10.7: Using Tuples as Keys in Dictionaries

Because tuples are hashable and lists are not, if 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.

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