python file operation

file.open(name[,mode[,buffering]])

模式的类型有：

r　　默认只读

w     以写方式打开，如果文件不存在则会先创建，如果文件存在则先把文件内容清空（truncate the file first）
a     以追加模式打开 (从 EOF 开始, 必要时创建新文件)用seek也无用。打开的文件也是不能读的。
r+     以读写模式打开，如果文件不存在则报错，文件可读可写，可写到文件的任何位置
w+    以读写模式打开 (参见 w )，和r+不同的是，它会truncate the file first
a+     以读写模式打开 (参见 a )，和r+不同的是，它只能写到文件末尾
rb      以二进制读模式打开
wb     以二进制写模式打开 (参见 w )
ab      以二进制追加模式打开 (参见 a )
rb+    以二进制读写模式打开 (参见 r+ )
wb+   以二进制读写模式打开 (参见 w+ )
ab+    以二进制读写模式打开 (参见 a+ )

file.read([size]) size未指定则返回整个文件,如果文件大小>2倍内存则有问题.f.read()读到文件尾时返回""(空字串)

file.readline( )：表示逐行读取,返回字符串

file.readlines(): 读取所有行,返回字符串列表

file.readline([size]) 返回包含size行的列表,size 未指定则返回全部行

file.write() 接收字符串并且写入到文件中

file.writelines() 接收一个字符串列表作为参数，将他们写入到文件中，换行符不会自动的加入，因此，需要显式的加入换行符

file.tell() 返回一个整数,表示当前文件指针的位置

file.seek(偏移量,[起始位置])

用来移动文件指针
偏移量:单位:比特,可正可负
起始位置:

0-文件头,默认值

1-当前位置

2-文件尾

f.truncate() 清空文件

file.close() 关闭文件

for line in f: print line #通过迭代器访问

Data descriptors defined here:

closed
True if the file is closed

encoding
file encoding

errors
Unicode error handler

mode
file mode ('r', 'U', 'w', 'a', possibly with 'b' or '+' added)

name
file name

newlines
end-of-line convention used in this file

softspace
flag indicating that a space needs to be printed; used by print

>>> fo=open("test.txt","r")
>>> fo.name
'test.txt'
>>> fo.mode
'r'
>>> fo.closed
False
>>> fo.encoding
>>> fo.errors
>>> fo.newlines
>>> fo.softspace
0

# -*- coding:utf-8 -*-

from sys import argv

script,filename=argv

txt=open(filename)

print "Here's your file %r:" % filename
print txt.read()

print "Type the filename again:"
file_again=raw_input(">>")

txt_again=open(file_again)

print txt_again.read()

> python ex15.py ex15_sample.txt
Here's your file 'ex15_sample.txt':
This is stuff I typed into a file.
It is really cool stuff.
Lots and lots of fun to have in here.
Type the filename again:
>>1.txt
test for sys.argv

>>> open("1.txt").read()
'test for sys.argv'
>>> f=open("test.txt","w")
>>> f.write("Hello \n")
>>> f.write("python!")
>>> f.close()
>>> f=open("test.txt","r")
>>> f.read()
'Hello \npython!'

##指针移动
>>> f=open("test.txt","r")
>>> f.read(3)
'Hel'
>>> f.read(3)
'lo '
>>> f.read(3)
'\npy'
>>> f.read(3)
'tho'
>>> f.read(3)
'n!'
>>> f.read(3)
''

>>> f=open("test.txt","r")
>>> f.readline()
'Hello \n'
>>> f.readline()
'python!'
>>> f.readline()
''

>>> f=open("test2.txt","w")
>>> f.writelines(["first line","+\n","second line"])
>>> f.close()
>>> f=open("test2.txt","r")
>>> f.readline()
'first line+\n'
>>> f.readline()
'second line'
>>> f.readline()
''

>>> f=open("test.txt",'a+')
>>> f.readline()
'dsadsa \n'
>>> f=open("test.txt",'w+')
>>> f.truncate()
>>> f.close()
>>> f=open("test.txt",'a+')
>>> f.readline()
''

# -*-coding:utf-8 -*-

from sys import argv
from os.path import exists

script,from_file,to_file =argv

print "Copying from %s to %s" % (from_file,to_file)

in_file=open(from_file)
indata=in_file.read()

print "The input file is %d bytes long" % len(indata)

print "Does the output file exist? %r" % exists(to_file)
print "Ready,hit RETURN to continue,CTRL-C to abort."
raw_input(">>")

out_file=open(to_file,"w")
out_file.write(indata)

print "Alright, all done."

out_file.close()
in_file.close()

> python ex17.py test.txt test2.txt
Copying from test.txt to test2.txt
The input file is 14 bytes long
Does the output file exist? False
Ready,hit RETURN to continue,CTRL-C to abort.
>>
Alright, all done.
> python
Enthought Canopy Python 2.7.11 | 64-bit | (default, Jun 11 2016, 11:33:47) [MSC v.1500 64 bit
Type "help", "copyright", "credits" or "license" for more information.
>>> f=open("test2.txt")
>>> f.read()
'dsads\naaa\nddd\n'
>>>

>>> fo=open("test.txt","r")
>>> fo.readline()
'dsads\n'
>>> fo.tell()
7L
>>> fo.seek(0,0)
>>> fo.readline()
'dsads\n'
>>> fo.tell()
7L
>>> fo.seek(8,0)
>>> fo.readline()
'aa\n'
>>> fo.seek(7,0)
>>> fo.readline()
'aaa\n'
>>> fo.seek(0,2)
>>> fo.tell()
17L
>>> fo.seek(-8,2)
>>> fo.tell()
9L
>>> fo.read()
'a\nddd\n'