For the Javascript version of this lesson, please click here.

Previously, we've studied lists and other data types that can only store a single value of an element. Sometimes, you may encounter situations during your program when you need to refer to your data in pairs. In that case, dictionaries are used. They are a data structure in Python that used a technique called hashing to store the data in key-value pairs.

Suppose you want to store information of students in a class, along with information identifiers. In this case, using variables or lists is not recommended as you will have to create too many variables and separating identifiers and their values from the list will be difficult. You will need something that stores and categorizes your data according to the identifiers. Dictionaries are excellent for this purpose as they allow programmers to categorize data into key-value pairs, that is, you can specify multiple values under a single identifier. This makes the problem of storing student information easier!

Dictionaries

Dictionaries allow for the storage of data in the form of key-value pairs. Keys are unique dictionary elements that are assigned a value. A key-value pair is termed as an item. You can consider keys as indices of a dictionary. That is, the values stored in a dictionary can be accessed using a key. Moreover, unlike lists, dictionaries can store a collection of elements (using a list or tuple) as a value for a single key.

Let's look at few examples of dictionaries in Python.

1student = {"ID": "01", "name":"Patrick", "age":18, "major":"Biology"}

Visualizing this example, we see that the data is stored somewhat like the illustration below.

If we want to store multiple values for a single key, all the values are grouped in a list (or a tuple) and assigned to the corresponding key.

1students = {"ID": ["01", "02", "03", "04"], "name":["Patrick", "William", "Elizabeth", "John"], "age":[18, 19, 19, 20], "major":["Biology", "CS", "Literature", "Mathematics"]}

Storing multiple values within a single key stores the data in the form of tables within the program.

To access values in a dictionary, keys are used as indices with the dictionary.

1student = {"ID": "01", "name":"Patrick", "age":18, "major":"Biology"}
2
3print(student["name"])

Properties of Dictionaries

• Dictionaries cannot have duplicate keys. That is, two keys of the same name in the same dictionary will generate an error.
• Values assigned to keys in a dictionary can be of any data type (as shown in the examples in the previous section).
• In Python, dictionaries are ordered. This means that they will be sorted in a defined order, and that order cannot change. For example, you cannot move the key-value pair from the first position in the dictionary to the last position.
• Keys of a dictionary must be immutable. This means that you can use strings, integers, or tuples as dictionary keys, but lists are not allowed.

  In programming, immutable means that the value of a variable cannot be changed over time. Strings, int, and tuples are examples of immutable data types in Python.

Dictionary Methods and Operations

Similar to lists and strings in Python, dictionaries also have built-in functions for insertion, deletion, updating elements, and other operations that need to be performed on dictionaries.

To get the total length of the dictionary, we use the len() function. The total length of the dictionary is the number of items (or key-value pairs) in the dictionary. We saw this in strings and lists as well.

1student = {"ID": "01", "name":"Patrick", "age":18, "major":"Biology"}
2
3print(len(student))

To add new items in a dictionary, there is no built-in method. Rather, they are directly assigned by specifying the key name and its value with the dictionary as shown in the code block below.

1student = {"ID": "01", "name":"Patrick", "age":18, "major":"Biology"}
2
3student["section"] = "A"
4
5print(student) #prints the updated dictionary

Another important operation in dictionaries is the removal of items. This is performed by using the del keyword by specifying the key name of the key-value pair with the dictionary name that needs to be removed.

1student = {"ID": "01", "name":"Patrick", "age":18, "major":"Biology"}
2
3del student["section"]
4
5print(student) #prints the updated dictionary

You can also remove the entire contents (all key-value pairs) of the dictionary using the dict.clear() method.

1student = {"ID": "01", "name":"Patrick", "age":18, "major":"Biology"}
2
3student.clear()
4
5print(student) #prints the updated dictionary

Some more commonly used dictionary methods are,

• To get all the items or key-value pairs from the dictionary, dict.items() is used. It returns the key-value pairs in the form of a tuple.
• To get all the keys in a dictionary, dict.keys() is used. This returns all the keys in the form of a list.
• dict.get(key) returns a value for the specified key. If the key is not in the dictionary, the method returns None. This is an alternate method (other than indexing) to get values for a key in the dictionary.

Summary

In this lesson, we learned about a new method to store data during the execution of a program. Variables, lists, and dictionaries are all different methods of storing data. Depending on the situation, these different data storage elements (or data structures) are used to store and manipulate data.

One Pager Cheat Sheet

• We will learn about dictionaries, a data structure in Python which uses hashing to store data in key-value pairs, allowing us to categorize data easily.
• Dictionaries allow for the storage and retrieval of data in the form of key-value pairs, where keys are used as indices.
• Dictionaries in Python are ordered collections of immutable key-value pairs, with no duplicate keys allowed.
• No, a dictionary cannot have two same keys, but two same values are allowed.
• Dictionaries have built-in methods for insertion, deletion, and other operations, as well as an len() function to get the total length of the dictionary.
• The code iterates through the items of a dictionary using dict.items() and prints them in the form of key, value, yielding the output "Peter 12".
• We learned about different data structures, such as variables, lists and dictionaries, to store and manipulate data during the execution of a program.