python head, tail and backward read by lines of a text file

Question

How to implement somethig like the 'head' and 'tail' commands in python and backward read by lines of a text file?

Answer 1

head is easy:

from itertools import islice
with open("file") as f:
    for line in islice(f, n):
        print line

tail is harder if you don't want to keep the whole file in memory. If the input is a file, you could start reading blocks beginning at the end of the file. The original tail also works if the input is a pipe, so a more general solution is to read and discard the whole input, except for the last few lines. An easy way to do this is collections.deque:

from collections import deque
with open("file") as f:
    for line in deque(f, maxlen=n):
        print line

In both these code snippets, n is the number of lines to print.

Answer 2

This is my personal file class ;-)

class File(file):
    """ An helper class for file reading  """

    def __init__(self, *args, **kwargs):
        super(File, self).__init__(*args, **kwargs)
        self.BLOCKSIZE = 4096

    def head(self, lines_2find=1):
        self.seek(0)                            #Rewind file
        return [super(File, self).next() for x in xrange(lines_2find)]

    def tail(self, lines_2find=1):  
        self.seek(0, 2)                         #Go to end of file
        bytes_in_file = self.tell()
        lines_found, total_bytes_scanned = 0, 0
        while (lines_2find + 1 > lines_found and
               bytes_in_file > total_bytes_scanned): 
            byte_block = min(
                self.BLOCKSIZE,
                bytes_in_file - total_bytes_scanned)
            self.seek( -(byte_block + total_bytes_scanned), 2)
            total_bytes_scanned += byte_block
            lines_found += self.read(self.BLOCKSIZE).count('\n')
        self.seek(-total_bytes_scanned, 2)
        line_list = list(self.readlines())
        return line_list[-lines_2find:]

    def backward(self):
        self.seek(0, 2)                         #Go to end of file
        blocksize = self.BLOCKSIZE
        last_row = ''
        while self.tell() != 0:
            try:
                self.seek(-blocksize, 1)
            except IOError:
                blocksize = self.tell()
                self.seek(-blocksize, 1)
            block = self.read(blocksize)
            self.seek(-blocksize, 1)
            rows = block.split('\n')
            rows[-1] = rows[-1] + last_row
            while rows:
                last_row = rows.pop(-1)
                if rows and last_row:
                    yield last_row
        yield last_row

Example usage:

with File('file.name') as f:
    print f.head(5)
    print f.tail(5)
    for row in f.backward():
        print row