Bug? On WindowsNT, f.seek() and f.tell() aren't symmetric

[Peter Stoehr]

Oliver Steele schrieb:
> 
> My reading of the Python Library documentation
> <http://www.python.org/doc/lib/bltin-file-objects.html> is that,
> regardless of file modes, f.seek(f.tell()) should be a noop (assuming f
> is bound to an open file):
>   f.seek(0); f.readline(); print `f.readline()`
> and
>   f.seek(0); f.readline(); f.seek(f.tell()); print `f.readline()`
> should print the same value.
> 
> Not so. On a Windows machine (specifically, an SGI running WindowsNT and
> Python 1.5.2 final), the function below prints 'a\012' for the first
> line and '\012' for the second.  Changing the file open mode to 'rb' or
> running the program on a Mac or UNIX machine works fine.
> 
> Note that it's not the fact that different platforms yield different results
> that's an issue -- that's expected, given the platform-dependent file that
> the write() statement creates.  It's the fact that that there's any platform
> such that the two lines below labeled 'line A' and 'line B' yield different
> results on the platform, that disagrees with my understanding of the Python
> documentation.
> 
> The upshot of this is that it's possible to develop a Python program under
> MacOS or UNIX, using the documentation available on those platforms, that
> fails under NT -- in other words, it demotes Python to the status of a
> 'write once test everywhere language'.  I ran across this in code I
> developed on the Mac as an interface to the WordNet lexical database; the
> fix was to open the local files in 'rb' mode on Windows, and 'r' mode
> elsewhere.
> 
> def testseek():
>     # create a two-line file
>     f = open('test.txt', 'wb')
>     f.write('a\nb\n')
>     f.close()
> 
>     # read it
>     f = open('test.txt', 'r')
>     f.seek(0); f.readline(); print `f.readline()` # line A
>     f.seek(0); f.readline(); f.seek(f.tell()); print `f.readline()` # line B
>     f.close()
> 
> NT>>> testseek()
> 'a\012'
> '\012'
> 
> UNIX>>> testseek()
> 'b\012'
> 'b\012'
> 
> MacOS>>> testseek()
> ''
> ''

Windows NT SP4 / Intel PII 266 
Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> def testseek():
    # create a two-line file
    f = open('test.txt', 'wb')
    f.write('a\nb\n')
    f.close()

    # read it
    f = open('test.txt', 'r')
    f.seek(0); f.readline(); print `f.readline()` # line A
    f.seek(0); f.readline(); f.seek(f.tell()); print `f.readline()` #
line B
    f.close()


>>> testseek()
'b\012'
'\012'

-- 
Dr. Peter Stoehr 		mailto:peter.stoehr at sdm.de
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