跟老齐学Python之不要红头文件(1)
这两天身体不给力，拖欠了每天发讲座的约定，看官见谅。
红头文件，是某国特别色的东西，在python里不需要，python里要处理的是计算机中的文件，包括文本的、图片的、音频的、视频的等等，还有不少没见过的扩展名的，在linux中，不是所有的东西都被保存到文件中吗？文件，在python中，是一种对象，就如同已经学习过的字符串、数字等一样。
先要在交互模式下查看一下文件都有哪些属性：
>>> dir(file) ['__class__', '__delattr__', '__doc__', '__enter__', '__exit__', '__format__', '__getattribute__', '__hash__', '__init__', '__iter__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'close', 'closed', 'encoding', 'errors', 'fileno', 'flush', 'isatty', 'mode', 'name', 'newlines', 'next', 'read', 'readinto', 'readline', 'readlines', 'seek', 'softspace', 'tell', 'truncate', 'write', 'writelines', 'xreadlines']
然后对部分属性进行详细说明，就是看官学习了。
打开文件
在某个文件夹下面建立了一个文件，名曰：130.txt，并且在里面输入了如下内容：
<code>learn python http://qiwsir.github.io qiwsir@gmail.com</code>
此文件以供三行。
下图显示了这个文件的存储位置：
在上面截图中，我在当前位置输入了python（我已经设置了环境变量，如果你没有，需要写全启动python命令路径），进入到交互模式。在这个交互模式下，这样操作：
>>> f = open("130.txt") #打开已经存在的文件
>>> for line in f:
... print line
...
learn python
http://qiwsir.github.io
qiwsir@gmail.com
将打开的文件，赋值个变量f，这样也就是变量f跟对象文件130.txt用线连起来了（对象引用）。
接下来，用for来读取文件中的内容，就如同读取一个前面已经学过的序列对象一样，如list、str、tuple，把读到的文件中的每行，赋值给变量line。也可以理解为，for循环是一行一行地读取文件内容。每次扫描一行，遇到行结束符号\n表示本行结束，然后是下一行。
从打印的结果看出，每一样跟前面看到的文件内容中的每一行是一样的。只是行与行之间多了一空行，前面显示文章内容的时候，没有这个空行。或许这无关紧要，但是，还要深究一下，才能豁然。
在原文中，每行结束有本行结束符号\n，表示换行。在for语句汇总，print line表示每次打印完line的对象之后，就换行，也就是打印完line的对象之后会增加一个\n。这样看来，在每行末尾就有两个\n，即：\n\n，于是在打印中就出现了一个空行。
>>> f = open('130.txt')
>>> for line in f:
... print line, #后面加一个逗号，就去掉了原来默认增加的\n了，看看，少了空行。
...
learn python
http://qiwsir.github.io
qiwsir@gmail.com
在进行上述操作的时候，有没有遇到这样的情况呢？
>>> f = open('130.txt')
>>> for line in f:
... print line,
...
learn python
http://qiwsir.github.io
qiwsir@gmail.com
>>> for line2 in f: #在前面通过for循环读取了文件内容之后，再次读取，
... print line2 #然后打印，结果就什么也显示，这是什么问题？
...
>>>
如果看官没有遇到上面问题，可以试试。遇到了，这就解惑。不是什么错误，是因为前一次已经读取了文件内容，并且到了文件的末尾了。再重复操作，就是从末尾开始继续读了。当然显示不了什么东西，但是python并不认为这是错误，因为后面就会讲到，或许在这次读取之前，已经又向文件中追加内容了。那么，如果要再次读取怎么办？就从新来一边好了。
特别提醒看官，因为当前的交互模式是在该文件所在目录启动的，所以，就相当于这个实验室和文件130.txt是同一个目录，这时候我们打开文件130.txt，就认为是在本目录中打开，如果文件不是在本目录中，需要写清楚路径。
比如：在上一级目录中（~/Documents/ITArticles/BasicPython），加入我进入到那个目录中，运行交互模式，然后试图打开130.txt文件。
>>> f = open("130.txt")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory: '130.txt'
>>> f = open("./codes/130.txt") #必须得写上路径了（注意，windows的路径是\隔开，需要转义。对转义符，看官看以前讲座）
>>> for line in f:
... print line
...
learn python
http://qiwsir.github.io
qiwsir@gmail.com
>>>
创建文件
上面的实验中，打开的是一个已经存在的文件。如何创建文件呢？
>>> nf = open("131.txt","w")
>>> nf.write("This is a file")
就这样创建了一个文件？并写入了文件内容呢？看看再说：
真的就这样创建了新文件，并且里面有那句话呢。
看官注意了没有，这次我们同样是用open()这个函数，但是多了个"w"，这是在告诉python用什么样的模式打开文件。也就是说，用open()打开文件，可以有不同的模式打开。看下表：
从表中不难看出，不同模式下打开文件，可以进行相关的读写。那么，如果什么模式都不写，像前面那样呢？那样就是默认为r模式，只读的方式打开文件。
>>> f = open("130.txt")
>>> f
<open file '130.txt', mode 'r' at 0xb7530230>
>>> f = open("130.txt","r")
>>> f
<open file '130.txt', mode 'r' at 0xb750a700>
可以用这种方式查看当前打开的文件是采用什么模式的，上面显示，两种模式是一样的效果。下面逐个对各种模式进行解释
"w":以写方式打开文件，可向文件写入信息。如文件存在，则清空该文件，再写入新内容
131.txt这个文件是存在的，前面建立的，并且在里面写了一句话：This is a file
>>> fp = open("131.txt")
>>> for line in fp: #原来这个文件里面的内容
... print line
...
This is a file
>>> fp = open("131.txt","w") #这时候再看看这个文件，里面还有什么呢？是不是空了呢？
>>> fp.write("My name is qiwsir.\nMy website is qiwsir.github.io") #再查看内容
>>> fp.close()
查看文件内容：
$ cat 131.txt #cat是linux下显示文件内容的命令，这里就是要显示131.txt内容 My name is qiwsir. My website is qiwsir.github.io
"a":以追加模式打开文件（即一打开文件，文件指针自动移到文件末尾），如果文件不存在则创建
>>> fp = open("131.txt","a")
>>> fp.write("\nAha,I like program\n") #向文件中追加
>>> fp.close() #这是关闭文件，一定要养成一个习惯，写完内容之后就关闭
查看文件内容：
$ cat 131.txt My name is qiwsir. My website is qiwsir.github.io Aha,I like program
其它项目就不一一讲述了。看官可以自己实验。
本讲先到这里，明天继续文件。感冒药吃了，昏昏欲睡。
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