在考虑到日历月中的不平等日的情况下，增加/扣除月份

Question

我正试图想出一种方法来创建一个日期列表--从给定的日期n到dt几个月前。然而，基于什么是dt，这似乎很棘手。下面我通过几个例子来说明这个困境(尤指)。请看下面棘手的案例--3)：

from datetime import datetime
from dateutil.relativedelta import relativedelta
# Simple case.
dt = datetime(2021, 2, 15)
dt - relativedelta(months=1)  # n=1 gives datetime.datetime(2021, 1, 15, 0, 0)
dt - relativedelta(months=2)  # n=2 gives datetime.datetime(2020, 12, 15, 0, 0)
# Simple case-2
dt = datetime(2021, 3, 31)
dt - relativedelta(months=1)  # n=1 gives datetime.datetime(2021, 2, 28, 0, 0)
dt - relativedelta(months=2)  # n=2 gives datetime.datetime(2021, 1, 31, 0, 0)
dt - relativedelta(months=3)  # n=3 gives datetime.datetime(2020, 12, 31, 0, 0)
dt - relativedelta(months=4)  # n=4 gives datetime.datetime(2020, 11, 30, 0, 0)
# Tricky case-3
dt = datetime(2021, 2, 28)
dt - relativedelta(months=1)  # n=1 gives datetime.datetime(2021, 1, 28, 0, 0) and not datetime.datetime(2021, 1, 31, 0, 0)
dt - relativedelta(months=2)  # n=2 gives datetime.datetime(2020, 12, 28, 0, 0) and not datetime.datetime(2020, 12, 31, 0, 0)
dt - relativedelta(months=3)  # n=3 gives datetime.datetime(2020, 11, 28, 0, 0) and not datetime.datetime(2020, 11, 30, 0, 0)
dt - relativedelta(months=4)  # n=4 gives datetime.datetime(2020, 10, 28, 0, 0) and not datetime.datetime(2020, 10, 31, 0, 0)

Answer 1

相对人塔似乎在月底的情况下失败了，而月有不到31天的时间。这里有个解决办法：

• 检查日期是否为月底
• 如果没有，只需使用relativedelta
• 如果是这样的话，请使用相对论month，但是通过显式地设置day属性来确保日期是月中的最后一天。

from datetime import datetime, timedelta
from dateutil.relativedelta import relativedelta

# add_month adds n months to datetime object dt
def add_month(dt, n):
    # we can add a day without month changing - not end of month:
    if (dt + timedelta(1)).month == dt.month:
        return dt + relativedelta(months=n)
    # implicit else: end of month
    return (dt + relativedelta(months=n+1)).replace(day=1) - timedelta(1)

示例：

d = datetime(2021, 3, 15)
print(add_month(d, -1).date(), d.date(), add_month(d, 1).date())
# 2021-02-15 2021-03-15 2021-04-15

d = datetime(2021, 3, 31)
print(add_month(d, -1).date(), d.date(), add_month(d, 1).date())
# 2021-02-28 2021-03-31 2021-04-30

d = datetime(2021,2,28)
print(add_month(d, -1).date(), d.date(), add_month(d, 1).date())
# 2021-01-31 2021-02-28 2021-03-31

d = datetime(2021,11,30)
print(add_month(d, -1).date(), d.date(), add_month(d, 1).date())
# 2021-10-31 2021-11-30 2021-12-31