Dictionaries

In this section, you’ll learn how to use dictionaries which allow you to connect pieces of related information. Dictionaries let you model a variety of real-world objects more accurately. We will create, modify and access elements of a dictionary.

Creating a dictionary

experiment = {"description": "resource optimization"}
print(type(experiment))

Above code snippet creates a simple dictionary and prints its type:

>>> Output

<class 'dict'>

In Python, a dictionary is wrapped in curly braces ({}), with a series of key-value pairs inside the braces. Each key is connected to a value, and you can use a key to access the value associated with that key. A key’s value can be any type, like a string, integer, list, or even another dictionary. In the above example, the key is "description" and its value "resource optimization"

Every key is connected to its value by a colon. Individual key-value pairs are separated by commas. You can store as many key-value pairs as you want in a dictionary.

experiment = {
    "description": "resource optimization",
    "sample_weight_in_grams": 5,
}

Accessing values

To get the value associated with a key, give the name of the dictionary and then place the key inside a set of square brackets.

experiment = {"sample_weight_in_grams": 5}
print(experiment["sample_weight_in_grams"])

>>> Output

5

Create a dictionary

Manage the cost of raw materials in a dictionary. The dictionary should contain the following key-value pairs:

• "steel": 100
• "aluminium": 150
• "copper": 200
• "plastic": 50

Create the dictionary and print the price of copper.

Adding key-value pairs

You can add new key-value pairs to a dictionary at any time. For example, to add a new key-value pair, you would give the name of the dictionary followed by the new key in square brackets along with the new value.

experiment = {}
experiment["description"] = "resource optimization"
print(experiment)

In the above example, we start with an empty dictionary and add a key-value pair to it.

>>> Output

{'description': 'resource optimization'}

However, we can't add the same key a second time to the dictionary. Every key is unique within the dictionary.

Modifying values

Values can be overwritten:

experiment = {"sample_weight_in_grams": 10}
experiment["sample_weight_in_grams"] = 10.2

Removing key-value pairs

We can remove key-value-pairs using the key and the del statement:

experiment = {
    "supervisor": "Alex",
    "sample_weight_in_grams": 10,
}

print(experiment)

del experiment["supervisor"]

print(experiment)

>>> Output

{'supervisor': 'Alex', 'sample_weight_in_grams': 10}
{'sample_weight_in_grams': 10}

Modify a dictionary

Remember that a value can hold any data type? You are given a dictionary with production data.

production = {
    "singapore": {"steel": 100, "aluminium": 150},
    "taipeh": {"steel": 200, "aluminium": 250},
    "linz": {"steel": 300, "aluminium": 350, "copper": 100},
}

Each key represents a location and has another dictionary as value. This dictionary contains the production quantity of different materials.

• Remove linz from the dictionary.
• Add a new location vienna with the production of 200 steel and 250 aluminium.
• Print the aluminium value of taipeh (try accessing it step by step and use variables for each step).

Info

At first, the above example might seem a bit too overcomplicated. However, nesting (in this example: storing a dictionary within a dictionary) is common practice and as already discussed, lets you represent more complex data structures. Even databases like Redis and MongoDB are at it's core key value stores, just like our dictionary above.

Recap

We have covered following topics in this section:

• Dictionaries store key-value pairs
• How to get and modify values
• Adding key-value pairs
• Removing key-value pairs with del

Lastly, as part of the Containers topic, we will have a look at tuples.