Python学习笔记 Day11 文件和异常

Day 11 文件和异常

• 从文件读取数据
  • 一次性读取全部文件内容

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

    • 函数open()用于打开文件，参数位文件名，返回值位文件对象；
    • 关键字with在不需要访问文件后自动完成关闭工作；
  • 逐行读取

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

    • 从输出结果看，open返回的file_object应该是一个列表类型的变量；
    • 关键字with结合open()返回的文件对象只在with代码块内可见，如需在with代码块外可见，则需创建一个列表对象，用于存储文件各行内容并留着在with代码块外使用；

      with open(file_name) as file_object:
      	lines = file_object.readlines()
      for line in lines:
      	print (line.rstrip())
      

  • 使用文件内容

  	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))
  

• 写入文件
  • with open(filename, ‘w’) as file_object:

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

  • open的第二个实参：
    • ‘r’：只读模式打开
    • ’w’：写入模式打开
    • ‘a’：附加模式打开
    • ’r+‘：读取和写入模式打开
    • 默认以只读方式打开文件。
• 异常
  • python程序执行过程中发生错误是，会产生一个异常对象，如果编写了处理该异常的代码，程序继续运行，否则，程序将停止，并显示一个traceback。
  • try expect else 代码块用于处理异常

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

• 分析文本

  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:
  	#计算文件大致包含多少个单词
  	words = contents.split()
  	num_words = len(words)
  	print("The file " + filename + " has about " +
  		str(num_words) + " words.")
  

• 同时处理多个文件：将需要处理的文件名用列表存储

#定义计算文件有多少个单词的函数
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:
		#计算文件大致包含多少个单词
		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)