文件基本操作
Python从文本读取数据时，一次会到达一个数据行。
sketch.txt文件
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!
需求：读取文件，将上面的对话都加上said
代码如下：
# coding:utf-8
import os # 查看当前工作目录
print(os.getcwd())
# 切换为包含数据文件的文件夹
os.chdir('../../HeadFirstPython/chapter03_except')
# 再次查看当前工作目录
print(os.getcwd()) # 打开文本
data = open('sketch.txt')
# 获取一个数据行，打印
print(data.readline(), end='')
# 再次获取一个数据行，打印
print(data.readline(), end='') # 回到文件原始位置
data.seek(0)
# data.tell() # 等价
for each_line in data:
# 当每一行都存在冒号时，执行if中的操作
if not each_line.find(':') == -1:
# 遇到冒号就切割字符串，返回一个字符串列表，赋至一个目变量元组，则称为多重赋值
# 由于一行字符串中可能有多个: 所以我们需要在使用split方法maxsplit参数
(role, line_spoken) = each_line.split(":", 1)
print(role, end='')
print(' said:', end='')
print(line_spoken, end='') # 关闭文件
data.close()
但是这段代码过于脆弱，但文件格式发生变化或者其他异常情况，还是出问题，这时需要用到异常机制。
处理异常
try/except机制
# coding:utf-8 # 如果文件不存在，异常处理
try:
data = open('sketch.txt')
for each_line in data:
try:
(role, line_spoken) = each_line.split(":", 1)
print(role, end='')
print(' said:', end='')
print(line_spoken, end='')
except:
# 如果出现异常，pass，继续执行代码
pass
data.close()
# 想要捕捉特性的异常，需要制定错误类型
except IOError:
print("the data file is missing!")
# 其他所有的异常
except:
print("the data file is broken!")
其他：
判断文件是否存在：
os.path.exists('sketch.txt')