Do not update sensor values on error

Currently, we update self._temperature and self._humidity, even if it
turns out that the data returned by the sensor was bogus. If the user
queries the data within two seconds after an error, they will actually
get wrong data.

Fix this by updating _temperature and _humidity attributes only if no
error was detected.

Author: michaellass

adafruit_dht.py

@@ -175,6 +175,9 @@ def measure(self):
                 (time.monotonic()-self._last_called) > delay_between_readings):
             self._last_called = time.monotonic()
 
+            new_temperature = 0
+            new_humidity = 0
+
             if _USE_PULSEIO:
                 pulses = self._get_pulses_pulseio()
             else:
@@ -188,19 +191,19 @@ def measure(self):
 
                 if self._dht11:
                     # humidity is 1 byte
-                    self._humidity = buf[0]
+                    new_humidity = buf[0]
                     # temperature is 1 byte
-                    self._temperature = buf[2]
+                    new_temperature = buf[2]
                 else:
                     # humidity is 2 bytes
-                    self._humidity = ((buf[0]<<8) | buf[1]) / 10
+                    new_humidity = ((buf[0]<<8) | buf[1]) / 10
                     # temperature is 2 bytes
                     # MSB is sign, bits 0-14 are magnitude)
                     raw_temperature = (((buf[2] & 0x7f)<<8) | buf[3]) / 10
                     # set sign
                     if buf[2] & 0x80:
                         raw_temperature = -raw_temperature
-                    self._temperature = raw_temperature
+                    new_temperature = raw_temperature
                 # calc checksum
                 chk_sum = 0
                 for b in buf[0:4]:
@@ -213,10 +216,14 @@ def measure(self):
 
             elif len(pulses) >= 10:
                 # We got *some* data just not 81 bits
-                raise RuntimeError("A full buffer was not returned.  Try again.")
+                raise RuntimeError("A full buffer was not returned. Try again.")
             else:
                 # Probably a connection issue!
                 raise RuntimeError("DHT sensor not found, check wiring")
+
+            self._temperature = new_temperature
+            self._humidity = new_humidity
+
     @property
     def temperature(self):
         """ temperature current reading.  It makes sure a reading is available