[New-bugs-announce] [issue36190] file object method .tell() sometimes returns large number when position is right before a line break

[Erik Wennstrom]

New submission from Erik Wennstrom <ewennstr at indiana.edu>:

Sometimes, when the position on a text file object is right before a line break, the file object method .tell() returns a bizarre large number (18446744073709551621) instead of the correct position.

The incorrect behavior occurs consistently for certain text files, but sometimes, a slight modification of the file will cause the behavior to revert to normal.

I can get this behavior in both Python 3.7.2 and 3.6.5. I've seen it on two different Windows X machines.

I've included two sample text files and a program that tests them both with the same code, which opens the file, reads 4 characters from the file, and then prints the result of the .tell() method. Both should print 4, but one of them prints 18446744073709551621. The only difference between the text files is that one of them has a single extra character before the last line break (which I should note is several lines away from the line where the weird behavior occurs).

Frankly, I don't even have a sliver of an inkling of a notion as to how this error might happen. I encountered it in the middle of teaching an intro programming lecture where I was showing them how file object positions work with .read(). Brought the entire class to a screeching halt.

----------
components: Windows
files: telltest.zip
messages: 337148
nosy: erwenn, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: file object method .tell() sometimes returns large number when position is right before a line break
type: behavior
versions: Python 3.6, Python 3.7
Added file: https://bugs.python.org/file48186/telltest.zip

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36190>
_______________________________________