• read：如果指定了参数 size，就按照该指定长度从文件中读取内容，否则，就读取全文。被读出来的内容，全部塞到一个字符串里面。这样有好处，就是东西都到内存里面了，随时取用；但如果文件内容太多了，内存会吃不消

• readline：可选参数 size 的含义同上。它是以行为单位返回字符串，也就是每次读一行，依次循环，如果不限定 size，直到最后一个返回的是空字符串，意味着到文件末尾了(EOF)。

• readlines：size 同上。它返回的是以行为单位的列表，即相当于先执行 readline() ，得到每一行，然后把这一行的字符串作为列表中的元素塞到一个列表中，最后将此列表返回。

分别用上述三种函数读取这个文件：

一、read

>>> f = open(“you.md”)
>>> content = f.read()
>>> content
‘You Raise Me Up\nWhen I am down and, oh my soul, so weary;\nWhen troubles come and my heart burdened be;\nThen, ……(此处省略)’
>>> f.close()

得到的就是一个很大的字符串，因为原文中有换行，所以这个字符串里面包含着一些符号 \n 。如果用 print 输出这个字符串，其中的\n 起作用了：

>>> print content
You Raise Me Up
When I am down and, oh my soul, so weary;
When troubles come and my heart burdened be;
Then, I am still and wait here in the silence,
Until you come and sit awhile with me.
You raise me up, so I can stand on mountains;
You raise me up, to walk on stormy seas;
I am strong, when I am on your shoulders;
You raise me up: To more than I can be.

二、readline()

>>> f = open(“you.md”)
>>> f.readline()
‘You Raise Me Up\n’
>>> f.readline()
‘When I am down and, oh my soul, so weary;\n’
>>> f.readline()
‘When troubles come and my heart burdened be;\n’
>>> f.close()

每操作一次f.readline() ，就读取一行，并且将指针向下移动一行，如此循环。这是一种循环/可迭代。因此，可以用循环语句来完成对全文的读取。

#!/usr/bin/env Python
# coding=utf-8
f = open(“you.md”)
while True:
line = f.readline()
if not line: #到 EOF，返回空字符串，则终止循环
break
print line ,
f.close()

三、readlines()

>>> f = open(“you.md”)
>>> content = f.readlines()
>>> content
[‘You Raise Me Up\n’, ‘When I am down and, oh my soul, so weary;\n’, ‘When troubles come and my heart burdened be;\n’,….(此处省略)]

返回的是一个列表，列表中每个元素都是一个字符串，每个字符串中的内容就是文件的一行文字，含行末的符号。

它是可以用 for 来循环的
>>> for line in content:
… print line ,
…
You Raise Me Up
When I am down and, oh my soul, so weary;
When troubles come and my heart burdened be;
Then, I am still and wait here in the silence,
Until you come and sit awhile with me.
You raise me up, so I can stand on mountains;
You raise me up, to walk on stormy seas;
I am strong, when I am on your shoulders;
You raise me up: To more than I can be.
>>> f.close()

四、读取大文件

如果文件太大，就不能用 read() 或者 readlines() 一次性将全部内容读入内存，可以使用 while 循环和 readlin() 来完成这个任务。

此外，还可以使用 fileinput 模块：
>>> import fileinput
>>> for line in fileinput.input(“you.md”):
… print line ,
…
You Raise Me Up
When I am down and, oh my soul, so weary;
When troubles come and my heart burdened be;
Then, I am still and wait here in the silence,
Until you come and sit awhile with me.
You raise me up, so I can stand on mountains;
You raise me up, to walk on stormy seas;
I am strong, when I am on your shoulders;
You raise me up: To more than I can be.

还有一种方法，更为常用：
>>> for line in f:
… print line ,
…
You Raise Me Up
When I am down and, oh my soul, so weary;
When troubles come and my heart burdened be;
Then, I am still and wait here in the silence,
Until you come and sit awhile with me.
You raise me up, so I can stand on mountains;
You raise me up, to walk on stormy seas;
I am strong, when I am on your shoulders;
You raise me up: To more than I can be.
之所以能够如此，是因为 file 是可迭代的数据类型，直接用 for 来迭代即可