un archivo de texto en el búfer circular pitón

Question

Necesito un script en Python, la implementación de un buffer circular para las filas en un archivo de texto limitado a N filas como esto:

 row 1 -> pop 
     row 2 
     row 3 
     | 
     | 
push -> row N 

Cuál es la mejor solución?

EDIT: Este script debe crear y mantener el archivo de texto que sólo contiene las últimas N líneas. Luego debería aparecer la primera línea introducida. Como un buffer fifo.

Answer 1

Prueba mi receta y lo siento por el uso italiano:

#!/usr/bin/env python 
# -*- coding: utf-8 -*- 
# 
#  fifo(.py) 
#  
#  Copyright 2011 Fabio Di Bernardini <fdb@altraqua.com> 
#  
#  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() 

Answer 2

Uso collections.deque. Es compatible con un parámetro maxlen.

d = collections.deque(maxlen=10) 
for line in f: 
    d.append(line) 
    # ... 

Answer 3

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]