问题描述
我已阅读文档，但是 readlines(n) 有什么作用呢?readlines(n) 是指 readlines(3) 或任何其他数字.
I've read the documentation, but what does readlines(n) do? By readlines(n), I mean readlines(3) or any other number.
当我运行 readlines(3) 时，它返回与 readlines() 相同的内容.
When I run readlines(3), it returns same thing as readlines().
推荐答案
可选参数应该表示从文件中读取了多少(大约)字节.文件将被进一步读取，直到当前行结束:
The optional argument should mean how many (approximately) bytes are read from the file. The file will be read further, until the current line ends:
readlines([size]) -> list of strings, each a line from the file.
Call readline() repeatedly and return a list of the lines so read.
The optional size argument, if given, is an approximate bound on the
total number of bytes in the lines returned.
另一个引用:
如果给定一个可选参数 sizehint，它会从文件中读取那么多字节和足够多的字节来完成一行，并从中返回行.
您说得对，它似乎对小文件没有太大作用，这很有趣:
You're right that it doesn't seem to do much for small files, which is interesting:
In [1]: open('hello').readlines()
Out[1]: ['Hello\n', 'there\n', '!\n']
In [2]: open('hello').readlines(2)
Out[2]: ['Hello\n', 'there\n', '!\n']
人们可能认为文档中的以下短语对此进行了解释:
One might think it's explained by the following phrase in the documentation:
使用 readline() 读取直到 EOF 并返回包含所读取行的列表.如果存在可选的 sizehint 参数，则不会读取到 EOF，而是读取总计大约 sizehint 字节(可能在向上舍入到内部缓冲区大小之后)的整行.实现类文件接口的对象可能会选择忽略 sizehint，如果它无法实现，或者无法有效实现.
然而，即使我尝试在没有缓冲的情况下读取文件，它似乎也没有改变任何东西，这意味着某种其他类型的内部缓冲区:
However, even when I try to read the file without buffering, it doesn't seem to change anything, which means some other kind of internal buffer is meant:
In [4]: open('hello', 'r', 0).readlines(2)
Out[4]: ['Hello\n', 'there\n', '!\n']
在我的系统上，这个内部缓冲区大小似乎约为 5k 字节/1.7k 行:
On my system, this internal buffer size seems to be around 5k bytes / 1.7k lines:
In [1]: len(open('hello', 'r', 0).readlines(5))
Out[1]: 1756
In [2]: len(open('hello', 'r', 0).readlines())
Out[2]: 28080
这篇关于Python 的函数 readlines(n) 行为的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！