无法使用dateutil relativedelta重建结束日期

Question

我正在使用dateutil.relativedelta来计算年龄。当我验证结果时，它似乎在某些情况下是不正确的。只有当开始日期的日期是1，月底是31时，才会出现问题，但不是所有的月份都是这样！奇怪的是，1月和8月的结果是正确的。我肯定遗漏了一些东西，但我不能确定是什么和在哪里。

我使用的是Win32上的Python2.7.10和dateutil2.7.5(但2.6.1也有同样的问题)

要运行的代码：

from __future__ import print_function
import datetime
from dateutil.relativedelta import relativedelta

# Main function -- later on, its result is used in dateutil.rrule
# (all irrelevant code expurged)
def age(start, end, includeFirstDay=False):
    # includeFirstDay: whether to include the start date in the result
    if includeFirstDay:
        start += relativedelta(days=-1)
    # Documentation tells nothing about the order of arguments when the first
    # two are dates. The following seems correct.
    return relativedelta(end, start)

# Utility in external module
def eomonth(dt):
    # from the beginning of month, add one month and substract one day
    eom = dt + relativedelta(days=-dt.day+1) + relativedelta(months=+1, days=-1)
    return eom

test_values = [
    # the 3 following lines fail
    (datetime.date(2018, 10, 1), datetime.date(2018, 10, 31)),
    (datetime.date(2018, 10, 1), datetime.date(2018, 11, 1)+relativedelta(days=-1)),
    (datetime.date(2018, 10, 1), eomonth(datetime.date(2018, 10, 1))),
    # the following lines pass
    (datetime.date(2018, 10, 5), eomonth(datetime.date(2018, 10, 5))),
    (datetime.date(2016, 2, 1), datetime.date(2016, 2, 29)),
    (datetime.date(2016, 2, 1), datetime.date(2016, 3, 1)+relativedelta(days=-1)),
    (datetime.date(2016, 2, 1), eomonth(datetime.date(2016, 2, 1))),
    ]

def test(start, end, includeFirstDay=False):
    rd = age(start, end, includeFirstDay=includeFirstDay)
    # calculate end date from age
    d = rd.days + (-1 if includeFirstDay else 0)
    fin = start + relativedelta(years=rd.years, months=rd.months, days=d)
    if fin != end:  # i.e. AssertionError
        print('expected %s, got %s (includeFirstDay=%s)' %(end, fin, includeFirstDay))

for start, end in test_values:
    test(start, end, includeFirstDay=False)
    test(start, end, includeFirstDay=True)

# trying with all months in a year

def make_test(year):
    t = []
    for month in range(1, 13):
        start = datetime.date(year, month, 1)
        # Three ways to find the end of the month
        t.append((start, eomonth(start)))
        t.append((start, start + relativedelta(months=+1, days=-1)))  # only if start day == 1
        for eom in [31, 30, 29, 28]:  # brute force
            try:
                t.append((start, datetime.date(year, month, eom)))
                break
            except:
                pass
    return sorted(list(set(t)))

from pprint import pprint

print('\n*** testing 2016 (leap year)')
test_values = make_test(2016)
pprint(test_values)  # verify ends of months are correct
for start, end in test_values:
    test(start, end, includeFirstDay=False)
    test(start, end, includeFirstDay=True)

print('\n*** testing 2017')
test_values = make_test(2017)
pprint(test_values)  # verify ends of months are correct
for start, end in test_values:
    test(start, end, includeFirstDay=False)
    test(start, end, includeFirstDay=True)

输出：

expected 2018-10-31, got 2018-11-01 (includeFirstDay=True)
expected 2018-10-31, got 2018-11-01 (includeFirstDay=True)
expected 2018-10-31, got 2018-11-01 (includeFirstDay=True)

