[New-bugs-announce] [issue43028] seeking past the end of a file unexpected behavior

[Cotton Seed]

New submission from Cotton Seed <cotton.seed at gmail.com>:

Seeking past the end of a file with file objects does not match the same code implemented in terms of file descriptors.  Is this the intended behavior?

Smallest example I could find:

f = open('new_file', 'ab')
print(f.seek(1))
print(f.write(b'foo'))
print(f.tell())
f.close()

This program outputs: 1, 3, 4 as expected, but only creates a 3-byte file:

and creates a 3-byte file:
$ hexdump -C new_file
00000000  66 6f 6f                                          |foo|
00000003

If I use open(..., buffering=0), or flush before the tell, it outputs: 1, 3, 3.

The obvious code with file descriptors:

fd = os.open('log', os.O_WRONLY | os.O_CREAT)
print(os.lseek(fd, 1, os.SEEK_SET))
os.write(fd, b'foo')
os.close(fd)

works as expected, creating a 4-byte file.

Could this be related this issue:

https://bugs.python.org/issue36411

?

----------
components: IO
messages: 385692
nosy: cotton.seed
priority: normal
severity: normal
status: open
title: seeking past the end of a file unexpected behavior
versions: Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43028>
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