Because tuples are hashable and lists are not, i f we want to create a composite key to use in a dictionary we must use a tuple as the key.
We would encounter a composite key if we wanted to create a telephone directory that maps from last-name, first-name pairs to telephone numbers. Assuming that we have defined the variables last, first and number, we could write a dictionary assignment statement as follows:
directory[last,first] = number
The expression in brackets is a tuple. We could use tuple assignment in a for loop to traverse this dictionary.
for last, first in directory: print first, last, directory[last,first]
This loop traverses the keys in directory, which are tuples. It assigns the elements of each tuple to last and first, then prints the name and corresponding telephone number.