Python-文件读写

#!/usr/bin/env python
# -*- coding:utf-8 -*-
# Author:woshinidaye

#读文件
'''
read() 每次读取整个文件，它通常用于将文件内容放到一个字符串变量中。如果文件大于可用内存，为了保险起见，可以反复调用read(size)方法，每次最多读取size个字节的内容。
readlines() 之间的差异是后者一次读取整个文件，象 .read() 一样。.readlines() 自动将文件内容分析成一个行的列表，该列表可以由 Python 的 for ... in ... 结构进行处理。
readline() 每次只读取一行，通常比readlines() 慢得多。仅当没有足够内存可以一次读取整个文件时，才应该使用 readline()。
file = open('user_pwd.txt','r') #r文本文件。rb,表示二进制文件
print(file.read())#读取整个文件
print(file.read(2))
print(file.readline())
stu1 123
print(file.readline(1))
print(file.readline(10))
readline是一行一行读取
print(file.readlines())#自动将文件给到列表里面，但是三种方式都有换行符，艹了

为了保障文件调用以后被正确关闭，需要通过try....finally来完成
'''

'''
try:                #try/finally
    file = open('user_pwd.txt','r')
    print(file.read())
finally:
    if file:
        file.close()
'''

'''
try....finally比较麻烦，可以使用with命令完成
'''

'''
with open('user_pwd.txt','r') as file:
    print(file.read())
'''

#readline:读取一行，以文本形式，2就代表前两列
'''
with open('user_pwd.txt','r') as file:
    print(file.readline())
with open('user_pwd.txt','r',encoding='utf-8') as file:
    print(file.readline(2)) 
'''

#readlines:以列表的形式读取，2就表示 0 1 两个元素

'''
with open('user_pwd.txt','r') as file:
    print(file.readlines())
with open('user_pwd.txt','r') as file:
    print(file.readlines(2))
'''
#['stu1     123\n', 'stu2     123\n', 'stu3     123\n', 'stu4     123']
#['stu1     123\n']
#三种读取方式都有换行符，另外两种看不到是因为print认为是换行，所以不是\n

'''
with open('user_pwd.txt','r') as file:
    list = file.readlines()
#去除换行符
for i in range(0,len(list)):
    list[i] = list[i].strip('\n')
    print(list)
'''

#写文件   w:写文本文件，wb，写二进制文件
'''
with open('user_pwd.txt','w') as file:
    file.write('stu05    123')
with open('user_pwd.txt','r') as read_file:
    print(read_file.read())
'''
#直接用w,若没有文件，创建一个新文件，然后写新文本进去；若文件存在，清空以后，写入文本文件；
#直接可以用a,不过需要注意格式！
'''
with open('user_pwd.txt','a') as file:
    file.write('stu06    123')
with open('user_pwd.txt','r') as read_file:
    print(read_file.read())
'''

'''
with open('user_pwd.txt','w') as file :
    file.writelines(['1','2','3'])
with open('user_pwd.txt','r') as read_file:
    print(read_file.read())
    #结果是123

with open('user_pwd.txt','w') as file :
    file.writelines('123')
with open('user_pwd.txt','r') as read_file:
    print(read_file.read())
    #结果还是123     #readlines接收列表

with open('user_pwd.txt','w') as file :
    file.writelines(['stu1    123\n','stu2    123\n','stu3    123\n'])
with open('user_pwd.txt','r') as read_file:
    print(read_file.readlines())
    #需要手动指定换行符
'''

'''
username = input('please enter your username:')
password = input('please enter your password:')
with open('user_pwd.txt','w') as file :
    file.writelines([username,password])
with open('user_pwd.txt','r') as read_file:
    print(read_file.readlines())
#这样写格式还是不对
'''

'''
username_list = []
username = input('please enter your username:')
username_list.append(username)
#print(username_list)
password_list = []
password = input('please enter your password:')
password_list.append(password)
#print(password_list)
with open('user_pwd.txt','a') as file :
    for i in range(max(len(username_list),len(password_list))):
        try:                #try的思路可以学习一下，try/except
            file.write("{}\t{}\n".format(username_list[i],password_list[i]))
        except IndexError:
            if len(username_list) > len(password_list):
                file.write('{}\t\n'.format(username_list[i]))
            else:
                file.write('\t{}\n'.format((password_list[i])))
with open('user_pwd.txt','r') as read_file:
    print(read_file.read())
'''


#open的模式
'''
mode is an optional string that specifies the mode in which the file
    is opened. It defaults to 'r' which means open for reading in text
    mode.  Other common values are 'w' for writing (truncating the file if
    it already exists), 'x' for creating and writing to a new file, and
    'a' for appending (which on some Unix systems, means that all writes
    append to the end of the file regardless of the current seek position).
    In text mode, if encoding is not specified the encoding used is platform
    dependent: locale.getpreferredencoding(False) is called to get the
    current locale encoding. (For reading and writing raw bytes use binary
    mode and leave encoding unspecified.) The available modes are:
'''


'''
    ========= ===============================================================
    Character Meaning
    --------- ---------------------------------------------------------------
    'r'       open for reading (default)
    'w'       open for writing, truncating the file first
    'x'       create a new file and open it for writing
    'a'       open for writing, appending to the end of the file if it exists
    'b'       binary mode
    't'       text mode (default)
    '+'       open a disk file for updating (reading and writing)
    'U'       universal newline mode (deprecated)
    ========= ===============================================================
    
    The default mode is 'rt' (open for reading text). For binary random
    access, the mode 'w+b' opens and truncates the file to 0 bytes, while
    'r+b' opens the file without truncation. The 'x' mode implies 'w' and
    raises an `FileExistsError` if the file already exists.
'''

with open('song.txt','r',encoding='utf-8') as f:
    #print(f.read())
    #print('====>',f.read(23))
    print(f.readline(3))
    '''
    for line in f:
        #print(line)
        print(line.rstrip())
        #print(line,end='')
    '''
    #print(f.readlines())
print(f.closed)                 #查看文件是否被关闭