using tsl.lux can produce unanticipated Exception

using tsl.lux without consideration of all possible values will eventually produce an unanticipated Exception. 
return values can be a float, 0, or None. 
implicit datatype handling provides float(0) == 0.0 but float(None) will produce an Exception. 
alternatively, one should check isinstance( tsl.lux, Float ) but that would not explicitly show tsl.lux can be None in this example.

Author: csylvain

examples/tsl2561_simpletest.py

@@ -34,7 +34,7 @@
 # Get raw (luminosity) readings using tuple unpacking
 #broadband, infrared = tsl.luminosity
 
-# Get computed lux value
+# Get computed lux value (tsl.lux can return None or a float)
 lux = tsl.lux
 
 # Print results
@@ -43,7 +43,10 @@
 print("Integration time = {}".format(tsl.integration_time))
 print("Broadband = {}".format(broadband))
 print("Infrared = {}".format(infrared))
-print("Lux = {}".format(lux))
+if lux != None:
+  print("Lux = {}".format(lux))
+else:
+  print("Lux = NaN")
 
 # Disble the light sensor (to save power)
 tsl.enabled = False