适用场景：
反向读取大文件的需求，多半出于想要获取超大的日志文件最后几百行内容，小文件直接通过 lins = file.readlines()[-200:] 就可以直接得出了，
大文件readlines() 会将文件内容全部读取到内存，在GB大小情况下，内存不一定够，列表切片性能差。 生成器反向读取方式更合适。
反向读取大文件，主要基于：<details><summary>*<font color='gray'>[En]</font>*</summary>*<font color='gray'>Read large files in reverse, mainly based on:</font>*</details>
- file对象的seek()函数，偏移文件指针
- re 模块finditer 搜寻文件行尾符，Match对象的end获取行尾符索引位置
- yield 生成器 节省内存
性能验证：
在windows10， RAM 16G，处理器 11th Gen Intel(R) Core(TM) i5-11400 @ 2.60GHz 2.59 GHz ， python 3.7.11 下简单验证，
import re
def read_reverse_bigfile(filepath, encoding='utf-8', separator=b'\n', single_size=1024 * 1024): """ :param filepath: 文件路径 :param encoding: 字符编码，默认utf-8 :param separator: 行尾分隔符，默认 '\n' :param single_size: 单次读取 字符量，默认 1024*1024 :return: generator """ with open(filepath, 'rb') as f: try: f.seek(0, 2) position = f.tell() if position > single_size: f.seek(-single_size, 2) else: f.seek(0, 0) except OSError as e: return 'Blank file' line = b'' while 1: chunk = f.read(single_size) index_list = [match.end() for match in re.finditer(separator, chunk)] index = None while index_list: target = index_list.pop() if index is None: line = chunk[target:] + line else: line = chunk[target:index] + line if line: yield line.decode(encoding=encoding) line = b'' index = target else: line = chunk[:index] + line position = f.tell() if position > 2 * single_size and single_size > 0: f.seek(-2 * single_size, 1) else: f.seek(0, 0) single_size = position - single_size if single_size yield line.decode(encoding=encoding) return 'End'
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if __name__ == '__main__': import time import os import psutil pid = os.getpid() p = psutil.Process(pid)
start_time = time.time() fp = r'./test30.txt' rrb = list(read_reverse_bigfile(fp)) print(len(rrb)) print(f'耗时0：{time.time() - start_time}') with open(fp, encoding='utf-8') as f: orl = f.readlines() print(len(orl)) print(f'耗时1：{time.time() - start_time}') info = p.memory_full_info().uss/(1024*1024) print(f'内存信息：{info}MB')
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Original: https://www.cnblogs.com/yougnen/p/16081877.html
Author: 阿伦来啦
Title: Python3 反向按行读取大文件、日志read_reverse_bigfile
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