Python dictionaries (dict): A guide with examples

Dictionary (dict) plays an essential role in Python. Unlike list, dictionary stores data in a key -> value format.

In this article, we’ll dive deep into how to use this collection in your programs with illustrative examples.

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What is a dictionary

A dictionary is a collection that stores items in a "key": "value" format, also known as an associative array. It’s similar in use to lists, but there are some differences:

1. Anything can be a key: from a simple string to complex objects.
2. In newer versions of Python (>= 3.7), items are stored in the order they were added.

How to create a dictionary

To create a dictionary, we can use curly braces:

ages = {"Yura": 18, "Masha": 22, "Pasha": 30}

print(ages)

Output:

{'Yura': 18, 'Masha': 22, 'Pasha': 30}

There’s a second method: we can use the dict constructor:

translations = dict(hello="hello", world="world")

print(translations)

Output:

{'hello': 'hello', 'world': 'world'}

In Python, you can also use the dict constructor to create a dictionary from a list of key-value pairs:

translations = dict([
  ["hello", "hello"],
  ["world", "world"]
])

print(translations)

Output:

{'hello': 'hello', 'world': 'world'}

It’s worth noting that Python doesn’t allow having multiple items with the same key in a dictionary:

letters = {
  "A": 1,
  "A": 2,
}

print(letters)

Output:

{'A': 2}

As you can see, "A": 2 overwrote "A": 1.

dictionary = dict([
    ["apple", "apple"],
    ["orange", "orange"],
    ["apple", "apple-fruit"]
])

print(dictionary)

List of dictionaries

Since a dict dictionary is just a value in Python, we can store dictionaries within other collections. For instance, we can create a list of dictionaries:

dictionaries = [
  {"hello": "hello", "world": "world"},
  {"A": 1, "B": 2, "C": 3},
]

print(dictionaries)

Output:

[{'hello': 'hello', 'world': 'world'}, {'A': 1, 'B': 2, 'C': 3}]

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Fetching an item from the dictionary

To get the value of an item from a dictionary by its key, we can use square brackets just like with lists:

letters = {"A": 1, "B": 2, "C": 3}

print(letters["B"]) # 2
print(letters["C"]) # 3

However, if we try to access a non-existing key, we’ll get an error:

empty_dict = {}

print(empty_dict["no key"])

Error:

Traceback (most recent call last):
  ...
KeyError: 'no key'

To avoid an error when accessing a non-existing key, we can use the dict.get() method:

names = {"Андрей": "Andrew", "Макс": "Max"}

print("Дима:", names.get("Дима"))

Output:

Дима: None

As you can see, the get() method returned None because there’s no “Дима” in the dictionary.

Dictionary length

To find out how many items are in a dictionary, just use the len() function:

letters = {"A": 1, "B": 2, "C": 3}
print("number of letters:", len(letters))

The output you’ll get:

number of letters: 3

Adding an item to a dictionary

If you want to add a new item to a dictionary (or change an old one), just use the equal sign:

lang = {"C++": "static", "Rascal": "unknown"}

lang["Python"] = "dynamic"
lang["Rascal"] = "hybrid"

print(lang)

Here’s what you’ll see:

{'C++': 'static', 'Rascal': 'hybrid', 'Python': 'dynamic'}

fruits = {"apple": "red", "banana": "yellow"}

fruits["mango"] = "yellow"
fruits["banana"] = "green"

print(fruits)

If you’re in the mood to add or change multiple items at once, you can use the dict.update() method:

lang = {"C++": "static", "Rascal": "unknown"}

lang.update(Python="dynamic", Rascal="hybrid")

print(lang)

Guess what? The output is the same as before:

{'C++': 'static', 'Rascal': 'hybrid', 'Python': 'dynamic'}

You can use another dictionary as an argument for the update method if you want.

There’s also this cool method that lets you add an item to a dictionary only if that key isn’t there already:

first_seen = {"Dima": "yesterday"}

first_seen.setdefault("Dima", "today")
first_seen.setdefault("Masha", "today")

print(first_seen)

Check it out:

{'Dima': 'yesterday', 'Masha': 'today'}

As you can see, setdefault added “Masha” without messing with “Dima”.

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Removing an item from a dictionary

Python’s got a bunch of ways to kick items out of a dictionary.

