Make netcdftime.datetime immutable and hashable. Fixes Unidata#255.

Author: Daniel da Silva

netcdftime.py

@@ -19,7 +19,7 @@
 )
 TIMEZONE_REGEX = re.compile("(?P<prefix>[+-])(?P<hours>[0-9]{1,2}):(?P<minutes>[0-9]{1,2})")
 
-class datetime:
+class datetime(object):
     """
 Phony datetime object which mimics the python datetime object,
 but allows for dates that don't exist in the proleptic gregorian calendar.
@@ -44,6 +44,8 @@ def __init__(self,year,month,day,hour=0,minute=0,second=0,dayofwk=-1,dayofyr=1):
         self.dayofyr=dayofyr
         self.second=second
         self.format='%Y-%m-%d %H:%M:%S'
+        self._immutable = True
+
     def strftime(self,format=None):
         if format is None:
             format = self.format
@@ -53,6 +55,15 @@ def timetuple(self):
     def __repr__(self):
         return self.strftime(self.format)
 
+    def __hash__(self):
+        return hash(self.timetuple())
+
+    def __setattr__(self, name, value):
+        if hasattr(self, '_immutable'):
+            raise AttributeError("attempt to change immutable instance")
+        else:
+            object.__setattr__(self, name, value)
+
     def _compare(self, comparison_op, other):
         if hasattr(other, 'strftime'):
             return comparison_op(self.strftime('%Y-%m-%d %H:%M:%S'),
@@ -78,6 +89,7 @@ def __ge__(self, other):
         return self._compare(operator.ge, other)
 
 
+
 def JulianDayFromDate(date,calendar='standard'):
 
     """

test/tst_netcdftime.py

@@ -198,6 +198,17 @@ def runTest(self):
         assert d2 > d1
         assert d2 >= d1
 
+        # check datetime hash
+        assert hash(d1) == hash(d1)
+        assert hash(d1) != hash(d3)
+
+        # check datetime immutability
+        try:
+            d1.year = 1999
+            self.fail("datetime is mutable")
+        except AttributeError:
+            pass
+
 
 class TestDate2index(unittest.TestCase):
 
@@ -370,5 +381,6 @@ def test_select_nc(self):
             assert_equal(date.replace(tzinfo=tzutc()), date2)
         f.close()
 
+
 if __name__ == '__main__':
     unittest.main()