How Can I Efficiently Retrieve the Last N Lines of a File in Python (with Offset Support)?

Get Last N Lines of a File, Similar to Tail

Introduction

Log file analysis often involves the ability to view the most recent entries. This is typically achieved using the "tail" command, which retrieves the last n lines of a file. In this article, we will explore an implementation of a Python method that emulates the tail command, with support for offsets.

Tail Implementation

The proposed tail() method operates as follows:

1. It reads n lines from the bottom of the file.
2. It provides an offset parameter to skip a specified number of lines from the bottom.

def tail(f, n, offset=0):
    """Reads a n lines from f with an offset of offset lines."""
    avg_line_length = 74
    to_read = n + offset
    while 1:
        try:
            f.seek(-(avg_line_length * to_read), 2)
        except IOError:
            f.seek(0)
        pos = f.tell()
        lines = f.read().splitlines()
        if len(lines) >= to_read or pos == 0:
            return lines[-to_read:offset and -offset or None]
        avg_line_length *= 1.3

This method estimates the average line length and adjusts it dynamically to optimize performance.

Alternative Approach

The original implementation makes assumptions about line length, which may not always hold true. Here's an alternative approach that avoids such assumptions:

def tail(f, lines=20):
    total_lines_wanted = lines

    BLOCK_SIZE = 1024
    f.seek(0, 2)
    block_end_byte = f.tell()
    lines_to_go = total_lines_wanted
    block_number = -1
    blocks = [] 
    while lines_to_go > 0 and block_end_byte > 0:
        if (block_end_byte - BLOCK_SIZE > 0):
            f.seek(block_number*BLOCK_SIZE, 2)
            blocks.append(f.read(BLOCK_SIZE))
        else:
            f.seek(0,0)
            blocks.append(f.read(block_end_byte))
        lines_found = blocks[-1].count('\n')
        lines_to_go -= lines_found
        block_end_byte -= BLOCK_SIZE
        block_number -= 1
    all_read_text = ''.join(reversed(blocks))
    return '\n'.join(all_read_text.splitlines()[-total_lines_wanted:])

This method seeks backwards through the file one block at a time, counting line breaks to find the desired lines.

Conclusion

Both methods provide viable solutions for retrieving the last n lines of a file with offset support. The alternative approach avoids assumptions about line length and might be more efficient for large files.

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