python文件读写和异常

1,文本文件

• 普通打开

  f.open

  f.close

  def main():
      f = None
      try:
          f = open('致橡树.txt', 'r', encoding='utf-8')
          print(f.read())
      except FileNotFoundError:
          print('无法打开指定的文件!')
      except LookupError:
          print('指定了未知的编码!')
      except UnicodeDecodeError:
          print('读取文件时解码错误!')
      finally:
          if f:
              f.close()
  
  if __name__ == '__main__':
      main()
  

• with关键字

  with open as f

  def main():
      try:
          with open('致橡树.txt', 'r', encoding='utf-8') as f:
              print(f.read())
      except FileNotFoundError:
          print('无法打开指定的文件!')
      except LookupError:
          print('指定了未知的编码!')
      except UnicodeDecodeError:
          print('读取文件时解码错误!')
  
  if __name__ == '__main__':
      main()
  

• for-in循环逐行读取或者用readlines方法将文件按行读取到一个列表容器中

  import time
  
  def main():
      # 一次性读取整个文件内容
      with open('致橡树.txt', 'r', encoding='utf-8') as f:
          print(f.read())
  
      # 通过for-in循环逐行读取
      with open('致橡树.txt', mode='r') as f:
          for line in f:
              print(line, end='')
              time.sleep(0.5)
      print()
  
      # 读取文件按行读取到列表中
      with open('致橡树.txt') as f:
          lines = f.readlines()
      print(lines)
  
  if __name__ == '__main__':
      main()
  

• 写入文本文件

  from math import  sqrt
  
  def is_prime(n):
       """判断市素数"""
       assert  n > 0
       for factor in range(2,int(sqrt(n) + 1)):
           if n %factor == 0:
               return  False
       return True if n != 1 else False
  
  def main():
       filenames = ('a.txt', 'b.txt', 'c.txt')
       fs_list = []
       try:
           for filename in filenames:
               fs_list.append(open(filename,'w',encoding='utf-8'))
  
           for num in range(1,10000):
               if is_prime(num):
                   if num < 100:
                       fs_list[0].write(str(num)+"\n")
                   elif num < 1000:
                       fs_list[1].write(str(num)+"\n")
                   else:
                       fs_list[2].write(str(num)+'\n')
       except IOError as e:
           print(e)
           print("文件写入错误")
       finally:
           for fs in fs_list:
               fs.close()
       print("操作完成")
  
  if __name__ == '__main__':
      main()
  

2,二进制文件

def main():
    try:
        with open('guido.jpg', 'rb') as fs1:
            data = fs1.read()
            print(type(data))  # <class 'bytes'>
        with open('吉多.jpg', 'wb') as fs2:
            fs2.write(data)
    except FileNotFoundError as e:
        print('指定的文件无法打开.')
    except IOError as e:
        print('读写文件时出现错误.')
    print('程序执行结束.')

if __name__ == '__main__':
    main()

3，json文件

JSON跟Python中的字典其实是一样一样的，事实上JSON的数据类型和Python的数据类型是很容易找到对应关系的，如下面两张表所示。

JSON	Python
object	dict
array	list
string	str
number (int / real)	int / float
true / false	True / False
null	None

Python	JSON
dict	object
list, tuple	array
str	string
int, float, int- & float-derived Enums	number
True / False	true / false
None	null

我们使用Python中的json模块就可以将字典或列表以JSON格式保存到文件中，代码如下所示。

import json

def main():
    mydict = {
        'name': '骆昊',
        'age': 38,
        'qq': 957658,
        'friends': ['王大锤', '白元芳'],
        'cars': [
            {'brand': 'BYD', 'max_speed': 180},
            {'brand': 'Audi', 'max_speed': 280},
            {'brand': 'Benz', 'max_speed': 320}
        ]
    }
    try:
        with open('data.json', 'w', encoding='utf-8') as fs:
            json.dump(mydict, fs)
    except IOError as e:
        print(e)
    print('保存数据完成!')

if __name__ == '__main__':
    main()