Dictionaries

Introduction to Python Dictionaries

In this lesson, you will learn about dictionaries, a data structure in Python used to efficiently organize data with key-value pairs. You'll discover how to create dictionaries, as well as how to access and update stored data.

We have already learned how to store multiple values using a list. For example:

tasks = [1, 2, 3, 4, 5]

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But, lists are not the only way to store multiple values. Another data structure that can be used for this purpose are dictionaries.

Let's create out first Python dictionary:

person = {
  'age': 30,
  'name': 'John',
  'gender': 'male'
}

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We have created a dictionary called person that stores data about John. The data is stored in so called key-value pairs.

The keys in the dictionary are: 'age', 'name', and 'gender'.

Each key is associated with a specific value.

Just as with lists and tuples, the values in dictionaries can be of any type, such as strings, integers, or even other dictionaries.

Dictionaries are defined using curly braces { } with pairs separated by commas. Keys and values are separated by colons :

It doesn't matter whether you create a dictionary in a single line or multiple lines. The following are all valid dictionaries:

# single line:
{ 'age': 12, 'name': 'John' }
 
# multiple lines:
{
  'age': 15,
  'name': 'Alice'
}
# indentation doesn't matter:
{
  'age': 6,
'name': 'Bob'
}

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To access a value in a dictionary, you need the key associated with that value.

Two methods can be used to retrieve a value from a dictionary using its key: either by using square brackets or by using the dict.get(key) method:

person = { 'age': 12, 'name': 'John' }
 
# get age using square brackets:
print(person['age'])
 
# get age using dict.get() method:
print(person.get('age'))

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Output

So, why should you use a dictionary instead of a list or a tuple to store multiple values?

Let's compare our dictionary to a list that stores the same values:

person = [12, 'John']
 
print(person[0])

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Output

This works, but you need to remember that our person's age is stored at index 0.

Additionally, the index may change when we new values are added to the list.

If you have values associated with specific keys and require quick access to those values based on their keys, a dictionary is the ideal choice.

Here are some examples:

employee = {
  'id': 12345,
  'name': 'Bob',
  'department': 'IT',
  'salary': 70000
}
 
inventory = {
  'apple': 10,
  'banana': 15,
  'orange': 20
}

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However, when you simply need to store a collection of data without associating each element with a unique key, you should use a list or a tuple.

Some examples:

scores = [85, 90, 75, 85, 80]
 
daily_steps = (8420, 11280, 4528, 16090)

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What will be the output?

employee = {
  'id': 12345,
  'name': 'Bob',
  'department': 'IT',
  'salary': 70000
}
 
print(employee['name'])

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What will be the output?

employee = {
  'id': 12345,
  'name': 'Bob',
  'department': 'IT',
  'salary': 70000
}
 
print(employee.get('id'))

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Dictionaries, unlike lists and tuples, are not ordered.

This means you cannot use an index to access a value.

person = { 'age': 12, 'name': 'John' }
 
print(person[0])

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Output

Python throws the error: KeyError: 0 if we try to access a value at index 0.

This is because Python tried to find the key 0 in our dictionary, but couldn't find it.

Yes, integers are valid keys in dictionaries. Like in this example:

my_dict = {
  'str': 10,
  # integer as key
  10: 'str'
}
 
print(my_dict[10])
print(my_dict[5])

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Output

The 10 in my_dict[10] has nothing to do with index 10. Instead, it refers to the key 10.

You can use any immutable data type as key in dictionaries. For instance:

my_dict = {
  # string as key
  'str': 10,
  # integer as key
  10: 11,
  # tuple as key
  (5,10): 99
}
 
print(my_dict['str'])
print(my_dict[10])
print(my_dict[(5,10)])

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Output

However, keys have to be unique. They can not be used more than once in the same dictionary.

Let's see what happens if we don't follow this rule:

my_dict = {
  'key': 10,
  # use key again
  'key': 99
}
 
print(my_dict)

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Output

As you can see, Python simply overwrites the first value.

Values, on the other hand, don't have to be unique:

my_dict = {
  'a': 10,
  # use same value again
  'b': 10
}
 
print(my_dict)

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Output

What will be the output?

my_dict = { 'a': 1, 'b': 2 }
 
print(my_dict[0])

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What will be the output?

my_dict = { 'a': 1, 0: 2 }
 
print(my_dict[0])

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What will be the output?

my_dict = { 'a': 1, 0: 2 }
 
print(my_dict[2])

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What will be the output?

my_dict = { 'a': 1, 0: 2 }
 
print(my_dict['a'])

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What will be the output?

my_dict = { 'a': 1, 0: 2, 'a': 2 }
 
print(my_dict['a'])

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What will be the output?

my_dict = { 'a': 1, 0: (1,2), (1,2): 2 }
 
print(my_dict.get((1,2)))

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