Dictionaries

You may be thinking, isn’t a dictionary just a list of words and their meanings? Yes, in real life it is! But in Python, a dictionary is a special data structure that allows you to store data in key-value pairs.

Key-value pairs means that each item in the dictionary has a unique key that is used to access its corresponding value.

In fact when you think about it, a real-life dictionary is also made up of key-value pairs, where the key is the word and the value is its meaning.

We use dictionaries in Python when we want to store and organize data that is related to each other in some way.

Dictionaries are very useful because they allow us to quickly look up values based on their keys, making data retrieval efficient and straightforward.

Let’s see how we can create and use dictionaries in Python.

Creating a dictionary

You can create a dictionary by placing key-value pairs inside curly braces {}, separated by commas. Each key is separated from its value by a colon :.

Let’s create a simple dictionary that represents a person:

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "hair_color": "brown"
}
 
print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York', 'hair_color': 'brown'}

In this example, we created a dictionary named person with three key-value pairs:

• The key "name" has the value "Alice"
• The key "age" has the value 30
• The key "city" has the value "New York"

Remember that keys must be unique within a dictionary.

If we tried to create another key "name" in the same dictionary, it would overwrite the previous value.

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "hair_color": "brown",
    "name": "Aly"  # This will overwrite the previous "name" key
}
 
print(person)  # Output: {'name': 'Aly', 'age': 30, 'city': 'New York', 'hair_color': 'brown'}

As you can see, the value for the key "name" is now "Aly" because it overwrote the previous value.

Values, on the other hand, can be of any data type and can be duplicated.

if we had another key eye_color with the value "brown", that would be perfectly fine.

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "hair_color": "brown",
    "eye_color": "brown"  # This is perfectly fine
}
print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York', 'hair_color': 'brown', 'eye_color': 'brown'}

Accessing dictionary values

You can access the value associated with a specific key in a dictionary by using square brackets [] with the key inside the brackets.

let’s say we want to access the age of the person in our person dictionary:

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "hair_color": "brown"
}
print(person["age"])  # Output: 30

In this example, we accessed the value associated with the key "age" in the person dictionary, which is 30.

You can also use the get() method to access dictionary values. This method is useful because it allows you to provide a default value if the key does not exist in the dictionary.

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "hair_color": "brown"
}
print(person.get("age"))          # Output: 30
print(person.get("occupation", "Not specified"))  # Output: Not specified

In this example, we used the get() method instead of square brackets [] to access the value associated with the key "age", which is 30. Then we tried to access the value for the key "occupation", which does not exist in the dictionary. Since we provided a default value of "Not specified", that is what was returned.

Modifying dictionary values

You can also modify the value associated with a specific key in a dictionary by using square brackets [] with the key inside the brackets and assigning a new value.

It’s Alice’s birthday and she just turned 31! Let’s update her age in the person dictionary:

person = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "hair_color": "brown"
}
 
person["age"] = 31  # Updating Alice's age
print(person)  # Output: {'name': 'Alice', 'age': 31, 'city': 'New York', 'hair_color': 'brown'}

In this example, we updated the value associated with the key "age" from 30 to 31.

Adding new key-value pairs

You can add new key-value pairs to a dictionary by using square brackets [] with the new key inside the brackets and assigning a value. Let’s say we want to add Alice’s occupation to the person dictionary:

person = {
    "name": "Alice",
    "age": 31,
    "city": "New York",
    "hair_color": "brown"
}
person["occupation"] = "Engineer"  # Adding a new key-value pair
print(person)  # Output: {'name': 'Alice', 'age': 31, 'city': 'New York', 'hair_color': 'brown', 'occupation': 'Engineer'}

Notice how this is the same syntax we used to modify an existing key’s value.

Removing key-value pairs

You can remove key-value pairs from a dictionary using the del statement or the pop() method. Let’s remove key-value pairs from the person dictionary using both methods:

person = {
    "name": "Alice",
    "age": 31,
    "city": "New York",
    "hair_color": "brown",
    "occupation": "Engineer"
}
 
# Using del statement
del person["hair_color"]
print(person)  # Output: {'name': 'Alice', 'age': 31, 'city': 'New York', 'occupation': 'Engineer'}
# Using pop() method
occupation = person.pop("occupation")
print(person)  # Output: {'name': 'Alice', 'age': 31, 'city': 'New York'}
print(occupation)  # Output: Engineer

In this example, we first used the del statement to remove the key-value pair with the key "hair_color".

Then we used the pop() method to remove the key-value pair with the key "occupation" and stored the removed value in the variable occupation.