On a fresh 64-bit install of Windows 2.7.3 running on Windows 7, f.tell() is producing unexpected (and unusable) results:

>>> f = open('sample.txt')
>>> f.read(3)
>>> f.read(3)
>>> f.tell()
-5L

What tell() returns before any reading? What about non-buffered files?

FWIW, on Windows only, "open('foo', 'a').tell()" always returns 0, which I think is broken too.  It usually works anyway, because it updates the position before the first .write().  But it does not work in case the writing occurs in another process, like described here:

http://stackoverflow.com/questions/13821708/

(Tested only on Python 2.6.2, but according to the link above, Python 2.7.3 has the same problem.)

What is in sample.txt? I cannot reproduce this with a source build
(Windows 7, 64-bit pgo build):

Python 2.7.3+ (default, Jan 17 2013, 20:26:24) [MSC v.1500 64 bit (AMD64)] on win32                          
Type "help", "copyright", "credits" or "license" for more information.                                       
>>> f = open('sample.txt')                                                                                   
>>> f.read(3)                                                                                                
'abc'                                                                                                        
>>> f.read(3)                                                                                                
'def'                                                                                                        
>>> f.tell()                                                                                                 
6L

I am afraid I cannot reproduce with the 2.7.3 64-bit installer either:

C:\>python27\python.exe
Python 2.7.3 (default, Apr 10 2012, 23:24:47) [MSC v.1500 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> f = open("sample.txt")
>>> f.read(3)
'xxx'
>>> f.read(3)
'xxx'
>>> f.tell()
6L

Could it be that Raymond's file had Unix line endings?

I tried to reproduce this, picked a file on my disk and indeed I got a negative number, but that file has Unix line endings. This is documented at http://docs.python.org/2/library/stdtypes.html#file.tell so probably there's nothing to do then.

As for Armin's report in msg180145, even though it's not intuitive, this matches ftell's behavior on Windows, as documented in the Remarks section of http://msdn.microsoft.com/en-us/library/0ys3hc0b%28v=vs.100%29.aspx. The tell() method on fileobjects is explicitly documented as matching ftell behavior: "Return the file’s current position, like stdio‘s ftell()". So even though it's not intuitive at all, it's probably better to leave it as is. tell() returns the intuitive non zero position when opening with 'a' on Python3 and on Python 2.7 when using io.open so it's fixed for the future anyway.

I can't find any mention of this behaviour in python3's documentation, nor any reference to ftell(). Is it only well hidden or was it deleted by accident?

Giacomo, which behaviour are you talking about?
FWIW, there is no such tell() issue in Python 3 and it doesn't use ftell().

I can reproduce a similar behaviour, but instead of negative values I obtain huge values(which may as well be a "negative" unsigned converted to a python int).

See this stackoverflow question: http://stackoverflow.com/questions/15934950/python-file-tell-giving-strange-numbers

If it doesn't use ftell() then this might mean that there is a new bug in the current implementation? I can reproduce the results of the questioner, on a windows7 machine with python3.3.0, with the following code:

    with open('C:/Users/Giacomo/test', 'wb') as f:
        f.write(b'hello\r\n\r\n-data1:blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah\r\n\r\n\r\n-data2:blah blah blah blah blah blah blah blah blah blah blah\r\n-data3: Empty\r\n\r\n-data4: Empty')
    with open('C:/Users/Giacomo/test', 'rt') as f:
        for line in iter(f.readline, ''):
            print(f.tell())

Which outputs:

    7
    9
    18446744073709551714
    99
    101
    164
    179
    181
    194

As I explained in my answer I thought the 1844... was due to the "ftell() bug", but if that's not it I'm at a loss. Could you explain this results with the current implementation?

By the way: it seems to be a python3.3 bug, since python3.2.3 is not affected.

Giacomo, that's not related to the 2.7 problem. In Python 3, the result of f.tell() is not a file position, but a special value that tells the decoder to pick up when you seek().

The documentation for python 3.3.1 states, at http://docs.python.org/3/tutorial/inputoutput.html#reading-and-writing-files, states:

    f.tell() returns an integer giving the file object’s current position in the file, **measured in bytes from the beginning of the file**.

And regarding files opened in text mode the only statement is:

    In text files (those opened without a b in the mode string), only seeks relative to the beginning of the file are allowed (the exception being seeking to the very file end with seek(0, 2)).

Even though in the io module it is documented that it may not be the number of bytes.

I think the former documentation should be changed, since it's simply wrong. Sorry for the disturb, but in the beginning I thought it was related.

Attaching a patch with a doc update to the tutorial, specifying that

1. The return value from f.tell is an opaque number for text files.
2. When seeking using text files the offset value needs to come from f.tell(). http://docs.python.org/3.4/library/io.html#io.TextIOBase.seek

New changeset 81bc2d64c006 by R David Murray in branch '3.3':
#16273: Fix tutorial discussion of seek/tell (opaque text-mode values).
http://hg.python.org/cpython/rev/81bc2d64c006

New changeset c7d9a2159c6c by R David Murray in branch 'default':
Merge: #16273: Fix tutorial discussion of seek/tell (opaque text-mode values).
http://hg.python.org/cpython/rev/c7d9a2159c6c

Committed the doc patch.  Thanks Sijin.