I need a python script implementing a circular buffer for rows in a text file limited to N rows like this:
row 1 -> pop
row 2
row 3
|
|
push -> row N
What's the best solution?
EDIT: This script should create and maintain the text file which only contains the latest N lines. Then it should pop the first line pushed in. Like a fifo buffer.
Use collections.deque. It supports a maxlen parameter.
d = collections.deque(maxlen=10)
for line in f:
d.append(line)
# ...
fd = open(filename, 'r+'); d = collections.deque(fd, maxlen=10)
d.append('string') the file is empty. Thank you.Try my recipe and sorry for italian usage:
#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# fifo(.py)
#
# Copyright 2011 Fabio Di Bernardini <[email protected]>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
# MA 02110-1301, USA.
def string_conditioned(string):
return string.decode('string_escape').rstrip() + '\n'
def pop(n, size, filename):
with open(filename, 'r+U') as fd:
rows = fd.readlines()
with open(filename, 'w') as fd:
n = int(n)
fd.writelines(rows[n:])
return ''.join(rows[:n])
def trim_fifo(row, size, filename):
size = int(size)
with open(filename, 'rU') as fd:
rows = fd.readlines()
num_rows = len(rows)
if num_rows >= size:
n = string_conditioned(row).count('\n')
pop(num_rows + n - size, size, filename)
def push(row, size, filename):
trim_fifo(row, size, filename)
with open(filename, 'a') as fd:
fd.write(string_conditioned(row))
return ''
def main():
import sys
try:
command = sys.argv[1]
param = sys.argv[2]
size = sys.argv[3]
filename = sys.argv[4]
sys.stdout.write({
'--push': push,
'--pop' : pop,
}[command](param, size, filename))
except Exception, e:
print r"""
Uso:
fifo --push ROW MAX_ROWS FILE
fifo --pop NUM MAX_ROWS FILE
fifo implementa un buffer ad anello di righe di testo, Quando viene inserita
una riga che fa superare il numero massimo di righe (MAX_ROWS) elimina la riga
più vecchia.
Comandi:
--push accoda la riga di testo ROW nel FILE rimuovendo le righe più vecchie
se il file supera MAX_ROWS. Usare '\n' per separare righe multiple.
--pop stampa le prime NUM righe e le rimuove dal FILE. MAX_ROWS viene
ignorato ma deve essere comunque specificato.
Esempi:
fifo --push 'row_one \n row_two' 10 fifo.txt
fifo --pop 2 10 fifo.txt
"""
print e
if __name__ == '__main__':
main()
import collections
def keep_last_n_and_return_first_of_last_n(filename, n):
with open(filename, "r") as inp:
lines= collections.deque(inp, maxlen=n)
with open(filename, "w") as out:
out.writelines(lines)
return lines[0]
