在 Python 中到达文件末尾时 readline return 的值是多少？

Question

可以使用经典循环

file_in = open('suppliers.txt', 'r')
line = file_in.readline()

while line:
    line = file_in.readline()

在 Python 中逐行读取文件。

但是当循环退出时'line'有什么价值呢？ Python 3 文档只读：

readline(size=-1)

Read and return one line from the stream. If size is specified, at most size bytes will be read.

The line terminator is always b'\n' for binary files; for text files, the newline argument to open() can be used to select the line terminator(s) recognized.

编辑:

在我的 Python (3.6.1) 版本中，如果以二进制模式打开文件，help(file_in.readline) 会给出

readline(size=-1, /) method of _io.BufferedReader instance

    Read and return a line from the stream.

    If size is specified, at most size bytes will be read.

    The line terminator is always b'\n' for binary files; for text
    files, the newlines argument to open can be used to select the line
    terminator(s) recognized.

与docs quoted above. But, as noted by 完全相同，如果您以文本模式打开文件，您会得到有用的注释。（糟糕！我的复制粘贴错误）

Answer 1

运行 (Python 3) 控制台中问题的代码片段显示它 returns 一个空字符串，或者如果以二进制形式打开文件则为一个空字节对象模式。

这在某处记录了吗？也许它是一种广泛的 python 标准？

Answer 2

来自教程：https://docs.python.org/3.6/tutorial/inputoutput.html#methods-of-file-objects

f.readline() reads a single line from the file; a newline character (\n) is left at the end of the string, and is only omitted on the last line of the file if the file doesn’t end in a newline. This makes the return value unambiguous; if f.readline() returns an empty string, the end of the file has been reached, while a blank line is represented by '\n', a string containing only a single newline.

Answer 3

在 python 控制台中打开一个文件，f，然后在它的 readline 方法上调用 help 准确地告诉你：

>>> f = open('temp.txt', 'w')
>>> help(f.readline)
Help on built-in function readline:

readline(size=-1, /) method of _io.TextIOWrapper instance
    Read until newline or EOF.

    Returns an empty string if EOF is hit immediately.

每个 readline 从当前点开始对文件的剩余部分进行操作，因此最终会遇到 EOF。

请注意，如果您以二进制模式打开文件，使用 rb 而不是 r，那么您将获得 <class '_io.BufferedReader'> 对象而不是 <class '_io.TextIOWrapper'> 对象- 那么帮助信息就不同了：

Help on built-in function readline:

readline(size=-1, /) method of _io.BufferedReader instance
    Read and return a line from the stream.

    If size is specified, at most size bytes will be read.

    The line terminator is always b'\n' for binary files; for text
    files, the newlines argument to open can be used to select the line
    terminator(s) recognized.

当此方法到达 EOF 时，它将 return 空字节数组，b'' 而不是空字符串。

请注意，以上所有内容均在 Win10 上使用 python 3.6 进行了测试。

在 Python 中到达文件末尾时 readline return 的值是多少？

What value does readline return when reaching the end of the file in Python?

python

readline