bpo-33812: Corrected astimezone for naive datetimes. (GH-7578)

A datetime object d is aware if d.tzinfo is not None and
d.tzinfo.utcoffset(d) does  not return None. If d.tzinfo is None,
or if d.tzinfo is not None but d.tzinfo.utcoffset(d) returns None,
 d is naive.

This commit ensures that instances with non-None d.tzinfo, but
d.tzinfo.utcoffset(d) returning None are treated as naive.

In addition, C acceleration code will raise TypeError if
d.tzinfo.utcoffset(d) returns an object with the type other than
timedelta.

* Updated the documentation.

Assume that the term "naive" is defined elsewhere and remove the
not entirely correct clarification.  Thanks, Tim.

2 files changed, 22 insertions, 12 deletions

diff --git a/Lib/datetime.py b/Lib/datetime.py
index 5e9aab9700..5e922c80b0 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -1773,14 +1773,17 @@ class datetime(date):
         mytz = self.tzinfo
         if mytz is None:
             mytz = self._local_timezone()
+            myoffset = mytz.utcoffset(self)
+        else:
+            myoffset = mytz.utcoffset(self)
+            if myoffset is None:
+                mytz = self.replace(tzinfo=None)._local_timezone()
+                myoffset = mytz.utcoffset(self)
 
         if tz is mytz:
             return self
 
         # Convert self to UTC, and attach the new time zone object.
-        myoffset = mytz.utcoffset(self)
-        if myoffset is None:
-            raise ValueError("astimezone() requires an aware datetime")
         utc = (self - myoffset).replace(tzinfo=tz)
 
         # Convert from UTC to tz's local time.
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index a7e5e0b424..7d4cdac9f4 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -2414,25 +2414,24 @@ class TestDateTime(TestDate):
         base = cls(2000, 2, 29)
         self.assertRaises(ValueError, base.replace, year=2001)
 
+    @support.run_with_tz('EDT4')
     def test_astimezone(self):
-        return  # The rest is no longer applicable
-        # Pretty boring!  The TZ test is more interesting here.  astimezone()
-        # simply can't be applied to a naive object.
         dt = self.theclass.now()
-        f = FixedOffset(44, "")
-        self.assertRaises(ValueError, dt.astimezone) # naive
+        f = FixedOffset(44, "0044")
+        dt_utc = dt.replace(tzinfo=timezone(timedelta(hours=-4), 'EDT'))
+        self.assertEqual(dt.astimezone(), dt_utc) # 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
-        self.assertRaises(ValueError, dt.astimezone, tz=f)  # naive
+        dt_f = dt.replace(tzinfo=f) + timedelta(hours=4, minutes=44)
+        self.assertEqual(dt.astimezone(f), dt_f) # naive
+        self.assertEqual(dt.astimezone(tz=f), dt_f) # naive
 
         class Bogus(tzinfo):
             def utcoffset(self, dt): return None
             def dst(self, dt): return timedelta(0)
         bog = Bogus()
         self.assertRaises(ValueError, dt.astimezone, bog)   # naive
-        self.assertRaises(ValueError,
-                          dt.replace(tzinfo=bog).astimezone, f)
+        self.assertEqual(dt.replace(tzinfo=bog).astimezone(f), dt_f)
 
         class AlsoBogus(tzinfo):
             def utcoffset(self, dt): return timedelta(0)
@@ -2440,6 +2439,14 @@ class TestDateTime(TestDate):
         alsobog = AlsoBogus()
         self.assertRaises(ValueError, dt.astimezone, alsobog) # also naive
 
+        class Broken(tzinfo):
+            def utcoffset(self, dt): return 1
+            def dst(self, dt): return 1
+        broken = Broken()
+        dt_broken = dt.replace(tzinfo=broken)
+        with self.assertRaises(TypeError):
+            dt_broken.astimezone()
+
     def test_subclass_datetime(self):
 
         class C(self.theclass):