enum_value as a class attribute, bytes as a property

nptdms/timestamp.py

@@ -16,12 +16,11 @@ class TdmsTimestamp(object):
         :ivar ~.seconds: Seconds since the epoch as a signed integer
         :ivar ~.second_fractions: A positive number of 2^-64 fractions of a second
     """
+    enum_value = 0x44
 
     def __init__(self, seconds, second_fractions):
         self.seconds = seconds
         self.second_fractions = second_fractions
-        self.enum_value = 0x44
-        self.bytes = _struct_pack('<Qq', second_fractions, seconds)
 
     def __repr__(self):
         return "TdmsTimestamp({0}, {1})".format(self.seconds, self.second_fractions)
@@ -53,6 +52,10 @@ def as_datetime(self):
         microseconds = (self.second_fractions / fractions_per_us)
         return datetime(1904, 1, 1, 0, 0, 0) + timedelta(seconds=self.seconds) + timedelta(microseconds=microseconds)
 
+    @property
+    def bytes(self):
+        return _struct_pack('<Qq', self.second_fractions, self.seconds)
+
 
 class TimestampArray(np.ndarray):
     """ A numpy array of TDMS timestamps