Python 的函数 readlines(n) 行为 [英] Python's function readlines(n) behavior

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.

If given an optional parameter sizehint, it reads that many bytes from the file and enough more to complete a line, and returns the lines from that.

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.

Read until EOF using readline() and return a list containing the lines thus read. If the optional sizehint argument is present, instead of reading up to EOF, whole lines totalling approximately sizehint bytes (possibly after rounding up to an internal buffer size) are read. Objects implementing a file-like interface may choose to ignore sizehint if it cannot be implemented, or cannot be implemented efficiently.

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IT屋！