dict(A.get(B), A, B) is a conveniance function for accessing dictionaries and avoiding exceptions. If the first paramter -- the key you are looking for -- does not exist in the dictionary, than the second paramter is returned instead. No value.index exception is raised ever.
dict(A.get(B) d, a) is a conveniance function for accessing dictionary that avoids value.index exceptions by returning the null if the looked for key does not exist in the dictionary. In most cases the neutral value is null. The neutral values of integer, float and boolean is 0, 0.0 or false resp.