*** testing 2016 (leap year)
[(datetime.date(2016, 1, 1), datetime.date(2016, 1, 31)),
 (datetime.date(2016, 2, 1), datetime.date(2016, 2, 29)),
 (datetime.date(2016, 3, 1), datetime.date(2016, 3, 31)),
 (datetime.date(2016, 4, 1), datetime.date(2016, 4, 30)),
 (datetime.date(2016, 5, 1), datetime.date(2016, 5, 31)),
 (datetime.date(2016, 6, 1), datetime.date(2016, 6, 30)),
 (datetime.date(2016, 7, 1), datetime.date(2016, 7, 31)),
 (datetime.date(2016, 8, 1), datetime.date(2016, 8, 31)),
 (datetime.date(2016, 9, 1), datetime.date(2016, 9, 30)),
 (datetime.date(2016, 10, 1), datetime.date(2016, 10, 31)),
 (datetime.date(2016, 11, 1), datetime.date(2016, 11, 30)),
 (datetime.date(2016, 12, 1), datetime.date(2016, 12, 31))]
expected 2016-03-31, got 2016-04-02 (includeFirstDay=True)
expected 2016-05-31, got 2016-06-01 (includeFirstDay=True)
expected 2016-07-31, got 2016-08-01 (includeFirstDay=True)
expected 2016-10-31, got 2016-11-01 (includeFirstDay=True)
expected 2016-12-31, got 2017-01-01 (includeFirstDay=True)

*** testing 2017
[(datetime.date(2017, 1, 1), datetime.date(2017, 1, 31)),
 (datetime.date(2017, 2, 1), datetime.date(2017, 2, 28)),
 (datetime.date(2017, 3, 1), datetime.date(2017, 3, 31)),
 (datetime.date(2017, 4, 1), datetime.date(2017, 4, 30)),
 (datetime.date(2017, 5, 1), datetime.date(2017, 5, 31)),
 (datetime.date(2017, 6, 1), datetime.date(2017, 6, 30)),
 (datetime.date(2017, 7, 1), datetime.date(2017, 7, 31)),
 (datetime.date(2017, 8, 1), datetime.date(2017, 8, 31)),
 (datetime.date(2017, 9, 1), datetime.date(2017, 9, 30)),
 (datetime.date(2017, 10, 1), datetime.date(2017, 10, 31)),
 (datetime.date(2017, 11, 1), datetime.date(2017, 11, 30)),
 (datetime.date(2017, 12, 1), datetime.date(2017, 12, 31))]
expected 2017-03-31, got 2017-04-03 (includeFirstDay=True)
expected 2017-05-31, got 2017-06-01 (includeFirstDay=True)
expected 2017-07-31, got 2017-08-01 (includeFirstDay=True)
expected 2017-10-31, got 2017-11-01 (includeFirstDay=True)
expected 2017-12-31, got 2018-01-01 (includeFirstDay=True)

此外，3月份的结束日期甚至是错误的，如果这是可能的话(这个词确实存在)。

如果有人能开导我，我将不胜感激。提前谢谢。

Answer 1

实际上，我做错了事。1月和8月表现“正常”的原因:对于这两个月，前几个月也有31天。3月份对我来说很奇怪的原因:2月份比31天少了2到3天

例如：

start = datetime.date(2018, 3, 1)
end = start + relativedelta(months=+1, days=-1)
a = age(start, end, True)
print 'start:', start
print 'end  :', end
print 'age  :', a

输出：

start: 2018-03-01
end  : 2018-03-31
age  : relativedelta(months=+1, days=+3)

我期待的是relativedelta(days=+31)之类的东西。

根据开始日期和结束日期的提供方式，relativedelta会返回不同的结果。例如：

relativedelta(datetime.date(2018, 4, 1), datetime.date(2018, 3, 1))

给出

relativedelta(months=+1)

和

relativedelta(datetime.date(2018, 3, 31), datetime.date(2018, 2, 28))

给出

relativedelta(months=+1, days=+3)

无论如何，开始日期和结束日期之间的差值应始终等于31天。

我的结论是不使用relativedelta来计算从1日到最后一天(包括所有日期)之间的单个月的年龄，当计算其中一个日期时。

或者修改age()函数，我可能会这样做。

编辑

在不久的将来，我将删除这个问题，因为我认为它提出的观点应该包含在dateutil文档中的“转角案例”类别中。

Answer 2

from __future__ import print_function
import datetime
from datetime import date
from dateutil.relativedelta import relativedelta

today= date.today()
end_day=today.replace(day=1)
end_day += relativedelta(months=1)
end_day -= relativedelta(days=1)

print(end_day)

Answer 3

我的函数age()写得很糟糕。下面是正确的答案：

def age(start, end, includeFirstDay=False):
    r = relativedelta(end, start)
    if includeFirstDay:
        r += relativedelta(days=1)
    return r

说明:将start偏移-1天将日期移至上个月，并欺骗了我的意图混淆的relativedelta。