tail

[Dennis Lee Bieber]

On Sun, 8 May 2022 22:48:32 +0200, Marco Sulla
<Marco.Sulla.Python at gmail.com> declaimed the following:

>
>Emh. I re-quote
>
>seek(offset, whence=SEEK_SET)
>Change the stream position to the given byte offset.
>
>And so on. No mention of differences between text and binary mode.

	You ignore that, underneath, Python is just wrapping the C API... And
the documentation for C explicitly specifies that other then SEEK_END with
offset 0, and SEEK_SET with offset of 0, for a text file one can only rely
upon SEEK_SET using an offset previously obtained with (C) ftell() /
(Python) .tell() .

https://docs.python.org/3/library/io.html
"""
class io.IOBase

    The abstract base class for all I/O classes.
"""
 seek(offset, whence=SEEK_SET)

    Change the stream position to the given byte offset. offset is
interpreted relative to the position indicated by whence. The default value
for whence is SEEK_SET. Values for whence are:
"""

	Applicable to BINARY MODE I/O: For UTF-8 and any other multibyte
encoding, this means you could end up positioning into the middle of a
"character" and subsequently read garbage. It is on you to handle
synchronizing on a valid character position, and also to handle different
line ending conventions.

"""
class io.TextIOBase

    Base class for text streams. This class provides a character and line
based interface to stream I/O. It inherits IOBase.
"""
 seek(offset, whence=SEEK_SET)

    Change the stream position to the given offset. Behaviour depends on
the whence parameter. The default value for whence is SEEK_SET.

        SEEK_SET or 0: seek from the start of the stream (the default);
offset must either be a number returned by TextIOBase.tell(), or zero. Any
other offset value produces undefined behaviour.

        SEEK_CUR or 1: “seek” to the current position; offset must be zero,
which is a no-operation (all other values are unsupported).

        SEEK_END or 2: seek to the end of the stream; offset must be zero
(all other values are unsupported).
"""

EMPHASIS: "offset must either be a number returned by TextIOBase.tell(), or
zero." 

	TEXT I/O, with a specified encoding, will return Unicode data points,
and will handle converting line ending to the internal (<lf> represents
new-line) format.

	Since your code does not specify BINARY mode in the open statement,
Python should be using TEXT mode.



-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
	wlfraed at ix.netcom.com    http://wlfraed.microdiversity.freeddns.org/