python之文件操作
一、文件操作基本流程
计算机系统分为：计算机硬件，操作系统，应用程序三部分。
我们用python或其他语言编写的应用程序若想要把数据永久保存下来，必须要保存于硬盘中，这就涉及到应用程序要操作硬件，众所周知，应用程序是无法直接操作硬件的，这就用到了操作系统。操作系统把复杂的硬件操作封装成简单的接口给用户/应用程序使用，其中文件就是操作系统提供给应用程序来操作硬盘虚拟概念，用户或应用程序通过操作文件，可以将自己的数据永久保存下来。
有了文件的概念，我们无需再去考虑操作硬盘的细节，只需要关注操作文件的流程：
#1. 打开文件，得到文件句柄并赋值给一个变量
f=open('a.txt','r',encoding='utf-8') #默认打开模式就为r
#2. 通过句柄对文件进行操作
data=f.read()
#3. 关闭文件
f.close()
关闭文件的注意事项：
打开一个文件包含两部分资源：操作系统级打开的文件+应用程序的变量。在操作完毕一个文件时，必须把与该文件的这两部分资源一个不落地回收，回收方法为：
1、f.close() #回收操作系统级打开的文件
2、del f #回收应用程序级的变量
其中del f一定要发生在f.close()之后，否则就会导致操作系统打开的文件还没有关闭，白白占用资源，
而python自动的垃圾回收机制决定了我们无需考虑del f，这就要求我们，在操作完毕文件后，一定要记住 f.close()
虽然我这么说，但是很多同学还是会很不要脸地忘记f.close(),对于这些不长脑子的同学，我们推荐傻瓜式操作方式：使用with关键字来帮我们管理上下文
with open('a.txt','w') as f:
pass
with open('a.txt','r') as read_f,open('b.txt','w') as write_f:
data=read_f.read()
write_f.write(data)
注意
二、文件编码
f=open(...)是由操作系统打开文件，那么如果我们没有为open指定编码，那么打开文件的默认编码很明显是操作系统说了算了，操作系统会用自己的默认编码去打开文件，在windows下是gbk，在linux下是utf-8。
#若要保证不乱码，文件以什么方式存的，就要以什么方式打开。
f=open('a.txt','r',encoding='utf-8')
三、文件的打开模式
文件句柄 = open（‘文件路径’，‘模式’）
#1. 打开文件的模式有(默认为文本模式)：
r ，只读模式【默认模式，文件必须存在，不存在则抛出异常】
w，只写模式【不可读；不存在则创建；存在则清空内容】
a， 只追加写模式【不可读；不存在则创建；存在则只追加内容】
#2. 对于非文本文件，我们只能使用b模式，"b"表示以字节的方式操作（而所有文件也都是以字节的形式存储的，使用这种模式无需考虑文本文件的字符编码、图片文件的jgp格式、视频文件的avi格式）
rb
wb
ab
注：以b方式打开时，读取到的内容是字节类型，写入时也需要提供字节类型，不能指定编码
#3,‘+’模式（就是增加了一个功能）
r+， 读写【可读，可写】
w+，写读【可写，可读】
a+， 写读【可写，可读】
#4，以bytes类型操作的读写，写读，写读模式
r+b， 读写【可读，可写】
w+b，写读【可写，可读】
a+b， 写读【可写，可读】
四、文件操作方法
4.1常用操作方法
read（3）：
1. 文件打开方式为文本模式时，代表读取3个字符
2. 文件打开方式为b模式时，代表读取3个字节
其余的文件内光标移动都是以字节为单位的如：seek，tell，truncate
注意：
1. seek有三种移动方式0，1，2，其中1和2必须在b模式下进行，但无论哪种模式，都是以bytes为单位移动的
2. truncate是截断文件，所以文件的打开方式必须可写，但是不能用w或w+等方式打开，因为那样直接清空文件了，所以truncate要在r+或a或a+等模式下测试效果。
4.2所有操作方法（了解）
class file(object)
def close(self): # real signature unknown; restored from __doc__
关闭文件
"""
close() -> None or (perhaps) an integer. Close the file.
Sets data attribute .closed to True. A closed file cannot be used for
further I/O operations. close() may be called more than once without
error. Some kinds of file objects (for example, opened by popen())
may return an exit status upon closing.
