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"It runs like _x, where _x is something unsavory" -- Prof. Romas Aleliunas, CS 435

Re: tail

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From: cs...@cskk.id.au (Cameron Simpson)
Newsgroups: comp.lang.python
Subject: Re: tail
Date: Mon, 2 May 2022 08:18:39 +1000
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by: Cameron Simpson - Sun, 1 May 2022 22:18 UTC

On 01May2022 18:55, Marco Sulla <Marco.Sulla.Python@gmail.com> wrote:
>Something like this is OK?
[...]
>def tail(f):
> chunk_size = 100
> size = os.stat(f.fileno()).st_size

I think you want os.fstat().

> positions = iter(range(size, -1, -chunk_size))
> next(positions)

I was wondering about the iter, but this makes sense. Alternatively you
could put a range check in the for-loop.

> chunk_line_pos = -1
> pos = 0
>
> for pos in positions:
> f.seek(pos)
> chars = f.read(chunk_size)
> chunk_line_pos = chars.rfind(b"\n")
>
> if chunk_line_pos != -1:
> break

Normal text file _end_ in a newline. I'd expect this to stop immediately
at the end of the file.

> if chunk_line_pos == -1:
> nbytes = pos
> pos = 0
> f.seek(pos)
> chars = f.read(nbytes)
> chunk_line_pos = chars.rfind(b"\n")

I presume this is because unless you're very lucky, 0 will not be a
position in the range(). I'd be inclined to avoid duplicating this code
and special case and instead maybe make the range unbounded and do
something like this:

if pos < 0:
pos = 0
... seek/read/etc ...
if pos == 0:
break

around the for-loop body.

> if chunk_line_pos == -1:
> line_pos = pos
> else:
> line_pos = pos + chunk_line_pos + 1
> f.seek(line_pos)
> return f.readline()
>
>This is simply for one line and for utf8.

And anything else where a newline is just an ASCII newline byte (10) and
can't be mistaken otherwise. So also ASCII and all the ISO8859-x single
byte encodings. But as Chris has mentioned, not for other encodings.

Seems sane. I haven't tried to run it.

Cheers,
Cameron Simpson <cs@cskk.id.au>

"It runs like _x, where _x is something unsavory" -- Prof. Romas Aleliunas, CS 435

devel / comp.lang.python / Re: tail

"It runs like _x, where _x is something unsavory" -- Prof. Romas Aleliunas, CS 435