Implement packing of time zone aware native types

Partially implements #4.

temporenc/temporenc.py

@@ -80,6 +80,35 @@ def _detect_type(first):
         return 'DTSZ', precision, DTSZ_LENGTHS[precision]
 
 
+class _FixedOffset(datetime.tzinfo):
+    """Time zone information for a fixed offset from UTC."""
+
+    # Python 2 does not have any concrete tzinfo implementations in its
+    # standard library, hence this implementation. This implementation
+    # is based on the examples in the Python docs, in particular:
+    # https://docs.python.org/3.4/library/datetime.html#tzinfo-objects
+
+    ZERO = datetime.timedelta(0)
+
+    def __init__(self, minutes):
+        self._offset = datetime.timedelta(minutes=minutes)
+        sign = '+' if minutes >= 0 else '-'
+        hours, minutes = divmod(minutes, 60)
+        self._name = 'UTC{0}{1:02d}:{2:02d}'.format(sign, hours, minutes)
+
+    def utcoffset(self, dt):
+        return self._offset
+
+    def tzname(self, dt):
+        return self._name
+
+    def dst(self, dt):
+        return self.ZERO
+
+    def __repr__(self):
+        return '<{}>'.format(self._name)
+
+
 #
 # Public API
 #
@@ -432,6 +461,17 @@ def packb(
 
         if isinstance(value, (datetime.datetime, datetime.time)):
             handled = True
+
+            if tz_offset is None:
+                # Extract time zone information for tz aware values.
+                delta = value.utcoffset()
+                if delta is not None:
+                    # This is a tz aware value. Convert to UTC and
+                    # obtain the offset from UTC. The tzinfo attribute
+                    # is not used, so don't bother setting it.
+                    value = value - delta
+                    tz_offset = int(delta.total_seconds()) // 60
+
             if hour is None:
                 hour = value.hour
             if minute is None:

tests/test_temporenc.py

@@ -378,6 +378,29 @@ def test_native_packing():
     assert actual == expected
 
 
+def test_native_packing_time_zone():
+
+    # Python < 3.2 doesn't have concrete tzinfo implementations, so
+    # use the internal helper class instead to avoid depending on newer
+    # Python versions (or on pytz).
+    from temporenc.temporenc import _FixedOffset
+    tz = _FixedOffset(60)  # UTC +01:00
+
+    # DTZ
+    actual = temporenc.packb(
+        datetime.datetime(1983, 1, 15, 18, 25, 12, 0, tzinfo=tz),
+        type='DTZ')
+    expected = from_hex('cf 7e 0e 8b 26 44')
+    assert actual == expected
+
+    # DTSZ (microsecond, since native types have that precision)
+    actual = temporenc.packb(
+        datetime.datetime(1983, 1, 15, 18, 25, 12, 123456, tzinfo=tz),
+        type='DTSZ')
+    dtsz_us = from_hex('eb df 83 a2 c9 83 c4 81 10')
+    assert actual == dtsz_us
+
+
 def test_native_packing_with_overrides():
     actual = temporenc.packb(
         datetime.datetime(1984, 1, 16, 18, 26, 12, 123456),