"""
def fileno(self): # real signature unknown; restored from __doc__
文件描述符
"""
fileno() -> integer "file descriptor".
This is needed for lower-level file interfaces, such os.read().
"""
return 0
def flush(self): # real signature unknown; restored from __doc__
刷新文件内部缓冲区
""" flush() -> None. Flush the internal I/O buffer. """
pass
def isatty(self): # real signature unknown; restored from __doc__
判断文件是否是同意tty设备
""" isatty() -> true or false. True if the file is connected to a tty device. """
return False
def next(self): # real signature unknown; restored from __doc__
获取下一行数据，不存在，则报错
""" x.next() -> the next value, or raise StopIteration """
pass
def read(self, size=None): # real signature unknown; restored from __doc__
读取指定字节数据
"""
read([size]) -> read at most size bytes, returned as a string.
If the size argument is negative or omitted, read until EOF is reached.
Notice that when in non-blocking mode, less data than what was requested
may be returned, even if no size parameter was given.
"""
pass
def readinto(self): # real signature unknown; restored from __doc__
读取到缓冲区，不要用，将被遗弃
""" readinto() -> Undocumented. Don't use this; it may go away. """
pass
def readline(self, size=None): # real signature unknown; restored from __doc__
仅读取一行数据
"""
readline([size]) -> next line from the file, as a string.
Retain newline. A non-negative size argument limits the maximum
number of bytes to return (an incomplete line may be returned then).
Return an empty string at EOF.
"""
pass
def readlines(self, size=None): # real signature unknown; restored from __doc__
读取所有数据，并根据换行保存值列表
"""
readlines([size]) -> list of strings, each a line from the file.
Call readline() repeatedly and return a list of the lines so read.
The optional size argument, if given, is an approximate bound on the
total number of bytes in the lines returned.
"""
return []
def seek(self, offset, whence=None): # real signature unknown; restored from __doc__
指定文件中指针位置
"""
seek(offset[, whence]) -> None. Move to new file position.
Argument offset is a byte count. Optional argument whence defaults to
(offset from start of file, offset should be >= 0); other values are 1
(move relative to current position, positive or negative), and 2 (move
relative to end of file, usually negative, although many platforms allow
seeking beyond the end of a file). If the file is opened in text mode,
only offsets returned by tell() are legal. Use of other offsets causes
undefined behavior.
Note that not all file objects are seekable.
"""
pass
def tell(self): # real signature unknown; restored from __doc__
获取当前指针位置
""" tell() -> current file position, an integer (may be a long integer). """
pass
def truncate(self, size=None): # real signature unknown; restored from __doc__
截断数据，仅保留指定之前数据
"""
truncate([size]) -> None. Truncate the file to at most size bytes.
Size defaults to the current file position, as returned by tell().
"""
pass
def write(self, p_str): # real signature unknown; restored from __doc__
写内容
"""
write(str) -> None. Write string str to file.
Note that due to buffering, flush() or close() may be needed before
the file on disk reflects the data written.
"""
pass
def writelines(self, sequence_of_strings): # real signature unknown; restored from __doc__
将一个字符串列表写入文件
"""
writelines(sequence_of_strings) -> None. Write the strings to the file.
Note that newlines are not added. The sequence can be any iterable object
producing strings. This is equivalent to calling write() for each string.
"""
pass
def xreadlines(self): # real signature unknown; restored from __doc__
可用于逐行读取文件，非全部
"""
xreadlines() -> returns self.
For backward compatibility. File objects now include the performance
optimizations previously implemented in the xreadlines module.
"""
pass
2.x
class TextIOWrapper(_TextIOBase):
"""
Character and line based layer over a BufferedIOBase object, buffer.
encoding gives the name of the encoding that the stream will be
decoded or encoded with. It defaults to locale.getpreferredencoding(False).
errors determines the strictness of encoding and decoding (see
help(codecs.Codec) or the documentation for codecs.register) and
defaults to "strict".
newline controls how line endings are handled. It can be None, '',
'\n', '\r', and '\r\n'. It works as follows:
* On input, if newline is None, universal newlines mode is
enabled. Lines in the input can end in '\n', '\r', or '\r\n', and
these are translated into '\n' before being returned to the
caller. If it is '', universal newline mode is enabled, but line
endings are returned to the caller untranslated. If it has any of
the other legal values, input lines are only terminated by the given
string, and the line ending is returned to the caller untranslated.
