python 文件操作（三）
九 文件操作
9.1 对文件操作流程
- 打开文件，得到文件句柄并赋值给一个变量
- 通过句柄对文件进行操作
- 关闭文件
现有文件如下：
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昨夜寒蛩不住鸣。惊回千里梦，已三更。起来独自绕阶行。人悄悄，帘外月胧明。白首为功名，旧山松竹老，阻归程。欲将心事付瑶琴。知音少，弦断有谁听。 |
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f = open('小重山') #打开文件data=f.read()#获取文件内容f.close() #关闭文件 |
注意 if in the win，hello文件是utf8保存的，打开文件时open函数是通过操作系统打开的文件，而win操作系统
默认的是gbk编码，所以直接打开会乱码，需要f=open('hello',encoding='utf8')，hello文件如果是gbk保存的，则直接打开即可。
9.2 文件打开模式
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========= =============================================================== 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) ========= =============================================================== |
先介绍三种最基本的模式：
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# f = open('小重山2','w') #打开文件# f = open('小重山2','a') #打开文件# f.write('莫等闲1\n')# f.write('白了少年头2\n')# f.write('空悲切!3') |
9.3 文件具体操作
def read(self, size=-1): # known case of _io.FileIO.read
"""
注意，不一定能全读回来
Read at most size bytes, returned as bytes.
Only makes one system call, so less data may be returned than requested.
In non-blocking mode, returns None if no data is available.
Return an empty bytes object at EOF.
"""
return ""
def readline(self, *args, **kwargs):
pass
def readlines(self, *args, **kwargs):
pass
def tell(self, *args, **kwargs): # real signature unknown
"""
Current file position.
Can raise OSError for non seekable files.
"""
pass
def seek(self, *args, **kwargs): # real signature unknown
"""
Move to new file position and return the file position.
Argument offset is a byte count. Optional argument whence defaults to
SEEK_SET or 0 (offset from start of file, offset should be >= 0); other values
are SEEK_CUR or 1 (move relative to current position, positive or negative),
and SEEK_END or 2 (move relative to end of file, usually negative, although
many platforms allow seeking beyond the end of a file).
Note that not all file objects are seekable.
"""
pass
def write(self, *args, **kwargs): # real signature unknown
"""
Write bytes b to file, return number written.
Only makes one system call, so not all of the data may be written.
The number of bytes actually written is returned. In non-blocking mode,
returns None if the write would block.
"""
pass
def flush(self, *args, **kwargs):
pass
def truncate(self, *args, **kwargs): # real signature unknown
"""
Truncate the file to at most size bytes and return the truncated size.
Size defaults to the current file position, as returned by tell().
The current file position is changed to the value of size.
"""
pass
def close(self): # real signature unknown; restored from __doc__
"""
Close the file.
A closed file cannot be used for further I/O operations. close() may be
called more than once without error.
"""
pass
##############################################################less usefull
def fileno(self, *args, **kwargs): # real signature unknown
""" Return the underlying file descriptor (an integer). """
pass
def isatty(self, *args, **kwargs): # real signature unknown
""" True if the file is connected to a TTY device. """
pass
def readable(self, *args, **kwargs): # real signature unknown
""" True if file was opened in a read mode. """
pass
def readall(self, *args, **kwargs): # real signature unknown
"""
Read all data from the file, returned as bytes.
In non-blocking mode, returns as much as is immediately available,
or None if no data is available. Return an empty bytes object at EOF.
