


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:
- It reads n lines from the bottom of the file.
- 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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