10.3 Dictionary operations

Learning objectives

By the end of this section you should be able to

• Recognize that a dictionary object is mutable.
• Evaluate dictionary items, keys, and values.
• Demonstrate the ability to access, evaluate, and modify dictionary items.
• Modify a dictionary by adding items.
• Modify a dictionary by removing items.

Accessing dictionary items

In Python, values associated with keys in a dictionary can be accessed using the keys as indexes. Here are two ways to access dictionary items in Python:

• Square bracket notation: Square brackets with the key inside access the value associated with that key. If the key is not found, an exception will be thrown.
• get method: The get method is called with the key as an argument to access the value associated with that key. If the key is not found, the method returns None by default, or a default value specified as the second argument.

Ex: In the code below, a dictionary object my_dict is initialized with items {"apple": 2, "banana": 3, "orange": 4}. The square bracket notation and get method are used to access values associated with the keys "banana" and "apple", respectively. When accessing the dictionary to obtain the key "pineapple", -1 is returned since the key does not exist in the dictionary.

my_dict = {"apple": 2, "banana": 3, "orange": 4}
print(my_dict["banana"]) # Prints: 3
print(my_dict.get("apple")) # Prints: 2
print(my_dict.get("pineapple", -1)) # Prints: -1

Obtaining dictionary keys and values

Dictionary keys, values, and both keys and values can be obtained using keys, values, and items function calls, respectively. The return type of keys, values, and items are dict_keys, dict_values, and dict_items, which can be converted to a list object using the list constructor list.

Dictionary mutability

In Python, a dictionary is a mutable data type, which means that a dictionary's content can be modified after creation. Dictionary items can be added, updated, or deleted from a dictionary after a dictionary object is created.

To add an item to a dictionary, either the square bracket notation or update function can be used.

• Square bracket notation: When using square brackets to create a new key object and assign a value to the key, the new key-value pair will be added to the dictionary. my_dict = {"apple": 2, "banana": 3, "orange": 4} my_dict["pineapple"] = 1 print(my_dict) # Prints: {"apple": 2, "banana": 3, "orange": 4, "pineapple": 1}
• update method: the update method can be called with additional key-value pairs to update the dictionary content. my_dict = {"apple": 2, "banana": 3, "orange": 4} my_dict.update({"pineapple": 1, "cherry": 0}) print(my_dict) # Prints: {"apple": 2, "banana": 3, "orange": 4, "pineapple": 1, "cherry": 0}

To modify a dictionary item, the two approaches above can be used on an existing dictionary key along with the updated value. Ex:

• Square bracket notation: my_dict = {"apple": 2, "banana": 3, "orange": 4} my_dict["apple"] = 1 print(my_dict) # Prints: {"apple": 1, "banana": 3, "orange": 4}
• update method: my_dict = {"apple": 2, "banana": 3, "orange": 4} my_dict.update({"apple": 1}) print(my_dict) # Prints: {"apple": 1, "banana": 3, "orange": 4}

Items can be deleted from a dictionary using the del keyword or the pop method.

• del keyword: my_dict = {"apple": 2, "banana": 3, "orange": 4} del my_dict["orange"] print(my_dict) # Prints: {"apple": 2, "banana": 3}
• pop method: my_dict = {"apple": 2, "banana": 3, "orange": 4} deleted_value = my_dict.pop("banana") print(deleted_value) # Prints: 3 print(my_dict) # Output: {"apple": 2, "orange": 4}