python获取当前时间的前一天，前一周，前一个月。
实用python的datetime.timedelta方法，避免了有的月份是30和31等不同的情况。
获取前一个月的时间，方法实现：首先datetime.datetime.now获取当前时间，然后通过datetime.timedelta获取上一个月最后一天的datetime对象dayto，最后用dayto的数据初始化这个月的第一个天和最后一天的datetime对象。
import datetimed = datetime.datetime.now()def day_get(d):oneday = datetime.timedelta(days=1)day = d - onedaydate_from = datetime.datetime(day.year, day.month, day.day, 0, 0, 0)date_to = datetime.datetime(day.year, day.month, day.day, 23, 59, 59)print '---'.join([str(date_from), str(date_to)])def week_get(d):dayscount = datetime.timedelta(days=d.isoweekday())dayto = d - dayscountsixdays = datetime.timedelta(days=6)dayfrom = dayto - sixdaysdate_from = datetime.datetime(dayfrom.year, dayfrom.month, dayfrom.day, 0, 0, 0)date_to = datetime.datetime(dayto.year, dayto.month, dayto.day, 23, 59, 59)print '---'.join([str(date_from), str(date_to)])def month_get(d):""" 返回上个月第一个天和最后一天的日期时间:returndate_from: 2016-01-01 00:00:00date_to: 2016-01-31 23:59:59"""dayscount = datetime.timedelta(days=d.day)dayto = d - dayscountdate_from = datetime.datetime(dayto.year, dayto.month, 1, 0, 0, 0)date_to = datetime.datetime(dayto.year, dayto.month, dayto.day, 23, 59, 59)print '---'.join([str(date_from), str(date_to)])return date_from, date_to参考：
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