python datetime和time的一些疑惑解答 及 获取上年同期、上月等日期

#!/usr/bin/env python
# -*- coding:utf-8 -*-
# Datetime:2018/7/13 0:54
# Author:Xzs

"""
功能：传入日期类似“201807”格式，年份及月份参数，例如
    date_before("201807", year=1, month=7)——返回上年同期7月前的日期，得到“201612”
    date_after("201807", year=1, month=6)——返回下年同期6月后的日期，得到“202001”
    date_before("201807", year=1, month=0)——上年同期
"""


import datetime
from datetime import timedelta
import calendar

import sys
reload(sys)
sys.setdefaultencoding("utf-8")


# 返回传入日期的上月
def last_one_month(date):
    year = int(date[:4])
    month = int(date[4:])
    date_now = datetime.datetime(year=year, month=month, day=1)  # 构造本月1号datetime
    date_last_month = date_now - timedelta(days=1)  # 上月datetime
    return date_last_month.strftime("%Y%m")


# 返回传入日期的下一个月
def next_one_month(date):
    year = int(date[:4])
    month = int(date[4:])
    a, b = calendar.monthrange(year, month)  # a,b——weekday的第一天是星期几（0-6对应星期一到星期天）和这个月的所有天数
    date_now = datetime.datetime(year=year, month=month, day=b)  # 构造本月1号datetime
    date_next_month = date_now + timedelta(days=1)  # 上月datetime
    return date_next_month.strftime("%Y%m")


def date_before(date, year=None, month=None):
    print u"%s年%s月前的日期是：" % (year if year else "-", month if month else "-"),
    if year >= 1:
        month = 12 * year + month

    if month > 1:
        for m in range(1, month + 1):
            new_date = last_one_month(date)  # 返回上个月，再以上个月为基础，循环计算得到最终月
            date = new_date
    elif month == 1:
        new_date = last_one_month(date)
    elif month == 0:
        new_date = date

    # 如果不输入参数，默认返回本日期
    if year is None and month is None:
        new_date = date
    print new_date
    return new_date


def date_after(date, year=None, month=None):
    print u"%s年%s月后的日期是：" % (year if year else "-", month if month else "-"),
    if year >= 1:
        month = 12 * year + month

    if month > 1:
        for m in range(1, month + 1):
            new_date = next_one_month(date)  # 返回下个月，再以下个月为基础，循环计算得到最终月
            date = new_date
    elif month == 1:
        new_date = next_one_month(date)
    elif month == 0:
        new_date = date

    # 如果不输入参数，默认返回本日期
    if year is None and month is None:
        new_date = date
    print new_date
    return new_date


if __name__ == ‘__main__‘:
    # next_day("20180501",day=5)
    # last_day("20160301",day=1,year=5)
    date_before("201801")
    date_after("201807")