對于某個城市的出租車數(shù)據(jù)，一天就有33210000條記錄，如何將每輛車的數(shù)據(jù)單獨拎出來放到一個專屬的文件中呢？

思路很簡單：

就是循環(huán)33210000條記錄，將每輛車的數(shù)據(jù)搬運到它該去的文件中。

但是對于3000多萬條數(shù)據(jù)，一個一個循環(huán)太消耗時間，我花了2個小時才搬運了60萬數(shù)據(jù)，算算3000萬我需要花費100個小時，也就需要4-5天。并且還需要保證這五天全天開機，不能出現(xiàn)卡機的事故。

因此，需要使用并行進行for循環(huán)的技巧：

由于3000萬數(shù)據(jù)放到csv中導致csv打不開，因此我就把一個csv通過split軟件將其切分成每份60萬，共53個csv。

我原來的思路是讀取文件夾，獲取由每一個60萬的csv文件組成的列表，再分別對每一個60萬的csv進行處理。實質(zhì)上還是循環(huán)33210000次，并行for循環(huán)就是同時處理幾個60萬的csv文件，就能成倍的減少時間消耗。

并行進行for循環(huán)是受下面的方法啟發(fā)：

我之前的做法類似這樣：

words = [’apple’, ’bananan’, ’cake’, ’dumpling’] for word in words: print word

并行for循環(huán)類似這樣：

from multiprocessing.dummy import Pool as ThreadPoolitems = list()pool = ThreadPool()pool.map(process, items)pool.close()pool.join()

其中,process是進行處理的函數(shù)

實例代碼如下：

# -*- coding: utf-8 -*-import timefrom multiprocessing.dummy import Pool as ThreadPooldef process(item): print(’正在并行for循環(huán)’) print(item) time.sleep(5)items = [’apple’, ’bananan’, ’cake’, ’dumpling’]pool = ThreadPool()pool.map(process, items)pool.close()pool.join()

補充知識：Python3用多線程替代for循環(huán)提升程序運行速度

優(yōu)化前后新老代碼如下：

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): ''' 獲取項目代碼行語言相關(guān)統(tǒng)計——新方法（提升效率） 應用多線程替代for循環(huán) 并發(fā)訪問共享外部資源 :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): ''' 獲取項目代碼行語言相關(guān)統(tǒng)計——老方法（耗時嚴重） 使用最基本的思路進行編程 雙層for循環(huán)嵌套并且每層都包含耗時操作 :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): ''' 測試方法 :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)

老方法里外層for循環(huán)和內(nèi)層for循環(huán)里均存在耗時操作：

1）git.get_project_members()

2）git.get_user_info(member_name, debug=False)

分兩步來優(yōu)化，先里后外或先外后里都行。用多線程替換for循環(huán)，并發(fā)共享外部資源，加鎖避免寫沖突。

測試結(jié)果通過，函數(shù)運行時間裝飾器顯示（單位秒）：

get_projects_lang_code_lines execution took up time:1.85294

get_projects_lang_code_lines_old execution took up time:108.604177

速度提升了約58倍

以上這篇如何提高python 中for循環(huán)的效率就是小編分享給大家的全部內(nèi)容了，希望能給大家一個參考，也希望大家多多支持好吧啦網(wǎng)。