python学习笔记文件操作（六）

1、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

如下文件：

2017-03-24 11:25:06:349 - info: [debug] [AndroidBootstrap] Sending command to android: {"cmd":"shutdown"}
2017-03-24 11:25:06:355 - info: [debug] [AndroidBootstrap] Received command result from bootstrap
2017-03-24 11:25:06:356 - info: [debug] [UiAutomator] Shutting down UiAutomator
2017-03-24 11:25:06:357 - info: [debug] [UiAutomator] Moving to state 'stopping'
2017-03-24 11:25:06:360 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got data from client: {"cmd":"shutdown"}
2017-03-24 11:25:06:361 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got command of type SHUTDOWN

操作流程：

f = open('log.txt','r')
print(f.read())
f.close()

注意： 在win系统中log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。

2、文件打开模式

# ========= ===============================================================
#
#     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)
#
# ========= ===============================================================

3、文件操作方法

获取文件内容

f = open('log.txt','r')
data = f.read()
print(data)
f.close()

输出：
2017-03-24 11:25:06:349 - info: [debug] [AndroidBootstrap] Sending command to android: {"cmd":"shutdown"} 2017-03-24 11:25:06:355 - info: [debug] [AndroidBootstrap] Received command result from bootstrap 2017-03-24 11:25:06:356 - info: [debug] [UiAutomator] Shutting down UiAutomator 2017-03-24 11:25:06:357 - info: [debug] [UiAutomator] Moving to state 'stopping' 2017-03-24 11:25:06:360 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got data from client: {"cmd":"shutdown"} 2017-03-24 11:25:06:361 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got command of type SHUTDOWN
===========================================================================================

f = open('log.txt','r')
data1 = f.read(10) #读取10个字符。（在这里汉字也只占一个单位）
print(data1)
f.close()

输出：
2017-03-24

readline和readlines
===========================================================================================

f = open('log.txt','r')
print(f.readlines()) #返回的是一个列表，注意换行符
f.close()

输出：
['2017-03-24 11:25:06:349 - info: [debug] [AndroidBootstrap] Sending command to android: {"cmd":"shutdown"}\n', '2017-03-24 11:25:06:355 - info: [debug] [AndroidBootstrap] Received command result from bootstrap\n', '2017-03-24 11:25:06:356 - info: [debug] [UiAutomator] Shutting down UiAutomator\n', "2017-03-24 11:25:06:357 - info: [debug] [UiAutomator] Moving to state 'stopping'\n", '2017-03-24 11:25:06:360 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got data from client: {"cmd":"shutdown"}\n', '2017-03-24 11:25:06:361 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got command of type SHUTDOWN']
===========================================================================================

f = open('log.txt','r')
print(f.readline()) #逐行读取
print(f.readline())
print(f.readline()，f.readline()，f.readline()) #逐行读取，每行末尾换行符
f.close()

输出：（注意每行末尾的换行符）
2017-03-24 11:25:06:349 - info: [debug] [AndroidBootstrap] Sending command to android: {"cmd":"shutdown"}

2017-03-24 11:25:06:355 - info: [debug] [AndroidBootstrap] Received command result from bootstrap

2017-03-24 11:25:06:356 - info: [debug] [UiAutomator] Shutting down UiAutomator
2017-03-24 11:25:06:357 - info: [debug] [UiAutomator] Moving to state 'stopping'
2017-03-24 11:25:06:360 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got data from client: {"cmd":"shutdown"}
===========================================================================================

f = open('log.txt','r')
for line in f.readlines():
print(line.strip())
f.close()

输出：
2017-03-24 11:25:06:349 - info: [debug] [AndroidBootstrap] Sending command to android: {"cmd":"shutdown"} 2017-03-24 11:25:06:355 - info: [debug] [AndroidBootstrap] Received command result from bootstrap 2017-03-24 11:25:06:356 - info: [debug] [UiAutomator] Shutting down UiAutomator 2017-03-24 11:25:06:357 - info: [debug] [UiAutomator] Moving to state 'stopping' 2017-03-24 11:25:06:360 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got data from client: {"cmd":"shutdown"} 2017-03-24 11:25:06:361 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got command of type SHUTDOWN
===========================================================================================

f = open('log.txt','r')
print(f)
for i in f:
print(i.strip())
f.close()

输出：
<_io.TextIOWrapper name='log.txt' mode='r' encoding='cp936'>
2017-03-24 11:25:06:349 - info: [debug] [AndroidBootstrap] Sending command to android: {"cmd":"shutdown"} 2017-03-24 11:25:06:355 - info: [debug] [AndroidBootstrap] Received command result from bootstrap 2017-03-24 11:25:06:356 - info: [debug] [UiAutomator] Shutting down UiAutomator 2017-03-24 11:25:06:357 - info: [debug] [UiAutomator] Moving to state 'stopping' 2017-03-24 11:25:06:360 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got data from client: {"cmd":"shutdown"} 2017-03-24 11:25:06:361 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got command of type SHUTDOWN
===========================================================================================

n = 0
f = open('log.txt','r')
for i in f:
if n == 3:
i = ''.join([i.strip(),' this is line 4'])
print(i.strip())
n += 1
f.close()

输出：
2017-03-24 11:25:06:349 - info: [debug] [AndroidBootstrap] Sending command to android: {"cmd":"shutdown"}
2017-03-24 11:25:06:355 - info: [debug] [AndroidBootstrap] Received command result from bootstrap
2017-03-24 11:25:06:356 - info: [debug] [UiAutomator] Shutting down UiAutomator
2017-03-24 11:25:06:357 - info: [debug] [UiAutomator] Moving to state 'stopping' this is line 4
2017-03-24 11:25:06:360 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got data from client: {"cmd":"shutdown"}
2017-03-24 11:25:06:361 - info: [AndroidBootstrap] [BOOTSTRAP LOG] [debug] Got command of type SHUTDOWN
===========================================================================================

tell和seek

f = open('log.txt','r')
print(f.read(25))
print(f.tell())  #取出光标所在位置
print(f.seek(0))  #移动光标到指定的位置
print(f.read(50))
f.close()

输出：
2017-03-24 11:25:06:349 -
25
0
2017-03-24 11:25:06:349 - info: [debug] [AndroidBo
注意：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；而seek和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.
===========================================================================================

flush：同步把数据从缓存移动到磁盘上去

#进度条实例
import sys,time
for i in range(30):
sys.stdout.write("*")
sys.stdout.flush()
time.sleep(0.1)

#print的flush
import sys,time
for i in range(30):
print('*',end='',flush=True)
time.sleep(0.1)

===========================================================================================
其他扩展：

#truncate（）：截断数据(不能在r模式下)
#在w模式下：先清空，再写，再截断
#在a模式下：直接将指定位置后的内容截断

# r+:光标默认在0位置开始写，从0开始覆盖数据
# w+:先清空，再写读
# a+:光标默认在最后位置

===========================================================================================
with：

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.txt','r') as f:
print(f.readline())

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：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。

#with 同时管理多个文件对象
with open('log1','r') as f_read, open('log2','w') as f_write:
for line in f_read:
f_write.write(line)