Issue #9527: datetime.astimezone() method will now supply a class

timezone instance corresponding to the system local timezone when
called with no arguments.

2 files changed, 44 insertions, 5 deletions

diff --git a/Lib/datetime.py b/Lib/datetime.py
index 6ab2499..ce88d85 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -1493,8 +1493,32 @@ class datetime(date):
         return datetime(year, month, day, hour, minute, second,
                           microsecond, tzinfo)
 
-    def astimezone(self, tz):
-        if not isinstance(tz, tzinfo):
+    def astimezone(self, tz=None):
+        if tz is None:
+            if self.tzinfo is None:
+                raise ValueError("astimezone() requires an aware datetime")
+            ts = (self - _EPOCH) // timedelta(seconds=1)
+            localtm = _time.localtime(ts)
+            local = datetime(*localtm[:6])
+            try:
+                # Extract TZ data if available 
+                gmtoff = localtm.tm_gmtoff
+                zone = localtm.tm_zone
+            except AttributeError:
+                # Compute UTC offset and compare with the value implied
+                # by tm_isdst.  If the values match, use the zone name
+                # implied by tm_isdst.
+                delta = local - datetime(*_time.gmtime(ts)[:6])
+                dst = _time.daylight and localtm.tm_isdst > 0
+                gmtoff = _time.altzone if dst else _time.timezone
+                if delta == timedelta(seconds=-gmtoff):
+                    tz = timezone(delta, _time.tzname[dst])
+                else:
+                    tz = timezone(delta)
+            else:
+                tz = timezone(timedelta(seconds=-gmtoff), zone)
+                
+        elif not isinstance(tz, tzinfo):
             raise TypeError("tz argument must be an instance of tzinfo")
 
         mytz = self.tzinfo
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 048d63c..e045447 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -1972,7 +1972,7 @@ class TestDateTime(TestDate):
         # simply can't be applied to a naive object.
         dt = self.theclass.now()
         f = FixedOffset(44, "")
-        self.assertRaises(TypeError, dt.astimezone) # not enough args
+        self.assertRaises(ValueError, dt.astimezone) # naive
         self.assertRaises(TypeError, dt.astimezone, f, f) # too many args
         self.assertRaises(TypeError, dt.astimezone, dt) # arg wrong type
         self.assertRaises(ValueError, dt.astimezone, f) # naive
@@ -3253,8 +3253,6 @@ class TestDateTimeTZ(TestDateTime, TZInfoBase, unittest.TestCase):
         self.assertTrue(dt.tzinfo is f44m)
         # Replacing with degenerate tzinfo raises an exception.
         self.assertRaises(ValueError, dt.astimezone, fnone)
-        # Ditto with None tz.
-        self.assertRaises(TypeError, dt.astimezone, None)
         # Replacing with same tzinfo makes no change.
         x = dt.astimezone(dt.tzinfo)
         self.assertTrue(x.tzinfo is f44m)
@@ -3274,6 +3272,23 @@ class TestDateTimeTZ(TestDateTime, TZInfoBase, unittest.TestCase):
         self.assertTrue(got.tzinfo is expected.tzinfo)
         self.assertEqual(got, expected)
 
+    @support.run_with_tz('UTC')
+    def test_astimezone_default_utc(self):
+        dt = self.theclass.now(timezone.utc)
+        self.assertEqual(dt.astimezone(None), dt)
+        self.assertEqual(dt.astimezone(), dt)
+
+    @support.run_with_tz('EST+05EDT,M3.2.0,M11.1.0')
+    def test_astimezone_default_eastern(self):
+        dt = self.theclass(2012, 11, 4, 6, 30, tzinfo=timezone.utc)
+        local = dt.astimezone()
+        self.assertEqual(dt, local)
+        self.assertEqual(local.strftime("%z %Z"), "+0500 EST") 
+        dt = self.theclass(2012, 11, 4, 5, 30, tzinfo=timezone.utc)
+        local = dt.astimezone()
+        self.assertEqual(dt, local)
+        self.assertEqual(local.strftime("%z %Z"), "+0400 EDT") 
+
     def test_aware_subtract(self):
         cls = self.theclass