"""
pass
def seekable(self, *args, **kwargs): # real signature unknown
""" True if file supports random-access. """
pass
def writable(self, *args, **kwargs): # real signature unknown
""" True if file was opened in a write mode. """
pass
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f = open('小重山') #打开文件# data1=f.read()#获取文件内容# data2=f.read()#获取文件内容## print(data1)# print('...',data2)# data=f.read(5)#获取文件内容# data=f.readline()# data=f.readline()# print(f.__iter__().__next__())# for i in range(5):# print(f.readline())# data=f.readlines()# for line in f.readlines():# print(line)# 问题来了:打印所有行,另外第3行后面加上:'end 3'# for index,line in enumerate(f.readlines()):# if index==2:# line=''.join([line.strip(),'end 3'])# print(line.strip())#切记:以后我们一定都用下面这种# count=0# for line in f:# if count==3:# line=''.join([line.strip(),'end 3'])# print(line.strip())# count+=1# print(f.tell())# print(f.readline())# print(f.tell())#tell对于英文字符就是占一个,中文字符占三个,区分与read()的不同.# print(f.read(5))#一个中文占三个字符# print(f.tell())# f.seek(0)# print(f.read(6))#read后不管是中文字符还是英文字符,都统一算一个单位,read(6),此刻就读了6个中文字符#terminal上操作:f = open('小重山2','w')# f.write('hello \n')# f.flush()# f.write('world')# 应用:进度条# import time,sys# for i in range(30):# sys.stdout.write("*")# # sys.stdout.flush()# time.sleep(0.1)# f = open('小重山2','w')# f.truncate()#全部截断# f.truncate(5)#全部截断# print(f.isatty())# print(f.seekable())# print(f.readable())f.close() #关闭文件 |
接下来我们继续扩展文件模式：
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# f = open('小重山2','w') #打开文件# f = open('小重山2','a') #打开文件# f.write('莫等闲1\n')# f.write('白了少年头2\n')# f.write('空悲切!3')# f.close()#r+,w+模式# f = open('小重山2','r+') #以读写模式打开文件# print(f.read(5))#可读# f.write('hello')# print('------')# print(f.read())# f = open('小重山2','w+') #以写读模式打开文件# print(f.read(5))#什么都没有,因为先格式化了文本# f.write('hello alex')# print(f.read())#还是read不到# f.seek(0)# print(f.read())#w+与a+的区别在于是否在开始覆盖整个文件# ok,重点来了,我要给文本第三行后面加一行内容:'hello 岳飞!'# 有同学说,前面不是做过修改了吗? 大哥,刚才是修改内容后print,现在是对文件进行修改!!!# f = open('小重山2','r+') #以写读模式打开文件# f.readline()# f.readline()# f.readline()# print(f.tell())# f.write('hello 岳飞')# f.close()# 和想的不一样,不管事!那涉及到文件修改怎么办呢?# f_read = open('小重山','r') #以写读模式打开文件# f_write = open('小重山_back','w') #以写读模式打开文件# count=0# for line in f_read: # if count==3: # f_write.write('hello,岳飞\n') # # else: # f_write.write(line) # another way: # if count==3: # # line='hello,岳飞2\n' # f_write.write(line) # count+=1# #二进制模式# f = open('小重山2','wb') #以二进制的形式读文件# # f = open('小重山2','wb') #以二进制的形式写文件# f.write('hello alvin!'.encode())#b'hello alvin!'就是一个二进制格式的数据,只是为了观看,没有显示成010101的形式 |
注意1: 无论是py2还是py3，在r+模式下都可以等量字节替换，但没有任何意义的！
注意2：有同学在这里会用readlines得到内容列表，再通过索引对相应内容进行修改，最后将列表重新写会该文件。
这种思路有一个很大的问题，数据若很大，你的内存会受不了的，而我们的方式则可以通过迭代器来优化这个过程。
补充：rb模式以及seek
在py2中：
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#昨夜寒蛩不住鸣.f = open('test','r',) #以写读模式打开文件f.read(3)# f.seek(3)# print f.read(3) # 夜# f.seek(3,1)# print f.read(3) # 寒# f.seek(-4,2)# print f.read(3) # 鸣 |
在py3中：
# test:
昨夜寒蛩不住鸣.
f = open('test','rb',) #以写读模式打开文件
f.read(3)
# f.seek(3)
# print(f.read(3)) # b'\xe5\xa4\x9c'
# f.seek(3,1)
# print(f.read(3)) # b'\xe5\xaf\x92'
# f.seek(-4,2)
# print(f.read(3)) # b'\xe9\xb8\xa3'
#总结: 在py3中,如果你想要字符数据,即用于观看的,则用r模式,这样我f.read到的数据是一个经过decode的
# unicode数据; 但是如果这个数据我并不需要看,而只是用于传输,比如文件上传,那么我并不需要decode
# 直接传送bytes就好了,所以这个时候用rb模式.
# 在py3中,有一条严格的线区分着bytes和unicode,比如seek的用法,在py2和py3里都是一个个字节的seek,
# 但在py3里你就必须声明好了f的类型是rb,不允许再模糊.
#建议: 以后再读写文件的时候直接用rb模式,需要decode的时候仔显示地去解码.
9.4 with语句
为了避免打开文件后忘记关闭，可以通过管理上下文，即：
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with open('log','r') as f: pass |
如此方式，当with代码块执行完毕时，内部会自动关闭并释放文件资源。
在Python 2.7 后，with又支持同时对多个文件的上下文进行管理，即：
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with open('log1') as obj1, open('log2') as obj2: pass |
