11.7. 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

Write code to create a dictionary called ‘d1’, and in it give the tuple (1, ‘a’) a value of “tuple”.

==== from unittest.gui import TestCaseGui class MyTests(TestCaseGui): def testOne(self): self.assertEqual(d1[(1, "a")], "tuple", "Checking to make sure the key has the correct value.") MyTests().main()

11-9-2: Which of these options can be keys of a dictionary? Select all that apply.

• Dictionaries
• Incorrect! Dictionaries cannot be the keys of other dictionaries. Try again.
• Tuples
• Correct! It is fine to use tuples as keys in a dictionary.
• Strings
• Correct! Strings are used as keys of dictionaries all the time!
• Integers
• Correct! Integers are perfectly acceptable to be keys of dictionaries.
• Lists
• Incorrect! Lists cannot be used as the keys of dictionaries. Try again.

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.

11-9-3: Which of the following lines of code correctly prints the value associated with (‘Go’, ‘Blue’)? Select all that apply.

my_dict = {}
my_dict[('Go', 'Blue')] = True

• my_dict['Go']
• Incorrect! You need both values in the tuple for the dictionary to recognize it as the correct key. Try again.
• my_dict['Blue']
• Incorrect! You need both values in the tuple for the dictionary to recognize it as the correct key. Try again.
• my_dict['Go', 'Blue']
• Correct! In this case, the parentheses of the tuple are not required in order to properly call its value.
• my_dict[('Go', 'Blue')]
• Correct! This is one way to grab the value associated with the tuple.

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