Dictionary Keys

Using Numbers as Keys

You can use integers or even floats as dictionary keys. This is useful when the "label" for your data is naturally a number, but you still want the properties of a dictionary rather than the indexed order of a list.

• Print the dictionary with the integer key.
• Change one of the keys to True, and print the dictionary.
• Now try changing one of the keys to a list, new_dict = {[100] : 'balloons', 'color' : 'red'}. print(new_dict[[100]])
• print the color key in this new dictionary; new_dict = {'99' : 'balloons', 'color' : 'red', 'color' : 'blue'}

Key Rules to Remember

• Immutability: Keys must be a data type that cannot be changed (Strings, Integers, and Tuples are common keys). You cannot use a list as a key because lists can be modified.
• Uniqueness: Every key in a dictionary must be unique. If you try to use the same key twice, the second one will simply overwrite the first.

Why use an Integer key instead of a List index?

It might look like new_dict[99] is the same as accessing a list at index 99, but it is very different:

1. In a list: To have an item at index 99, you must have 98 other items before it.
2. In a dictionary: You can have a single item with the key 99 without needing any other data. It is a label, not a position.

Exercise 1: Integer Keys

Practice using whole numbers as identifiers instead of strings.

• Task: Create a dictionary called prices where the key is the item ID (an integer) and the value is the price (a float).
• Goal: Print the price by targeting the key.

Exercise 2: Mixed Key Types

Dictionaries can handle a variety of immutable types at once.

• Starting Code: user_data = {1: "Admin", "status": "Active", 2.5: "Version"}
• Task: Access and print the value associated with the float key 2.5.
• Goal: Prove that decimals (floats) can also function as keys.

Exercise 3: The Uniqueness Rule

See what happens when you try to reuse a key.

• Task: Create a dictionary: points = {"team_a": 10, "team_a": 20}.
• Goal: Print points["team_a"] and observe which value remains.

Exercise 4: The "List as Key" Error

Identify what types are not allowed as keys.

• Task: Try to create this dictionary: bad_dict = {[1, 2]: "numbers"}.
• Goal: Run the code and identify the error (it should be a TypeError: unhashable type: 'list').

Exercise 5: Real-world Mapping

Use integers as keys for a simple ID system.

• Task: Create a dictionary employees where the keys are employee IDs (1001, 1002) and the values are their names ("Sarah", "John").
• Goal: Print a sentence: "Employee 1001 is " + employees[1001].