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 ThreadPoolitems = 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 timefrom multiprocessing.dummy import Pool as ThreadPooldef 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_Gitfrom threading import Thread, Lockimport datetimebase_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_timedef 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_timedef 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_timedef 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: continueuser_commits_lang_info_dict[member_name] = user_commits_lang_infoproject_members_commits_lang_info[git.project] = user_commits_lang_info_dict except:pass return project_members_commits_lang_infodef 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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