Python Dictionary

In Python, a dictionary is a collection that allows us to store data in key-value pairs.

Create a Dictionary

We create dictionaries by placing key:value pairs inside curly brackets {}, separated by commas. For example,

# creating a dictionary
country_capitals = {
  "United States": "Washington D.C.", 
  "Italy": "Rome", 
  "England": "London"
}

# printing the dictionary
print(country_capitals)

Output

{'United States': 'Washington D.C.', 'Italy': 'Rome', 'England': 'London'}

The country_capitals dictionary has three elements (key-value pairs).

Note: Dictionary keys must be immutable, such as tuples, strings, integers, etc. We cannot use mutable (changeable) objects such as lists as keys.

# Valid dictionary

my_dict = {
  1: "Hello", 
  (1, 2): "Hello Hi", 
  3: [1, 2, 3]
}

print(my_dict)

# Invalid dictionary
# Error: using a list as a key is not allowed

my_dict = {
  1: "Hello", 
  [1, 2]: "Hello Hi", 
}

print(my_dict)

Tip: We can also use Python's dict() function to create dictionaries.

Python Dictionary Length

We can get the size of a dictionary by using the len() function.

country_capitals = {
  "United States": "Washington D.C.", 
  "Italy": "Rome", 
  "England": "London"
}

# get dictionary's length
print(len(country_capitals)) # 3

Access Dictionary Items

We can access the value of a dictionary item by placing the key inside square brackets.

country_capitals = {
  "United States": "Washington D.C.", 
  "Italy": "Rome", 
  "England": "London"
}

print(country_capitals["United States"])  # Washington D.C.

print(country_capitals["England"]) # London

Note: We can also use the get() method to access dictionary items.

Change Dictionary Items

Python dictionaries are mutable (changeable). We can change the value of a dictionary element by referring to its key. For example,

country_capitals = {
  "United States": "New York", 
  "Italy": "Naples", 
  "England": "London"
}

# change the value of "Italy" key to "Rome"
country_capitals["Italy"] = "Rome"

print(country_capitals)

Output

{'United States': 'Washington D.C.', 'Italy': 'Rome', 'England': 'London'}

Add Items to a Dictionary

We can add an item to the dictionary by assigning a value to a new key (that does not exist in the dictionary). For example,

country_capitals = {
  "United States": "New York", 
  "Italy": "Naples" 
}

# add an item with "Germany" as key and "Berlin" as its value
country_capitals["Germany"] = "Berlin"

print(country_capitals)

Output

{'United States': 'Washington D.C.', 'Italy': 'Rome', 'Germany': 'Berlin'}

Note: We can also use the update method() to add or change dictionary items.

Remove Dictionary Items

We use the del statement to remove an element from the dictionary. For example,

country_capitals = {
  "United States": "New York", 
  "Italy": "Naples" 
}

# delete item having "United States" key
del country_capitals["United States"]


print(country_capitals)

Output

{'Italy': 'Naples'}

Note: We can also use the pop method() to remove an item from the dictionary.

If we need to remove all items from the dictionary at once, we can use the clear() method.

country_capitals = {
  "United States": "New York", 
  "Italy": "Naples" 
}

country_capitals.clear()

print(country_capitals) # {}

Python Dictionary Methods

Here are some of the commonly used dictionary methods.

Dictionary Membership Test

We can check whether a key exists in a dictionary using the in operator.

my_list = {1: "Hello", "Hi": 25, "Howdy": 100}

print(1 in my_list) # True

# the not in operator checks whether key doesn't exist
print("Howdy" not in my_list) # False

print("Hello" in my_list) # False

Note: The in operator checks whether a key exists; it doesn't check whether a value exists or not.

Iterating Through a Dictionary

A dictionary is an ordered collection of items (starting from Python 3.7). Meaning a dictionary maintains the order of its items.

We can iterate through dictionary keys one by one using a for loop.

country_capitals = {
  "United States": "New York", 
  "Italy": "Naples" 
}

# print dictionary keys one by one
for country in country_capitals:
    print(country)

print("----------")

# print dictionary values one by one
for country in country_capitals:
    capital = country_capitals[country]
    print(capital)