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Re: tail

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From: Marco.Su...@gmail.com (Marco Sulla)
Newsgroups: comp.lang.python
Subject: Re: tail
Date: Fri, 6 May 2022 21:21:36 +0200
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by: Marco Sulla - Fri, 6 May 2022 19:21 UTC

I have a little problem.

I tried to extend the tail function, so it can read lines from the bottom
of a file object opened in text mode.

The problem is it does not work. It gets a starting position that is lower
than the expected by 3 characters. So the first line is read only for 2
chars, and the last line is missing.

import os

_lf = "\n"
_cr = "\r"
_lf_ord = ord(_lf)

def tail(f, n=10, chunk_size=100):
n_chunk_size = n * chunk_size
pos = os.stat(f.fileno()).st_size
chunk_line_pos = -1
lines_not_found = n
binary_mode = "b" in f.mode
lf = _lf_ord if binary_mode else _lf

while pos != 0:
pos -= n_chunk_size

if pos < 0:
pos = 0

f.seek(pos)
chars = f.read(n_chunk_size)

for i, char in enumerate(reversed(chars)):
if char == lf:
lines_not_found -= 1

if lines_not_found == 0:
chunk_line_pos = len(chars) - i - 1
print(chunk_line_pos, i)
break

if lines_not_found == 0:
break

line_pos = pos + chunk_line_pos + 1

f.seek(line_pos)

res = b"" if binary_mode else ""

for i in range(n):
res += f.readline()

return res

Maybe the problem is 1 char != 1 byte?

Computers are not intelligent. They only think they are.

devel / comp.lang.python / Re: tail