* On output, if newline is None, any '\n' characters written are
translated to the system default line separator, os.linesep. If
newline is '' or '\n', no translation takes place. If newline is any
of the other legal values, any '\n' characters written are translated
to the given string.
If line_buffering is True, a call to flush is implied when a call to
write contains a newline character.
"""
def close(self, *args, **kwargs): # real signature unknown
关闭文件
pass
def fileno(self, *args, **kwargs): # real signature unknown
文件描述符
pass
def flush(self, *args, **kwargs): # real signature unknown
刷新文件内部缓冲区
pass
def isatty(self, *args, **kwargs): # real signature unknown
判断文件是否是同意tty设备
pass
def read(self, *args, **kwargs): # real signature unknown
读取指定字节数据
pass
def readable(self, *args, **kwargs): # real signature unknown
是否可读
pass
def readline(self, *args, **kwargs): # real signature unknown
仅读取一行数据
pass
def seek(self, *args, **kwargs): # real signature unknown
指定文件中指针位置
pass
def seekable(self, *args, **kwargs): # real signature unknown
指针是否可操作
pass
def tell(self, *args, **kwargs): # real signature unknown
获取指针位置
pass
def truncate(self, *args, **kwargs): # real signature unknown
截断数据，仅保留指定之前数据
pass
def writable(self, *args, **kwargs): # real signature unknown
是否可写
pass
def write(self, *args, **kwargs): # real signature unknown
写内容
pass
def __getstate__(self, *args, **kwargs): # real signature unknown
pass
def __init__(self, *args, **kwargs): # real signature unknown
pass
@staticmethod # known case of __new__
def __new__(*args, **kwargs): # real signature unknown
""" Create and return a new object. See help(type) for accurate signature. """
pass
def __next__(self, *args, **kwargs): # real signature unknown
""" Implement next(self). """
pass
def __repr__(self, *args, **kwargs): # real signature unknown
""" Return repr(self). """
pass
buffer = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
closed = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
encoding = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
errors = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
line_buffering = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
name = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
newlines = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
_CHUNK_SIZE = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
_finalizing = property(lambda self: object(), lambda self, v: None, lambda self: None) # default
3.x
五、文件的修改
文件的数据是存放于硬盘上的，因而只存在覆盖、不存在修改这么一说，我们平时看到的修改文件，都是模拟出来的效果，具体的说有两种实现方式：
方式一：将硬盘存放的该文件的内容全部加载到内存，在内存中是可以修改的，修改完毕后，再由内存覆盖到硬盘（word，vim，nodpad++等编辑器）
import os # 调用系统模块
with open('a.txt') as read_f,open('a.txt.swap','w') as write_f:
data=read_f.read() #全部读入内存,如果文件很大,会很卡
data=data.replace('alex','SB') #在内存中完成修改
write_f.write(data) #一次性写入新文件
os.remove('a.txt') #删除原文件
os.rename('.a.txt.swap','a.txt') #将新建的文件重命名为原文件
方式二：将硬盘存放的该文件的内容一行一行地读入内存，修改完毕就写入新文件，最后用新文件覆盖源文件
import os
with open('a.txt') as read_f,open('.a.txt.swap','w') as write_f:
for line in read_f:
line=line.replace('alex','SB')
write_f.write(line)
os.remove('a.txt')
os.rename('.a.txt.swap','a.txt')
六、练习
1、 文件a.txt内容：每一行内容分别为商品名字，价钱，个数。
apple 10 3
tesla 100000 1
mac 3000 2
lenovo 30000 3
chicken 10 3
通过代码，将其构建成这种数据类型：[{'name':'apple','price':10,'amount':3},{'name':'tesla','price':1000000,'amount':1}......] 并计算出总价钱。
2、有如下文件：
-------
alex是头上长了个包。
alex其实是人妖。
谁说alex是sb？
你们真逗，alex再牛逼，也掩饰不住资深屌丝的气质。
----------
将文件中所有的alex都替换成大写的SB。
