誤那行顯示一個箭頭，例如下面的範例在print()函式找到了錯誤，在print()函式之前應該要有一個冒號 ＂:＂

while True print('Hello world')
    File "", line 1 
    while True print('Hello world') 
                   ^ 
SyntaxError: invalid syntax

異常

有時候在程式中即使語法正確但在執行時還是有可能發生錯誤，我們稱為異常(例外)，例如下列範例中的錯誤訊息ZeroDivisionError、NameError、TypeError，分別代表除法分母為零錯誤、變數名稱錯誤、型別錯誤

>>> 10 * (1/0)    #除法錯誤，分母為零
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

>>> 4 + spam*3    #變數名稱錯誤，變數name還未定義
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'spam' is not defined

>>> '2' + 2    #型別錯誤，string及int之間不提供加法運算
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't convert 'int' object to str implicitly

內建常見異常

寫程式最重要的能力之一是要會看懂錯誤訊息(Error Message)，才有辦法快速找出程式中的錯誤，官網文件中也提供了各種異常及它們代表的意義

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

如何處理異常?

python中處理異常的機制為使用try-except statement，用法為將有可能出現例外的程式碼放進try程式區塊，except程式區塊則是例外發生時的處理，關於例外處理相關的關鍵詞有:

下列兩個例子分別展示有例外情況與沒有例外情況用法

# 例外發生情況
try:
    print('Pass:', pass)
except:
        print('except happened!')
else:
        print('else happened!')
finally:
        print('finally pass the 30 days challenge!')


# 沒有發生例外
passChallenge= True
try:
    print('Pass:', passChallenge)
except:
    print('except happened!')
else:
    print('else happened!')
finally:
    print('finally pass the 30 days challenge!')

Reference

Python - 錯誤與異常
https://docs.python.org/zh-cn/3/tutorial/errors.html
Python - 內建異常
https://docs.python.org/zh-cn/3/library/exceptions.html#bltin-exceptions