model: set UTC timezone to DateTimes read rom JSON too

I came a cross this difference during testing of SAP#95

I decided to set tzinfo of JSON DateTimes to simplify my tests and it
also make sense to me to have the values configured the same way.

Author: filak-sap

pyodata/v2/model.py

@@ -26,6 +26,17 @@
 TypeInfo = collections.namedtuple("TypeInfo", "namespace name is_collection")
 
 
+def current_timezone():
+    """Default Timezone for Python datetime instances when parsed from
+       Edm.DateTime values and vice versa.
+
+       OData V2 does not mention Timezones in the documentation of
+       Edm.DateTime and UTC was chosen because it is universal.
+    """
+
+    return datetime.timezone.utc
+
+
 def modlog():
     return logging.getLogger(LOGGER_NAME)
 
@@ -429,11 +440,26 @@ def to_json(self, value):
 
         # Converts datetime into timestamp in milliseconds in UTC timezone as defined in ODATA specification
         # https://www.odata.org/documentation/odata-version-2-0/json-format/
-        return f"/Date({int(value.replace(tzinfo=datetime.timezone.utc).timestamp()) * 1000})/"
+        return f'/Date({int(value.replace(tzinfo=current_timezone()).timestamp()) * 1000})/'
 
     def from_json(self, value):
 
-        return self.from_literal(value)
+        if value is None:
+            return None
+
+        matches = re.match(r"^/Date\((.*)\)/$", value)
+        if not matches:
+            raise PyODataModelError(
+                "Malformed value {0} for primitive Edm type. Expected format is /Date(value)/".format(value))
+        value = matches.group(1)
+
+        try:
+            # https://stackoverflow.com/questions/36179914/timestamp-out-of-range-for-platform-localtime-gmtime-function
+            value = datetime.datetime(1970, 1, 1, tzinfo=current_timezone()) + datetime.timedelta(milliseconds=int(value))
+        except ValueError:
+            raise PyODataModelError('Cannot decode datetime from value {}.'.format(value))
+
+        return value
 
     def from_literal(self, value):
 
@@ -458,7 +484,7 @@ def from_literal(self, value):
                             "Cannot decode datetime from value {}.".format(value)
                         )
 
-        return value
+        return value.replace(tzinfo=current_timezone())
 
 
 class EdmStringTypTraits(TypTraits):

tests/test_model_v2.py

@@ -516,7 +516,7 @@ def test_traits_datetime():
 
     # 1. direction Python -> OData
 
-    testdate = datetime(2005, 1, 28, 18, 30, 44, 123456, tzinfo=timezone.utc)
+    testdate = datetime(2005, 1, 28, 18, 30, 44, 123456, tzinfo=current_timezone())
     assert typ.traits.to_literal(testdate) == "datetime'2005-01-28T18:30:44.123456'"
 
     # without miliseconds part
@@ -539,19 +539,22 @@ def test_traits_datetime():
     assert testdate.minute == 33
     assert testdate.second == 6
     assert testdate.microsecond == 654321
+    assert testdate.tzinfo == current_timezone()
 
     # parsing without miliseconds
     testdate = typ.traits.from_literal("datetime'1976-11-23T03:33:06'")
     assert testdate.year == 1976
     assert testdate.second == 6
     assert testdate.microsecond == 0
+    assert testdate.tzinfo == current_timezone()
 
     # parsing without seconds and miliseconds
     testdate = typ.traits.from_literal("datetime'1976-11-23T03:33'")
     assert testdate.year == 1976
     assert testdate.minute == 33
     assert testdate.second == 0
     assert testdate.microsecond == 0
+    assert testdate.tzinfo == current_timezone()
 
     # parsing invalid value
     with pytest.raises(PyODataModelError) as e_info:
@@ -573,19 +576,22 @@ def test_traits_datetime():
     assert testdate.minute == 33
     assert testdate.second == 6
     assert testdate.microsecond == 10000
+    assert testdate.tzinfo == current_timezone()
 
     # parsing without miliseconds
     testdate = typ.traits.from_json("/Date(217567986000)/")
     assert testdate.year == 1976
     assert testdate.second == 6
     assert testdate.microsecond == 0
+    assert testdate.tzinfo == current_timezone()
 
     # parsing without seconds and miliseconds
     testdate = typ.traits.from_json("/Date(217567980000)/")
     assert testdate.year == 1976
     assert testdate.minute == 33
     assert testdate.second == 0
     assert testdate.microsecond == 0
+    assert testdate.tzinfo == current_timezone()
 
     # parsing the lowest value
     with pytest.raises(OverflowError):
@@ -599,6 +605,7 @@ def test_traits_datetime():
     assert testdate.minute == 0
     assert testdate.second == 0
     assert testdate.microsecond == 0
+    assert testdate.tzinfo == current_timezone()
 
     # parsing the highest value
     with pytest.raises(OverflowError):
@@ -612,6 +619,7 @@ def test_traits_datetime():
     assert testdate.minute == 59
     assert testdate.second == 59
     assert testdate.microsecond == 999000
+    assert testdate.tzinfo == current_timezone()
 
     # parsing invalid value
     with pytest.raises(PyODataModelError) as e_info: