Closes bpo-28281: Remove year (1-9999) limits on the weekday() function. (#4109)

Patch by Mark Gollahon.

5 files changed, 74 insertions, 5 deletions

diff --git a/Doc/library/calendar.rst b/Doc/library/calendar.rst
index 8fe93fd4de3..56b75ef0f85 100644
--- a/Doc/library/calendar.rst
+++ b/Doc/library/calendar.rst
@@ -19,11 +19,13 @@ the week to Sunday (6) or to any other weekday.  Parameters that specify dates
 are given as integers. For related
 functionality, see also the :mod:`datetime` and :mod:`time` modules.
 
-Most of these functions and classes rely on the :mod:`datetime` module which
-uses an idealized calendar, the current Gregorian calendar extended
+The functions and classes defined in this module
+use an idealized calendar, the current Gregorian calendar extended indefinitely
 in both directions.  This matches the definition of the "proleptic Gregorian"
 calendar in Dershowitz and Reingold's book "Calendrical Calculations", where
-it's the base calendar for all computations.
+it's the base calendar for all computations.  Zero and negative years are
+interpreted as prescribed by the ISO 8601 standard.  Year 0 is 1 BC, year -1 is
+2 BC, and so on.
 
 
 .. class:: Calendar(firstweekday=0)
diff --git a/Lib/calendar.py b/Lib/calendar.py
index fb594e0f5b0..3828c43ed27 100644
--- a/Lib/calendar.py
+++ b/Lib/calendar.py
@@ -111,8 +111,9 @@ def leapdays(y1, y2):
 
 
 def weekday(year, month, day):
-    """Return weekday (0-6 ~ Mon-Sun) for year (1970-...), month (1-12),
-       day (1-31)."""
+    """Return weekday (0-6 ~ Mon-Sun) for year, month (1-12), day (1-31)."""
+    if not datetime.MINYEAR <= year <= datetime.MAXYEAR:
+        year = 2000 + year % 400
     return datetime.date(year, month, day).weekday()
 
 
diff --git a/Lib/test/test_calendar.py b/Lib/test/test_calendar.py
index ad8b6bb6b93..6241d114d33 100644
--- a/Lib/test/test_calendar.py
+++ b/Lib/test/test_calendar.py
@@ -9,6 +9,56 @@ import sys
 import datetime
 import os
 
+# From https://en.wikipedia.org/wiki/Leap_year_starting_on_Saturday
+result_0_02_text = """\
+     February 0
+Mo Tu We Th Fr Sa Su
+    1  2  3  4  5  6
+ 7  8  9 10 11 12 13
+14 15 16 17 18 19 20
+21 22 23 24 25 26 27
+28 29
+"""
+
+result_0_text = """\
+                                   0
+
+      January                   February                   March
+Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
+                1  2          1  2  3  4  5  6             1  2  3  4  5
+ 3  4  5  6  7  8  9       7  8  9 10 11 12 13       6  7  8  9 10 11 12
+10 11 12 13 14 15 16      14 15 16 17 18 19 20      13 14 15 16 17 18 19
+17 18 19 20 21 22 23      21 22 23 24 25 26 27      20 21 22 23 24 25 26
+24 25 26 27 28 29 30      28 29                     27 28 29 30 31
+31
+
+       April                      May                       June
+Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
+                1  2       1  2  3  4  5  6  7                1  2  3  4
+ 3  4  5  6  7  8  9       8  9 10 11 12 13 14       5  6  7  8  9 10 11
+10 11 12 13 14 15 16      15 16 17 18 19 20 21      12 13 14 15 16 17 18
+17 18 19 20 21 22 23      22 23 24 25 26 27 28      19 20 21 22 23 24 25
+24 25 26 27 28 29 30      29 30 31                  26 27 28 29 30
+
+        July                     August                  September
+Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
+                1  2          1  2  3  4  5  6                   1  2  3
+ 3  4  5  6  7  8  9       7  8  9 10 11 12 13       4  5  6  7  8  9 10
+10 11 12 13 14 15 16      14 15 16 17 18 19 20      11 12 13 14 15 16 17
+17 18 19 20 21 22 23      21 22 23 24 25 26 27      18 19 20 21 22 23 24
+24 25 26 27 28 29 30      28 29 30 31               25 26 27 28 29 30
+31
+
+      October                   November                  December
+Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
+                   1             1  2  3  4  5                   1  2  3
+ 2  3  4  5  6  7  8       6  7  8  9 10 11 12       4  5  6  7  8  9 10
+ 9 10 11 12 13 14 15      13 14 15 16 17 18 19      11 12 13 14 15 16 17
+16 17 18 19 20 21 22      20 21 22 23 24 25 26      18 19 20 21 22 23 24
+23 24 25 26 27 28 29      27 28 29 30               25 26 27 28 29 30 31
+30 31
+"""
+
 result_2004_01_text = """\
     January 2004
 Mo Tu We Th Fr Sa Su
@@ -343,12 +393,20 @@ class OutputTestCase(unittest.TestCase):
             self.normalize_calendar(calendar.calendar(2004)),
             self.normalize_calendar(result_2004_text)
         )
+        self.assertEqual(
+            self.normalize_calendar(calendar.calendar(0)),
+            self.normalize_calendar(result_0_text)
+        )
 
     def test_output_textcalendar(self):
         self.assertEqual(
             calendar.TextCalendar().formatyear(2004),
             result_2004_text
         )
+        self.assertEqual(
+            calendar.TextCalendar().formatyear(0),
+            result_0_text
+        )
 
     def test_output_htmlcalendar_encoding_ascii(self):
         self.check_htmlcalendar_encoding('ascii', 'ascii')
@@ -393,6 +451,10 @@ class OutputTestCase(unittest.TestCase):
             calendar.TextCalendar().formatmonth(2004, 1),
             result_2004_01_text
         )
+        self.assertEqual(
+            calendar.TextCalendar().formatmonth(0, 2),
+            result_0_02_text
+        )
 
     def test_formatmonthname_with_year(self):
         self.assertEqual(
diff --git a/Misc/ACKS b/Misc/ACKS
index 7b174c63535..ac7a258df18 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -545,6 +545,7 @@ Karan Goel
 Jeroen Van Goey
 Christoph Gohlke
 Tim Golden
+Mark Gollahon
 Guilherme Gonçalves
 Tiago Gonçalves
 Chris Gonnerman
diff --git a/Misc/NEWS.d/next/Library/2017-10-26-14-54-38.bpo-28281.7ZN5FG.rst b/Misc/NEWS.d/next/Library/2017-10-26-14-54-38.bpo-28281.7ZN5FG.rst
new file mode 100644
index 00000000000..e7681dd4e8f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-10-26-14-54-38.bpo-28281.7ZN5FG.rst
@@ -0,0 +1,3 @@
+Remove year (1-9999) limits on the Calendar.weekday() function.
+
+Patch by Mark Gollahon.