在Python中，f.readlines（）和list（f）之间的区别是什么？ [英] In Python, what is the difference between f.readlines() and list(f)

问题描述

从 Python2教程和< Python3教程中，有一条线在中点第7.2.1节说：

如果您要读取文件的所有行在列表中，也可以使用 list（f）或 f.readlines（）。所以我的问题是：这两种方法将文件对象转换为列表有什么区别？我很好奇在性能方面和在Python对象实现下（也许是Python2和Python3之间的区别）。

解决方案

在功能上，没有区别;这两个方法的结果都是完全相同的列表。

明智的做法是使用文件对象作为 iterator （直接调用 next（f）直到 StopIteration 被引发），另一个使用专门的方法读取整个文件。

Python 2和Python 3的区别在于，除非你使用 io.open（）使用隐藏缓冲区用于文件迭代，如果将文件对象迭代和 .readline（）或 .readlines（）来电。
$ b $ io 库（处理Python 3中的所有文件I / O）不使用这样一个隐藏的缓冲区，所有的缓冲都由一个 BufferedIOBase（）来处理， 包装类。实际上， io.IOBase .readlines（）实现将文件对象用作引擎盖下的迭代器 和 TextIOWrapper 迭代委托给 TextIOWrapper.readline ） ，所以 list（f）和 f.readlines（）基本上是同样的东西，真的。

性能方面，即使在Python 2中也没有什么区别，因为瓶颈是文件I / O。你能从磁盘读取多快？在微观层面上，性能可能取决于其他因素，比如操作系统是否已经缓冲了数据，行数是多少。

From both Python2 Tutorial and Python3 Tutorial, there is a line in the midpoint of section 7.2.1 saying:

If you want to read all the lines of a file in a list you can also use list(f) or f.readlines().

So my question is: What is the difference between these two ways to turn a file object to a list? I am curious both in performance aspect and in underneath Python object implementation (and maybe the difference between the Python2 and Python3).

解决方案

Functionally, there is no difference; both methods result in the exact same list.

Implementation wise, one uses the file object as an iterator (calls next(f) repeatedly until StopIteration is raised), the other uses a dedicated method to read the whole file.

Python 2 and 3 differ in what that means, exactly, unless you use io.open() in Python 2. Python 2 file objects use a hidden buffer for file iteration, which can trip you up if you mix file object iteration and .readline() or .readlines() calls.

The io library (which handles all file I/O in Python 3) does not use such a hidden buffer, all buffering is instead handled by a BufferedIOBase() wrapper class. In fact, the io.IOBase.readlines() implementation uses the file object as an iterator under the hood anyway, and TextIOWrapper iteration delegates to TextIOWrapper.readline(), so list(f) and f.readlines() essentially are the same thing, really.

Performance wise, there isn't really a difference even in Python 2, as the bottleneck is file I/O; how quickly can you read it from disk. At a micro level, performance can depend on other factors, such as if the OS has already buffered the data and how long the lines are.

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