The easiest one? Just use the del keyword:

population = {"Moscow": 12_000_000, "Abracadabra": 0}

del population["Abracadabra"]

print(population)

Here’s what you’ll get:

{'Moscow': 12000000}

But dictionaries also have this special pop method you can use instead of del:

population = {"Moscow": 12_000_000, "Abracadabra": 0}

value = population.pop("Abracadabra")

print(value)
print(population)

The output will look like:

0
{'Moscow': 12000000}

The pop() method is pretty smart. If it can’t find a key, it’ll just return a default value:

empty = {}
print(empty.pop("key", "default value")) # output: default value

If you’re feeling like removing all items from a dictionary, just go with the dict.clear() method:

letters = {"A": 1, "B": 2}
letters.clear()
print(letters)

Output:

{}

The dict.popitem() method removes the last item you added to the dictionary:

letters = {"B": 2}
letters["A"] = 1
letter = letters.popitem()

print("kicked out:", letter)
print("leftovers:", letters)

Output:

kicked out: ('A', 1)
leftovers: {'B': 2}

Just a heads up, before Python 3.7, popitem would randomly pick an item to remove.

Iterating over dictionary elements

The for loop allows us to iterate through all the keys in a dictionary:

students = {"Kate": 20, "John": 30, "Mark": 40}
for name in students:
    print(name, students[name])

Output:

Kate 20
John 30
Mark 40

dict.keys() method

The keys() method provides all the keys stored in a dictionary:

students = {"Kate": 20, "John": 30, "Mark": 40}
for name in students.keys():
    print(name)

Output:

Kate
John
Mark

By default, the for loop iterates over the dictionary’s keys.

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dict.values() method

To get values instead of keys, use the values() method:

students = {"Kate": 20, "John": 30, "Mark": 40}
for age in students.values():
    print(age)

Output:

20
30
40

dict.items() method

Often, it’s convenient to get both the key and its corresponding value when iterating over a dictionary. This is where the items() method comes in handy:

students = {"Kate": 20, "John": 30, "Mark": 40}
for name, age in students.items():
    print(name, age)

Output:

Kate 20
John 30
Mark 40

Dictionary within a dictionary

Since dictionaries in Python can hold any values, there’s nothing stopping us from creating a dictionary of dictionaries:

game = {
    "left": {
        "left": "empty",
        "right": "prize!",
    },
    "right": {
        "left": "empty",
        "right": "consolation prize",
    },
}

print(game["left"]["right"])  # prize!
print(game["right"]["right"]) # consolation prize

As you can see, working with nested dictionaries is no different than working with regular ones.

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Checking for a key in a dictionary

Often, there’s a need to execute code depending on whether a particular key exists in the dictionary. The in keyword lets you do just that:

country = {"name": "France", "population": 67_000_000}
if "name" in country:
    print("Country:", country["name"])
else:
    print("Nameless country")

Output:

Country: France

How to copy a dictionary

Python has a special method for copying dictionaries - copy():

original = {"A": 1, "B": 2}
copy = original.copy()
copy["C"] = 3

print("original:", original)
print("copy:    ", copy)

Output:

original: {'A': 1, 'B': 2}
copy:     {'A': 1, 'B': 2, 'C': 3}

If for some reason you don’t want to use the copy() method, you can create a new dictionary using the dict constructor:

original = {"A": 1, "B": 2}

copy = dict(original) # using the constructor instead of the copy method

copy["C"] = 3

print("original:", original)
print("copy:    ", copy)

The output remains the same:

original: {'A': 1, 'B': 2}
copy:     {'A': 1, 'B': 2, 'C': 3}

Dictionary comprehension

Python lets you create dictionaries from existing data using comprehension syntax:

sqr = {x: x * x for x in range(1, 4)}

print(sqr)

Output:

{1: 1, 2: 4, 3: 9}

Just like with list comprehension, you can throw in an if to filter out certain dictionary elements:

sqr = {x: x * x for x in range(1, 8) if x % 2 == 0}

print(sqr)

Output:

{2: 4, 4: 16, 6: 36}

items = {"a": 1, "b": 2, "c": 3}
doubled = {k: v * 2 for k, v in items.items() if v % 2 != 0}

print(doubled)

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Using the type() function for dictionaries

If you’re need to find out if a variable’s holding a dictionary, just use the type() function:

number = 1
d = {1: 2, 2: 3}

print("Is number a dict?", type(number) == dict)
print("Is d a dict?    ", type(d) == dict)

Output:

Is number a dict? False
Is d a dict?      True

Exercises

1. Creating a basic dictionary:
   Write a Python program that lets users store key -> value pairs in a dictionary. Once the user’s done typing, show them the whole dictionary.
2. Key search:
   Make a feature that lets users type in a key and the program displays the corresponding value from the dictionary.
3. Adding and removing elements:
   Change your program so users can add new key -> value pairs or remove existing ones by their keys. Every time something changes, display the updated dictionary on the screen.