Dictionaries in Python

Dictionaries are one of the most powerful and flexible types in Python.

Used to store key-value pairs, they allow organizing data in a clear, efficient, and fast-to-query manner.

They are omnipresent in Python programming: configurations, API results, counts, JSON structures, etc.

Dictionary syntax

As we have seen, a Python dictionary is an unordered, mutable, and indexed collection by unique keys.

Its syntax is very simple:

my_dict = {
    "name": "Alice",
    "age": 30,
    "city": "Paris"
}

Here:

• "name", "age", "city" are keys;
• "Alice", 30, "Paris" are the values.

Unlike lists, elements are accessed by their key, not their position.

Creating a dictionary

There are two methods to create a dictionary.

Method #1: with curly braces

The first method that allows creating a dictionary is with curly braces {}.

user = {"id": 101, "username": "magician", "subscribed": True}

Method #2: with the dict() method

Finally, a second way to create a dictionary is with the use of the dict() method.

user = dict(id=101, username="magician", subscribed=True)

The dict() constructor is convenient but only accepts valid key names as Python identifiers (no spaces or hyphens).

Accessing a value in a dictionary

Accessing a value is very simple in a dictionary with Python: just use their keys.

user = {"username": "hugo"}
print(user["username"])  # hugo

If we try to access a key that doesn't exist, an error will be triggered.

PYTHON

print(user["email"])  # ❌ KeyError

To avoid this, it is possible to use the get() method which allows avoiding an error if the key is missing (very useful for data coming from APIs).

PYTHON

print(user.get("email"))         # None
print(user.get("email", "N/A"))  # N/A

Here, "N/A" is displayed only if the email key doesn't exist.

Adding or modifying a value

Adding or updating an element in a dictionary is done with direct assignment:

profile = {"username": "arthur"}
profile["email"] = "arthur@example.com"  # Addition
profile["username"] = "arthur42"           # Modification

Easy, right? 😬

If the key doesn't exist, it is created. If it exists, its value is overwritten.

Adding multiple elements at once

Want to add multiple elements at once? It is possible to use update().

profile.update({"age": 29, "country": "France"})

Deleting a value

To delete an element from a dictionary, we can use one of the three available methods with the Python language: del, pop() and pop(key, default).

Method #1: del

del profile["email"]

This first method raises an error if the key doesn't exist (KeyError).

Method #2: pop()

This second method allows deleting a value without raising an error if the element doesn't exist.

username = profile.pop("username")  # Deletes and returns the value

Method #3: pop(key, default)

Finally, this last method takes method two and adds a subtlety: the possibility to define a default value to avoid any crash.

profile.pop("non_existent", "nothing")  # Doesn't raise an error

Deleting all elements from a dictionary in Python

To delete all elements in a dictionary, it is possible to use the clear() method.

profile.clear()

Iterating through a dictionary

Iterating through keys

profile = {"username": "arthur"}

for key in profile.keys():
    print(key) # username

It is also possible to avoid explicitly stating "keys()" but it is a good practice to keep to avoid any confusion problems.

PYTHON

for key in profile:
    print(key) # Same result here

Iterating through values

profile = {"username": "arthur"}

for value in profile.values():
    print(value) # arthur

Iterating through key/value pairs

profile = {"username": "arthur"}

for key, value in profile.items():
    print(f"{key} : {value}") # username : arthur

items() is essential when you need both the key and the value.

Checking for the presence of a key in a dictionary

Before accessing a value, we can test if a key exists in the dictionary using the in operator:

profile = {"username": "hugo", "email": "hugo@example.com"}

if "email" in profile:
    print("Address found")

This should be avoided to prevent a runtime error:

PYTHON

if profile["email"]:  # Risk KeyError if the key doesn't exist

Dictionary comprehensions

Python allows creating a dictionary in a single line thanks to a dictionary comprehension, similar to a list comprehension.

Syntax

{key: value for element in iterable}

Example #1: Creating a dictionary of squares

squares = {x: x**2 for x in range(5)}
print(squares)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Example #2: Filtering an existing dictionary

grades = {"Alice": 18, "Bob": 12, "Chloe": 8}
passed = {k: v for k, v in grades.items() if v >= 10}

Dictionary comprehension is ideal for transforming or filtering data in a concise and readable manner.

Copying a dictionary

There are two methods to copy a dictionary: copy() and dict().

Method #1: copy()

This first method makes a shallow copy (i.e., of the first level) of a dictionary.

copy = profile.copy()

To make a deep copy (with the contents of all nested elements) we must use copy.deepcopy():

import copy
total_copy = copy.deepcopy(original)

Method #2: dict()

To create a copy of a dictionary we also have the possibility to create... a dictionary of a dictionary! 👀

copy = dict(profile)

Merging two dictionaries

Python offers several ways to merge two dictionaries.

Method #1: with update()

a = {"name": "Alice"}
b = {"age": 30}

a.update(b)
print(a)  # {'name': 'Alice', 'age': 30}

This method modifies dictionary a directly.

Method #2: with the "|" operator

Since Python 3.9+ it is also possible to perform a merge between multiple dictionaries thanks to the | operator.

a = {"name": "Alice"}
b = {"age": 30}
merge = a | b
print(merge) # Same result

This method is different from the first method with update() because it returns a new dictionary without modifying a / b.

Sorting a dictionary

We can sort a dictionary by its keys or by its values.

Sorting by key

d = {"b": 2, "a": 1, "c": 3}
sorted_keys = dict(sorted(d.items()))

Sorting by value

sorted_values = dict(sorted(d.items(), key=lambda x: x[1]))

Useful methods on dictionaries

Here is an overview of the most frequently used methods on dictionaries.

Nested dictionaries

A nested dictionary is a dictionary whose some values are themselves dictionaries.

This is very common for representing complex data structures, like JSON or hierarchical configurations.

Example

user = {
    "name": "Alice",
    "profile": {
        "city": "Paris",
        "age": 30
    }
}

Accessing a nested value

print(user["profile"]["city"])  # Paris

Modifying a nested value

user["profile"]["age"] = 31

It is very easy to iterate through a nested dictionary using multiple loops.

Frequently asked questions about arrays

Let's review the questions that beginners with arrays in Python ask most!

What is the difference between dict() and {}?

Both create a dictionary, but:

• {} is faster and more readable;
• dict() is useful with pairs or keywords.

What's the difference between get() and []?

[] raises an error if the key is absent while get() returns None (or a default value).

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