如何提高python 中for循环的效率
对于某个城市的出租车数据,一天就有33210000条记录,如何将每辆车的数据单独拎出来放到一个专属的文件中呢?
思路很简单:
就是循环33210000条记录,将每辆车的数据搬运到它该去的文件中。
但是对于3000多万条数据,一个一个循环太消耗时间,我花了2个小时才搬运了60万数据,算算3000万我需要花费100个小时,也就需要4-5天。并且还需要保证这五天全天开机,不能出现卡机的事故。
因此,需要使用并行进行for循环的技巧:
由于3000万数据放到csv中导致csv打不开,因此我就把一个csv通过split软件将其切分成每份60万,共53个csv。
我原来的思路是读取文件夹,获取由每一个60万的csv文件组成的列表,再分别对每一个60万的csv进行处理。实质上还是循环33210000次,并行for循环就是同时处理几个60万的csv文件,就能成倍的减少时间消耗。
并行进行for循环是受下面的方法启发:
我之前的做法类似这样:
words = ['apple', 'bananan', 'cake', 'dumpling'] for word in words: print word
并行for循环类似这样:
from multiprocessing.dummy import Pool as ThreadPool items = list() pool = ThreadPool() pool.map(process, items) pool.close() pool.join()
其中,process是进行处理的函数
实例代码如下:
# -*- coding: utf-8 -*- import time from multiprocessing.dummy import Pool as ThreadPool def process(item): print('正在并行for循环') print(item) time.sleep(5) items = ['apple', 'bananan', 'cake', 'dumpling'] pool = ThreadPool() pool.map(process, items) pool.close() pool.join()
补充知识:Python3用多线程替代for循环提升程序运行速度
优化前后新老代码如下:
from git_tools.git_tool import get_collect_projects, QQNews_Git from threading import Thread, Lock import datetime base_url = "http://git.xx.com" project_members_commits_lang_info = {} lock = Lock() threads = [] ''' Author:zenkilan ''' def count_time(func): def took_up_time(*args, **kwargs): start_time = datetime.datetime.now() ret = func(*args, **kwargs) end_time = datetime.datetime.now() took_up_time = (end_time - start_time).total_seconds() print(f"{func.__name__} execution took up time:{took_up_time}") return ret return took_up_time def get_project_member_lang_code_lines(git, member, begin_date, end_date): global project_members_commits_lang_info global lock member_name = member["username"] r = git.get_user_info(member_name) if not r["id"]: return user_commits_lang_info = git.get_commits_user_lang_diff_between(r["id"], begin_date, end_date) if len(user_commits_lang_info) == 0: return lock.acquire() project_members_commits_lang_info.setdefault(git.project, dict()) project_members_commits_lang_info[git.project][member_name] = user_commits_lang_info lock.release() def get_project_lang_code_lines(project, begin_date, end_date): global threads git = QQNews_Git(project[1], base_url, project[0]) project_members = git.get_project_members() if len(project_members) == 0: return for member in project_members: thread = Thread(target=get_project_member_lang_code_lines, args=(git, member, begin_date, end_date)) threads.append(thread) thread.start() @count_time def get_projects_lang_code_lines(begin_date, end_date): """ 获取项目代码行语言相关统计——新方法(提升效率) 应用多线程替代for循环 并发访问共享外部资源 :return: """ global project_members_commits_lang_info global threads for project in get_collect_projects(): thread = Thread(target=get_project_lang_code_lines, args=(project, begin_date, end_date)) threads.append(thread) thread.start() @count_time def get_projects_lang_code_lines_old(begin_date, end_date): """ 获取项目代码行语言相关统计——老方法(耗时严重) 使用最基本的思路进行编程 双层for循环嵌套并且每层都包含耗时操作 :return: """ project_members_commits_lang_info = {} for project in get_collect_projects(): git = QQNews_Git(project[1], base_url, project[0]) project_members = git.get_project_members() user_commits_lang_info_dict = {} if len(project_members) == 0: continue for member in project_members: member_name = member["username"] r = git.get_user_info(member_name, debug=False) if not r["id"]: continue try: user_commits_lang_info = git.get_commits_user_lang_diff_between(r["id"], begin_date, end_date) if len(user_commits_lang_info) == 0: continue user_commits_lang_info_dict[member_name] = user_commits_lang_info project_members_commits_lang_info[git.project] = user_commits_lang_info_dict except: pass return project_members_commits_lang_info def test_results_equal(resultA, resultB): """ 测试方法 :param resultA: :param resultB: :return: """ print(resultA) print(resultB) assert len(str(resultA)) == len(str(resultB)) if __name__ == '__main__': from git_tools.config import begin_date, end_date get_projects_lang_code_lines(begin_date, end_date) for t in threads: t.join() old_result = get_projects_lang_code_lines_old(begin_date, end_date) test_results_equal(old_result, project_members_commits_lang_info)
老方法里外层for循环和内层for循环里均存在耗时操作:
1)git.get_project_members()
2)git.get_user_info(member_name, debug=False)
分两步来优化,先里后外或先外后里都行。用多线程替换for循环,并发共享外部资源,加锁避免写冲突。
测试结果通过,函数运行时间装饰器显示(单位秒):
get_projects_lang_code_lines execution took up time:1.85294
get_projects_lang_code_lines_old execution took up time:108.604177
速度提升了约58倍
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