Day 11 檔案和異常

• 從檔案讀取資料
  • 一次性讀取全部檔案內容

    with open('pi_digits.txt') as file_object:
    	contents =file_object.read()
    

    • 函式open()用於開啟檔案，引數位檔名，返回值位檔案物件；
    • 關鍵字with在不需要訪問檔案後自動完成關閉工作；
  • 逐行讀取

    file_name = "pi_digits.txt"
    with open(file_name) as file_object:
    	for line in
     
     file_object:
    		print (line.rstrip())
    

    • 從輸出結果看，open返回的file_object應該是一個列表型別的變數；
    • 關鍵字with結合open()返回的檔案物件只在with程式碼塊內可見，如需在with程式碼塊外可見，則需建立一個列表物件，用於儲存檔案各行內容並留著在with程式碼塊外使用；

      with open(file_name) as file_object:
      	lines = file_object.readlines()
      for line in lines:
      	print (line.rstrip())
      
  • 使用檔案內容

  	file_name = "pi_million_digits.txt"
  	with open(file_name) as file_object:
  		lines = file_object.readlines()
  	pi_string = ''
  	for line in lines:
  		pi_string += line.rstrip()
  	print (pi_string)
  	print (len(pi_string))
  

• 寫入檔案
  • with open(filename, ‘w’) as file_object:

  filename = 'programming.txt'
  with open(filename, 'w') as file_object:
  	file_object.write("I love programming!")
  

  • open的第二個實參：
    • ‘r’：只讀模式開啟
    • ’w’：寫入模式開啟
    • ‘a’：附加模式開啟
    • ’r+‘：讀取和寫入模式開啟
    • 預設以只讀方式開啟檔案。
• 異常
  • python程式執行過程中發生錯誤是，會產生一個異常物件，如果編寫了處理該異常的程式碼，程式繼續執行，否則，程式將停止，並顯示一個traceback。
  • try expect else 程式碼塊用於處理異常

  	try:
  		answer = int(first_number) / int(second_number)
  	except ZeroDivisionError:
  		print("You can't divide bu 0!")
  	else:
  		print(answer)
  

• 分析文字

  filename = 'alice.txt'
  try:
  	with open(filename) as f_obj:
  		contents = f_obj.read()
  except FileNotFoundError:
  	msg = "Sorry, the file " + filename + " does not exist."
  	print(msg)
  else:
  	#計算檔案大致包含多少個單詞
  	words = contents.split()
  	num_words = len(words)
  	print("The file " + filename + " has about " +
  		str(num_words) + " words.")
  

• 同時處理多個檔案：將需要處理的檔名用列表儲存

#定義計算檔案有多少個單詞的函式
def count_words(filename):
	try:
		with open(filename) as f_obj:
			contents = f_obj.read()
	except FileNotFoundError:
		msg = "Sorry, the file " + filename + " does not exist."
		print(msg)
	else:
		#計算檔案大致包含多少個單詞
		words = contents.split()
		num_words = len(words)
		print("The file " + filename + " has about " +
			str(num_words) + " words.")
			
filenames = ['alice.txt', 'siddhartha.txt', 'moby_dick.txt',
	'little_women.txt']
	
for filename in filenames:
	count_words(filename)