Using what you posted if SC is not None: but still have me the error "The field is not nullable". So I check the attributes filed and there are some blanks and nulls. I am trying to run this on the how feature class table not just the selected features.
def Reclass(SC):
if SC not in [None, "", " "]:
if (SC == "GR"):
return "GRAIN"
elif (SC == "WR"):
return "WRITE"
elif (SC == "PL"):
return "PLACE"
updateField = Reclass(!SC!)