Dictionaries in Python#

A dictionary is a collection of key-value pairs, i.e. key:value. Each key is unique and is used to access its corresponding value. Dictionaries are defined using curly braces {}.

See also

Here is a basic example of a dictionary:

# Example of a dictionary
my_dict = {
    'name': 'Alice',
    'age': 25,
    'city': 'New York'
}
print(my_dict)

{'name': 'Alice', 'age': 25, 'city': 'New York'}

Accessing Elements#

You can access elements in a dictionary by using their keys. If the key does not exist, Python will raise a KeyError.

# Accessing elements in a dictionary
print(my_dict['name'])
print(my_dict['age'])

Alice
25

# Accessing non-existing keys
print(my_dict['unknown'])

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[3], line 2
      1 # Accessing non-existing keys
----> 2 print(my_dict['unknown'])

KeyError: 'unknown'

Altering Elements#

You can alter elements in a dictionary by assigning a new value to an existing key, or add new key-value pairs by assigning a value to a new key.

# Altering elements in a dictionary
my_dict['age'] = 31
my_dict['city'] = 'Boston'
print(my_dict)

{'name': 'Alice', 'age': 31, 'city': 'Boston'}

Important Built-in Methods#

Here are some important built-in methods for dictionaries:

  • keys(): Returns a view object with all the keys in the dictionary.

  • values(): Returns a view object with all the values in the dictionary.

  • items(): Returns a view object with all the key-value pairs in the dictionary.

  • get(key): Returns the value for the specified key if it exists, else returns None.

# Using built-in methods
keys = my_dict.keys()
values = my_dict.values()
items = my_dict.items()
age = my_dict.get('age')

print('Keys:', keys)
print('Values:', values)
print('Items:', items)
print('Age:', age)

Keys: dict_keys(['name', 'age', 'city'])
Values: dict_values(['Alice', 31, 'Boston'])
Items: dict_items([('name', 'Alice'), ('age', 31), ('city', 'Boston')])
Age: 31

Using Dictionaries as Tables#

Dictionaries can also be used to store tabular data. For example, we can have a dictionary where the keys are column names and the values are lists of column data.

# Example of using dictionaries as tables
my_table = {
    'Name': ['Alice', 'Bob', 'Charlie'],
    'Age': [25, 30, 35],
    'City': ['New York', 'Los Angeles', 'Chicago']
}
print(my_table)

{'Name': ['Alice', 'Bob', 'Charlie'], 'Age': [25, 30, 35], 'City': ['New York', 'Los Angeles', 'Chicago']}

Dictionaries with variables#

It is also possible to store variables in a dictionary, which are evaluated when you access them

w1 = 5
h1 = 3
area1 = w1 * h1

w2 = 2
h2 = 4
area2 = w2 * h2

rectangles = {
    "width": [w1, w2],
    "height": [h1, h2],
    "area": [area1, area2]
}

print(rectangles)

{'width': [5, 2], 'height': [3, 4], 'area': [15, 8]}

Exercise#

You just measured the radius of three circles. Write them into a table and add a column with corresponding circle area measurements.

r1 = 12
r2 = 8
r3 = 15