when does newlines get set in universal newlines mode?

[Peter Otten]

arekfu at gmail.com wrote:

> Hi all,
> 
> I have a text file with Windows-style line terminators (\r\n) which I open
> in universal newlines mode (Python 2.7). I would expect the newlines
> attribute to be set after the first call to the readline() method, but
> apparently this is not the case:
> 
>>>> f=open('test_crlf', 'rU')
>>>> f.newlines
>>>> f.readline()
> 'foo\n'
>>>> f.newlines
>>>> f.readline()
> 'bar\n'
>>>> f.newlines
> '\r\n'
> On the other hand, the newlines attribute gets set after the first call to
> readline() on a file with Unix-style line endings.
> 
> Is this a bug or a feature?

According to

https://docs.python.org/2.7/library/functions.html#open

"""
If Python is built without universal newlines support a mode with 'U' is the 
same as normal text mode. Note that file objects so opened also have an 
attribute called newlines which has a value of None (if no newlines have yet 
been seen), '\n', '\r', '\r\n', or a tuple containing all the newline types 
seen.
"""

I tried:

>>> with open("tmp.txt", "wb") as f: f.write("alpha\r\nbeta\rgamma\n")
... 
>>> f = open("tmp.txt", "rU")
>>> f.newlines
>>> f.readline()
'alpha\n'
>>> f.newlines 
# expected: '\r\n'
>>> f.readline()
'beta\n'
>>> f.newlines
'\r\n' # expected: ('\r', '\r\n')
>>> f.readline()
'gamma\n'
>>> f.newlines
('\r', '\n', '\r\n')

I believe this is a bug.