1 異常處理

import sys
try:
    f = open("myfile.txt")
    s = f.readline()
    i = int(s.strip())
# except OSError as err:
#     print("OS error: {0}".format(err))
# except ValueError:
#     print("could not convert data to an integer")
except (OSError,ValueError): #(error1,error2),異常元組
    print("an error occurred")
except:#通配異常
    print("unexpected error:",sys.exc_info()[0]) #print prompt message
    raise    #將異常繼續丟擲,給上層處理
else:#沒有發生異常
    print("there is no error or execpetion in the programming code")
 

#python使用raise語句丟擲一個指定的異常
try:
    raise ValueError("HiThere")
except ValueError:
    print("an exception flew by")
    raise #如果你只想知道這是否丟擲了一個異常，
          # 並不想去處理它，那麼一個簡單的 raise 語句就可以再次把它丟擲

#使用者自定義異常
#你可以通過建立一個新的exception類來擁有自己的異常。
# 異常應該繼承自 Exception 類，或者直接繼承，或者間接繼承，
class MyError(Exception):
    def __init__(self,value):#類 Exception 預設的 __init__() 被覆蓋。
        self.value = value
    def __str__(self):
        return repr(self.value)


try:
    raise MyError(2*2)
except MyError as e:
    print("My Exception occurred,value:",e.value)
 

#當建立一個模組有可能丟擲多種不同的異常時，
# 一種通常的做法是為這個包建立一個基礎異常類，然後基於這個基礎類為不同的錯誤情況建立不同的子類:
class Error(Exception):
    """Base class for exceptions in this module."""
    pass

class InputError(Error):
    """ Exception raised for errors in the input
        Attributes:
            expression--input expression in which the error occurred
            message   --explanation of the error
    """
    def __init__(self,expression,message):
        self.expression = expression
        self.message = message

class TransitionError(Error):
    """Raised when an operation attempts a state transition  that's not allowed
       Attributes:
           previous -- state at beginning of transition
           next     -- attempted new state
           message  -- explanation of why the specific transition is not allowed
    """
    def __init__(self,previous,next,message):
        self.previoud = previous
        self.next = next
        self.message = message
 

>>> def divide(x, y):
        try:
            result = x / y
        except ZeroDivisionError:
            print("division by zero!")
        else:
            print("result is", result)
        finally:
            print("executing finally clause")
   
>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'

#一些物件定義了標準的清理行為，無論系統是否成功的使用了它，一旦不需要它了，那麼這個標準的清理行為就會執行
for line in open("myfile.txt"):
    print(line,end="")  #這段程式碼的問題是,當執行完畢後,檔案保持開啟的轉態,並沒有關閉

with open("myfile.txt") as f:
    for line in f:
        print(line,end="") #關鍵詞 with 語句就可以保證諸如檔案之類的物件在使用完之後一定會正確的執行他的清理方法