Merge pull request #9 from dhalbert/defer-packet_size

Defer packet size

Author: dhalbert

adafruit_ble_heart_rate.py

@@ -58,20 +58,20 @@
 )
 """Namedtuple for measurement values.
 
-.. :attribute:: heart_rate:
+* `HeartRateMeasurementValues.heart_rate`
 
         Heart rate (int), in beats per minute.
 
-.. :attribute:: contact:
+* `HeartRateMeasurementValues.contact`
 
         ``True`` if device is contacting the body, ``False`` if not,
         ``None`` if device does not support contact detection.
 
-.. :attribute:: energy_expended:
+* `HeartRateMeasurementValues.energy_expended`
 
         Energy expended (int), in kilo joules, or ``None`` if no value.
 
-.. :attribute:: rr_intervals:
+* `HeartRateMeasurementValues.rr_intervals`
 
         Sequence of RR intervals, measuring the time between
         beats. Oldest first, in ints that are units of 1024ths of a second.
@@ -139,9 +139,8 @@ class HeartRateService(Service):
 
     def __init__(self, service=None):
         super().__init__(service=service)
-        self._measurement_buf = bytearray(
-            self.heart_rate_measurement.packet_size  # pylint: disable=no-member
-        )
+        # Defer creating buffer until needed.
+        self._measurement_buf = None
 
     @property
     def measurement_values(self):
@@ -150,6 +149,10 @@ def measurement_values(self):
 
         Return ``None`` if no packet has been read yet.
         """
+        if self._measurement_buf is None:
+            self._measurement_buf = bytearray(
+                self.heart_rate_measurement.packet_size  # pylint: disable=no-member
+            )
         buf = self._measurement_buf
         packet_length = self.heart_rate_measurement.readinto(  # pylint: disable=no-member
             buf