calendar.prcal(9999) output has a problem

[lijp003163com]

BPO	28253
Nosy	@rhettinger, @abalkin, @serhiy-storchaka, @zhangyangyu, @lijp_003@163.com
Superseder	bpo-28292: Make Calendar.itermonthdates() behave consistently in edge cases
Files	20160923154147.png: output result calendar_pryear_9999.patch calendar_prcal_9999_demo.patch issue28253.diff issue28253-2.diff issue28253-3.diff issue28253-4.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/abalkin'
closed_at = <Date 2016-09-28.03:02:52.638>
created_at = <Date 2016-09-23.05:59:40.192>
labels = ['3.7', 'type-bug', 'library']
title = 'calendar.prcal(9999) output has a problem'
updated_at = <Date 2017-10-23.21:21:44.048>
user = 'https://github.com/lijp003163com'

bugs.python.org fields:

activity = <Date 2017-10-23.21:21:44.048>
actor = 'serhiy.storchaka'
assignee = 'belopolsky'
closed = True
closed_date = <Date 2016-09-28.03:02:52.638>
closer = 'belopolsky'
components = ['Library (Lib)']
creation = <Date 2016-09-23.05:59:40.192>
creator = 'jiangping.li'
dependencies = []
files = ['44788', '44789', '44793', '44834', '44835', '44836', '44845']
hgrepos = []
issue_num = 28253
keywords = ['patch']
message_count = 29.0
messages = ['277244', '277246', '277247', '277250', '277253', '277257', '277275', '277315', '277316', '277433', '277435', '277436', '277446', '277447', '277452', '277464', '277468', '277474', '277475', '277476', '277484', '277489', '277523', '277530', '277532', '277539', '277567', '277568', '277575']
nosy_count = 6.0
nosy_names = ['rhettinger', 'belopolsky', 'python-dev', 'serhiy.storchaka', 'xiang.zhang', 'jiangping.li']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = '28292'
type = 'behavior'
url = 'https://bugs.python.org/issue28253'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7']

[lijp003163com]

hi
I`m a python newer.
when I use python3.4.3 to learn about package calendar,I found a problem, Could you help me to confirm it which is is a bug.

Below is my env:
------------------------------
os: win7 x86
python version:3.4.3
------------------------------
problem:
when I use calendar.prcal() method to print detail calendar information. the display December of year 9999 `s localtion is wrong.
my python source:
--------------------------------------

import calendar
calendar.prcal(9999)

---

partial output:

but,when I use [print (calendar.month(9999,12))] to confirm, it`s OK .

please help me to confirm.
thank you.

[serhiy-storchaka]

Thank you for your report lijp. This is interesting issue. It is not specific for Windows.

itermonthdates() yields shortened sequence of dates at the end of maximal year. The formatted row of day numbers is centered. Since the number of days is less than expected, additional spaces are added at the left.

[zhangyangyu]

calendar internally relies on datetime and uses datetime.date. datetime.date can't represent date more than 9999.12.31 so some logic is broken for it(normally the week list should be of length 7, date outside the month will be (0, x) pair but this is not the case for 9999.12). This won't affect prmonth since it doesn't do center formatting. We can change to that behaviour to fix this. calendar_pryear_9999 tires to fix this.

[serhiy-storchaka]

Html calendar formats the last week differently for years 9982 and 9999.

Year 9982:
<tr><td class="mon">27</td><td class="tue">28</td><td class="wed">29</td><td class="thu">30</td><td class="fri">31</td><td class="noday"> </td><td class="noday"> </td></tr>

Year 9999:
<tr><td class="mon">27</td><td class="tue">28</td><td class="wed">29</td><td class="thu">30</td><td class="fri">31</td></tr>

I think this issue should be fixed on lower level than text calendar formatter.

[zhangyangyu]

Something like this?

diff -r 15f82b64eee0 Lib/calendar.py
--- a/Lib/calendar.py	Thu Sep 22 17:11:53 2016 -0700
+++ b/Lib/calendar.py	Fri Sep 23 16:27:03 2016 +0800
@@ -181,6 +181,9 @@
                 yield (0, date.weekday())
             else:
                 yield (date.day, date.weekday())
+                if year == 9999 and month == 12 and date.day == 31:
+                    yield (0, 6)
+                    yield (0, 7)
 
     def itermonthdays(self, year, month):
         """

Is there a more elegant way? I considered this but didn't quite like it so abandoned it.

[serhiy-storchaka]

This is not just not elegant, but it doesn't work if firstweekday is not 0.

The lowest level in the calendar module is itermonthdates(). But the problem is that dates outside supported range can't be represented.

While the output for December of the year 9999 looks badly formatted, calendar doesn't work at all with January of the year 1 if firstweekday is not 0:

>>> import calendar
>>> calendar.setfirstweekday(6)
>>> calendar.prmonth(1, 1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython-3.6/Lib/calendar.py", line 318, in prmonth
    print(self.formatmonth(theyear, themonth, w, l), end=' ')
  File "/home/serhiy/py/cpython-3.6/Lib/calendar.py", line 331, in formatmonth
    for week in self.monthdays2calendar(theyear, themonth):
  File "/home/serhiy/py/cpython-3.6/Lib/calendar.py", line 211, in monthdays2calendar
    days = list(self.itermonthdays2(year, month))
  File "/home/serhiy/py/cpython-3.6/Lib/calendar.py", line 179, in itermonthdays2
    for date in self.itermonthdates(year, month):
  File "/home/serhiy/py/cpython-3.6/Lib/calendar.py", line 162, in itermonthdates
    date -= datetime.timedelta(days=days)
OverflowError: date value out of range

[zhangyangyu]

How about the approach in calendar_prcal_9999_demo.patch? If it's not bad I can add tests then.

[serhiy-storchaka]

Having additional tests is always nice. After writing tests we can search whether there is other solution.

AFAIK the dummy data needs also the day attribute.