iterating over the lines of a file - difference between Python 2.7 and 3?
wolfgang.maier at biologie.uni-freiburg.de
Thu Jan 17 17:38:30 CET 2013
for this very helpful reply and for pointing out _pyio.py to me!
It's great to be able to check implementation details sometimes. So, if I understand you correctly, I can simply
and open files with io.open() - instead of open and although this is a bit a detour in Python3 - and this will ensure version-independent behavior of my code? That´s cool!
What will my IO object return then when I read from it in Python 2.7? str where Python3 gives bytes, and unicode instead of str ? This is what I understood from the Python 2.7 io module doc.
From: Peter Otten [mailto:__peter__ at web.de]
Sent: Thursday, January 17, 2013 1:04 PM
To: python-list at python.org
Subject: Re: iterating over the lines of a file - difference between Python 2.7 and 3?
You can get the Python 3 behaviour with io.open() in Python 2.7. There is an implementation in Python in _pyio.py:
return _BufferedIOMixin.tell(self) - len(self._read_buf) + self._read_pos
Wolfgang Maier wrote:
> I just came across an unexpected behavior in Python 3.3, which has to
> do with file iterators and their interplay with other methods of
> file/IO class methods, like readline() and tell(): Basically, I got
> used to the fact that it is a bad idea to mix them because the
> iterator would use that hidden read-ahead buffer, so what you got with
> subsequent calls to
> readline() or tell() was what was beyond that buffer, but not the next
> thing after what the iterator just returned.
> for line in in_file_object:
> print (line)
> if in_file_object.tell() > 300:
> # assuming that individual lines are
> # shorter
> This wouldn´t print anything in Python 2.7 since next(in_file_object)
> would read ahead beyond the 300 position immediately, as evidenced by
> a subsequent call to in_file_object.tell() (returning 8192 on my system).
> However, I find that under Python 3.3 this same code works: it prints
> some lines from my file and after completing in_file_object.tell()
> returns a quite reasonable 314 as the current position in the file.
> I couldn´t find this difference anywhere in the documentation. Is the
> 3.3 behavior official, and if so, when was it introduced and how is it
> implemented? I assume the read-ahead buffer still exists?
> By the way, the 3.3 behavior only works in binary mode. In text mode, the
> code will raise an OSError: telling position disabled by next() call. In
> Python 2.7 there was no difference between the binary and text mode
> behavior. Could not find this documented either.
More information about the Python-list