从零学python系列之从文件读取和保存数据

在HeadFirstPython网站中下载所有文件，解压后以chapter 3中的“sketch.txt”为例：

新建IDLE会话，首先导入os模块，并将工作目录却换到包含文件“sketch.txt”的文件夹，如C:\\Python33\\HeadFirstPython\\chapter3

代码如下:

>>> import os
>>> os.getcwd() #查看当前工作目录
'C:\\Python33'
>>> os.chdir('C:/Python33/HeadFirstPython/chapter3') #切换包含数据文件的文件夹
>>> os.getcwd() #查看切换后的工作目录
'C:\\Python33\\HeadFirstPython\\chapter3'

打开文件“sketch.txt”，读取并显示前两行：

代码如下:

>>> data=open('sketch.txt')
>>> print(data.readline(),end='')
Man: Is this the right room for an argument?
>>> print(data.readline(),end='')
Other Man: I've told you once.

回到文件起始位置，使用for语句处理文件中的每行，最后关闭文件：

代码如下:

>>> data.seek(0) #使用seek()方法回到文件起始位置
>>> for each_line in data:
print(each_line,end='')

Man: Is this the right room for an argument?
Other Man: I've told you once.
Man: No you haven't!
Other Man: Yes I have.
Man: When?
Other Man: Just now.
Man: No you didn't!
Other Man: Yes I did!
Man: You didn't!
Other Man: I'm telling you, I did!
Man: You did not!
Other Man: Oh I'm sorry, is this a five minute argument, or the full half hour?
Man: Ah! (taking out his wallet and paying) Just the five minutes.
Other Man: Just the five minutes. Thank you.
Other Man: Anyway, I did.
Man: You most certainly did not!
Other Man: Now let's get one thing quite clear: I most definitely told you!
Man: Oh no you didn't!
Other Man: Oh yes I did!
Man: Oh no you didn't!
Other Man: Oh yes I did!
Man: Oh look, this isn't an argument!
(pause)
Other Man: Yes it is!
Man: No it isn't!
(pause)
Man: It's just contradiction!
Other Man: No it isn't!
Man: It IS!
Other Man: It is NOT!
Man: You just contradicted me!
Other Man: No I didn't!
Man: You DID!
Other Man: No no no!
Man: You did just then!
Other Man: Nonsense!
Man: (exasperated) Oh, this is futile!!
(pause)
Other Man: No it isn't!
Man: Yes it is!
>>> data.close()

读取文件后，将不同role对应数据分别保存到列表man和other：

代码如下:

import os
print(os.getcwd())
os.chdir('C:\Python33\HeadFirstPython\chapter3')
man=[] #定义列表man接收Man的内容
other=[] #定义列表other接收Other Man的内容

try:
data=open("sketch.txt")
for each_line in data:
try:
(role, line_spoken)=each_line.split(':', 1)
line_spoken=line_spoken.strip()
if role=='Man':
man.append(line_spoken)
elif role=='Other Man':
other.append(line_spoken)
except ValueError:
pass
data.close()
except IOError:
print('The datafile is missing!')
print (man)
print (other)

Tips:

使用open()方法打开磁盘文件时，默认的访问模式为r，表示读，不需要特意指定；

要打开一个文件完成写，需要指定模式w，如data=open("sketch.txt","w")，如果该文件已经存在则会清空现有内容；

要追加到一个文件，需要指定模式a，不会清空现有内容；

要打开一个文件完成写和读，且不清空现有内容，需要指定模式w+；

例如，将上例中保存的man和other内容以文件方式保存时，可修改如下：

代码如下:

import os
print(os.getcwd())
os.chdir('C:\Python33\HeadFirstPython\chapter3')
man=[]
other=[]

try:
data=open("sketch.txt")
for each_line in data:
try:
(role, line_spoken)=each_line.split(':', 1)
line_spoken=line_spoken.strip()
if role=='Man':
man.append(line_spoken)
elif role=='Other Man':
other.append(line_spoken)
except ValueError:
pass
data.close()
except IOError:
print('The datafile is missing!')

try:
man_file=open('man.txt', 'w') #以w模式访问文件man.txt
other_file=open('other.txt','w') #以w模式访问文件other.txt
print (man, file=man_file) #将列表man的内容写到文件中
print (other, file=other_file)
except IOError:
print ('File error')
finally:
man_file.close()
other_file.close()

但是第26行print()为什么会报错？“syntax error while detecting tuple”，有大神能给解惑一下不