【Python3】文件操作

对文件操作流程

1. 打开文件，得到文件句柄并赋值给一个变量
2. 通过句柄对文件进行操作
3. 关闭文件

基本操作　

f =open('filename','r ',encoding='utf-8') #打开文件,通过文件句柄只读文件

data =f.read()   # 默认读取所有内容，可加参数指定字符数，文件大时不要用，指针从开始到末尾

print(data)   # 打印文件

---

f =open('filename','w',encoding='utf-8') #打开文件,通过文件句柄只写文件

f.write('需要写入的内容')  #先存到内存，积累到一定量写入硬盘

f.flush()   #刷，及时从内存写入硬盘

打印进度条：

import sys，time

for i in range(20):

　　sys.stout.write('#')

　　sys.stout.flush()

　　time.sleep(0.1)

---

f =open('filename','a',encoding='utf-8') #打开文件,通过文件句追加，不可读

f.write('需要写入的内容')  # 换行

---

first_line =f.readline()   # 读第一行

first_line =f.readline()   # 读第二行，指针移到第二行

for i in range(10):

　　print(f.readline())

---

for line in f：   #效率最高，每次读取一行，内存即时释放，优先使用

　　print(line)

---

for line in f.readlines():   #生成列表，适合小文件

　　print(line)   # print()默认含换行符，print(line.strip())，print(line,end='')

---

for index line in enumerate( f.readlines()):   #对于一个可迭代/可遍历对象（如列表、字符串），enumerate可以同时获得索引和值

　　if index==9:

　　print('-----')

　　continue   #实现第十行不输出

　　print(line)

---

f.tell()  #按字符计数，返回指针位置
f.seek()  #指针回到指定位置，seek(0)回到起始位置

---

f.encoding   #返回编码方式

f.truncate(20)   #从头截取20字符

---

f.close()   #关闭文件

打开文件的模式：

• r，只读模式（默认）。
• w，只写模式。【不可读；不存在则创建；存在则删除内容；】
• a，追加模式。【可读；   不存在则创建；存在则只追加内容；】

"+" 表示可以同时读写某个文件

• r+，读写文件。【可读；可写；可追加】
• w+，写读
• a+，同a

"U"表示在读取时，可以将 自动转换成 （与 r 或 r+ 模式同使用）， -windows系统换行，-nlinux系统换行

• rU
• r+U

"b"表示处理二进制文件（如：FTP发送上传ISO镜像文件，linux可忽略，windows处理二进制文件时需标注）

• rb
• wb，f.write(sentence.encode())转成utf-8
• ab

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：

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：

with open('log1') as obj1, open('log2') as obj2:

    pass

文件修改

• 先加载到内存，内存可以修改，硬盘直接覆盖难以插入
  
• 边读边写，同时打开两个文件

import sys

f=open(filename,'r',encoding='utf-8')

f_new=open(filename,’r’,encoding='utf-8')

find_str=sys.argv[1]

replace_str=sys.argv[2]

for line in f:

　　if find_str in line:

　　　　line = line.replace(find_str,replace_str)

　　　　f_new.write(line)

其它语法

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
    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 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 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 readinto(self): # real signature unknown; restored from __doc__
        """ Same as RawIOBase.readinto(). """
        pass #不要用,没人知道它是干嘛用的
    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 seekable(self, *args, **kwargs): # real signature unknown
        """ True if file supports random-access. """
        pass
    def tell(self, *args, **kwargs): # real signature unknown
        """
        Current file position.
        Can raise OSError for non seekable files.
        """
        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 writable(self, *args, **kwargs): # real signature unknown
        """ True if file was opened in a write mode. """
        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