Python 读取文本时的 read/readline/readlines

淡淡的烟草味﹌ 2022-05-31 06:29 181阅读 0赞

• 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