在Python中到达文件末尾时readline返回什么值？

Question

One can use the classic loop

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

while line:
    line = file_in.readline()

to read through a file line-by-line in Python.

But what value does 'line' have when the loop exits? The Python 3 docs only read:

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.

Edit :

In my version of Python (3.6.1), if you open a file in binary mode, help(file_in.readline) gives

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.

which is exactly the same as the docs quoted above . But, as noted by Steve Barnes , if you open the file in text mode, you get a useful comment. (Oops! Copy-paste error on my part)

Answer 1

From the tutorial: 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 2

In a python console opening a file, f , and then calling help on its readline method tells you exactly:

>>> 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.

Each readline operates on the remainder of the file from the current point onward so will eventually hit an EOF.

Note that if you open the file in binary mode, with rb rather than r , then rather than a <class '_io.TextIOWrapper'> object you will get a <class '_io.BufferedReader'> object - then the help message is different:

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.

And when this method reaches the EOF it will return an empty byte array, b'' rather than an empty string.

Note that all the above was tested with python 3.6 on Win10.

Answer 3

Running the code snippet from the question in a (Python 3) console shows that it returns an empty string, or an empty Bytes object if opening the file in binary mode.

Is this documented somewhere? Perhaps it is sort of a broad python standard?