Bugs item #1396471, was opened at 2006-01-03 20:53
Message generated for change (Comment added) made by tim_one
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Category: Windows
Group: Python 2.4
Status: Closed
Resolution: Fixed
Priority: 7
Submitted By: Tom Goddard (tom_goddard)
Assigned to: Nobody/Anonymous (nobody)
Summary: file.tell() returns illegal value on Windows
Initial Comment:
The file tell() method returns an illegal value that causes an exception
when passed to seek(). This happens on Windows when a file that
contains unix-style newlines '\n' is opened and read in text mode 'r'.
Below is code that produces the problem on Windows 2.4.2 in an IDLE
shell.
The bug does not occur when using mode 'rU' or 'rb'. But I expect
correct behaviour with mode 'r'. My understanding is that 'rU'
translates line endings to '\n' in the returned string while mode 'r' still
correctly reads the lines using readline(), recognizing all 3 common
endline conventions, but does not translate (ie includes \n\r or \r or \n
in returned string).
The error in the tell() return value depends on how long the file is.
Changing the 'more\n'*10 line in the example code will cause different
incorrect return values.
Previous bug reports have mentioned problems with tell/seek when
using file iterators, the file.next() method and the "for line in file:"
construct. This bug is different and just involves readline() and tell()
with mode 'r'.
Example code tellbug.py follows:
wf = open('testdata', 'wb')
wf.write('01234\n56789\n'+ 'more\n'*10)
wf.close()
f = open('testdata', 'r')
f.readline()
t = f.tell()
print t
f.seek(t)
-------
Running gives:
>>> execfile('tellbug.py')
-5
Traceback (most recent call last):
File "<pyshell#14>", line 1, in -toplevel-
execfile('tellbug.py')
File "tellbug.py", line 9, in -toplevel-
f.seek(t)
IOError: [Errno 22] Invalid argument
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>Comment By: Tim Peters (tim_one)
Date: 2006-01-20 10:35
Message:
Logged In: YES
user_id=31435
Well, it's not a Windows bug. Platforms are allowed to
insist on just about any restrictions they like for text
files, and it's on the user's head if they open a non-text
file in text mode. The C standard leaves all behavior
undefined in such cases. Text files on Windows have \r\n
line ends, and Windows isn't required to do anything in
particular if a file with even one \n line end is opened in
text mode. It so happens that a lot of things work the same
way they work on Linux anyway, but not all.
----------------------------------------------------------------------
Comment By: Georg Brandl (birkenfeld)
Date: 2006-01-20 04:15
Message:
Logged In: YES
user_id=1188172
Documented as a Windows bug in revisions 42102,42103.
----------------------------------------------------------------------
Comment By: Peter van Kampen (pterk)
Date: 2006-01-15 22:24
Message:
Logged In: YES
user_id=174455
I have done some additional research on this.
It appears the bug is in ftell of the windows c-library and
it seems to be a 'known problem'. See:
http://www.mathworks.com/access/helpdesk/help/techdoc/ref/ftell.html
More here:
http://www.cygwin.com/faq/faq.api.html
http://gnuwin32.sourceforge.net/compile.html (search for
ftell in both cases)
Also see:
http://sourceforge.net/tracker/index.php?func=detail&aid=1381717&group_id=5470&atid=105470
So even though the file isn't opened with 'rt' I gather from
Tim Peter's remark about 't' on windows that the default is
't' anyway unless you really try.
So this example is *exactly* what the matlab-people found.
That is nice to know I guess but it doesn't really help to
solve the problem.
It seems to me there are three solutions.
1. ftell could be fixed for unix-files on windows.* Not very
likely to happen soon.
2. Python could special-case this for this specific
scenario. Way beyond my capabilities and probably too much
hassle in any case especially since the 'fix' is so easy
(just open with 'rb').
3. Leave it as is but document it. I have submitted a
(doc-)patch 1407021
(http://sourceforge.net/tracker/index.php?func=detail&aid=1407021&group_id=5470&atid=305470)
for the library reference (libstdtypes.tex) that reads:
Please note that tell() can return illegal values (after an
fgets()) on Windows when reading files with UNIX-style
line-endings. Use 'rb'-mode to circumvent this
problem.
* MS provides this note in the compatibility section of the
c-runtime libraries: "Note: In this version of Visual C++,
UNIX compatibility information has been removed from the
function descriptions."
In case your wondering what 'this version' is: VS.NET and VS
6.0. Apperantly there were no other 'previous versions'.
----------------------------------------------------------------------
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