[issue36111] Negative `offset` values are no longer acceptable with implementation of `seek` with python3; should be per POSIX

[Enji Cooper]

Enji Cooper  added the comment:

?!

Being blunt: why should opening a file in binary vs text mode matter? POSIX 
doesn't make this distinction.

Per the pydoc (https://docs.python.org/2/library/functions.html#open):

> The default is to use text mode, which may convert '\n' characters to a 
> platform-specific representation on writing and back on reading.

If this is one of the only differentiators between binary and text mode, why 
should certain types of seeking be made impossible?

Having to stat the file, then set the cursor to the size of the file, minus the 
offset breaks the 'seek(..)' interface, and having to use 'rb', then convert 
from bytes to unicode overly complicates things :(.

--

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[issue36111] Negative `offset` values are no longer acceptable with implementation of `seek` with python3; should be per POSIX

[Steven D'Aprano]

Steven D'Aprano  added the comment:

I believe you will find that this is because you opened the file in text mode, 
which means Unicode, not bytes. If you open it in binary mode, the POSIX spec 
applies:

py> fp = open("sample", "rb"); fp.seek(-100, os.SEEK_END)
350

Supported values for seeking in text (Unicode) files are documented here:

https://docs.python.org/3/library/io.html#io.TextIOBase.seek

I don't believe this is a bug, or possible to be changed. Do you still think 
otherwise? If not, we should close this ticket.

--
nosy: +steven.daprano

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[issue36111] Negative `offset` values are no longer acceptable with implementation of `seek` with python3; should be per POSIX

[Enji Cooper]

New submission from Enji Cooper :

I tried using os.SEEK_END in a technical interview, but unfortunately, that 
didn't work with python 3.x:

pinklady:cpython ngie$ python3
Python 3.7.2 (default, Feb 12 2019, 08:15:36) 
[Clang 10.0.0 (clang-1000.11.45.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> fp = open("configure"); fp.seek(-100, os.SEEK_END)
Traceback (most recent call last):
  File "", line 1, in 
io.UnsupportedOperation: can't do nonzero end-relative seeks

It does however work with 2.x, which is aligned with the POSIX spec 
implementation, as shown below:

pinklady:cpython ngie$ python
Python 2.7.15 (default, Oct  2 2018, 11:47:18) 
[GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.11.45.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> fp = open("configure"); fp.seek(-100, os.SEEK_END)
>>> fp.tell()
501076
>>> os.stat("configure").st_size
501176
>>>

--
components: IO
messages: 336564
nosy: yaneurabeya
priority: normal
severity: normal
status: open
title: Negative `offset` values are no longer acceptable with implementation of 
`seek` with python3; should be per POSIX

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