? Python Nested Dictionaries

⚡ Quick Overview

A nested dictionary is a dictionary that contains one or more dictionaries as values. It is useful for storing structured or hierarchical data such as students, products, configuration settings, or JSON-like data.

? Key Concepts

A nested dictionary is simply “a dictionary inside a dictionary”.
The outer dictionary keys usually represent unique IDs like "student1", "student2".
The inner dictionary stores the details of that ID, such as name, age, city, etc.
You access values by chaining keys, for example: students["student1"]["name"].
You can add, modify, or delete inner keys just like in a normal dictionary.

? Syntax and Structure

A typical nested dictionary structure looks like this:

? Nested Dictionary Structure

# Outer dictionary with inner dictionaries as values
outer_dict = {
    "key1": {"inner_key1": "value1", "inner_key2": "value2"},
    "key2": {"inner_key1": "value3", "inner_key2": "value4"}
}

In our student example, each key like "student1" maps to another dictionary containing that student's details.

? Use Cases

Maintaining records of multiple users, students, or employees with several attributes.
Working with JSON data from web APIs (which maps naturally to nested dictionaries).
Grouping related configuration options by sections or categories.

? Code Examples

? Creating a Nested Dictionary

Here is a nested dictionary that stores information about two students:

# Create a nested dictionary for two students
students = {
    "student1": {"name": "Alice", "age": 25},
    "student2": {"name": "Bob", "age": 24}
}
# Print the entire nested dictionary
print(students)

? Accessing Values in a Nested Dictionary

Access nested values by using the outer key first, then the inner key:

print(students["student1"]["name"])  # Alice
print(students["student2"]["age"])   # 24

✏️ Modifying Values in Nested Dictionaries

Modify inner values just like updating a normal dictionary:

# Update the age of student1
students["student1"]["age"] = 26
print(students["student1"]["age"])  # 26

➕ Adding New Items to Nested Dictionaries

You can add a new student or new fields to an existing student:

# Add a new student entry
students["student3"] = {"name": "Charlie", "age": 23}
# Add a new field to an existing student
students["student1"]["city"] = "New York"
# Print the updated nested dictionary
print(students)

? Looping Through a Nested Dictionary

Use nested loops to go through each student and their details:

# Loop through each student and their details
for student_id, details in students.items():
    print("ID:", student_id)
    # Inner loop over each field in the student's info
    for key, value in details.items():
        print(" ", key + ":", value)

? Live Output and Explanation

This example creates the dictionary, updates it, and then prints each student's details:

▶️ Full Example with Output

# Create the initial nested dictionary of students
students = {
    "student1": {"name": "Alice", "age": 25},
    "student2": {"name": "Bob", "age": 24}
}

# Add a city for student1 and a new student3
students["student1"]["city"] = "New York"
students["student3"] = {"name": "Charlie", "age": 23}

# Loop through the dictionary and print each student's details
for sid, info in students.items():
    print("ID:", sid)
    print(" Name:", info["name"])
    print(" Age:", info["age"])
    if "city" in info:
        # Only print city if the key exists
        print(" City:", info["city"])
    print()

?️ Sample Output

The output in the terminal will look like:

ID: student1
 Name: Alice
 Age: 25
 City: New York

ID: student2
 Name: Bob
 Age: 24

ID: student3
 Name: Charlie
 Age: 23

The condition if "city" in info ensures we only print the city when that key exists, preventing a KeyError.

? Tips & Best Practices

Nested dictionaries are excellent for representing JSON-like structured data.
Use in or dict.get() to safely check keys before accessing nested values.
Use nested for loops to iterate over outer and inner dictionaries.
Be careful with copying: a shallow copy shares inner dictionaries. Use copy.deepcopy() for fully independent copies.
Keep your key naming consistent across all inner dictionaries (for example, always use "name", not a mix of "name" and "full_name").

? Try It Yourself

Create a nested dictionary for 3 students with keys like name and age.
Access and print the name of student2 using chained keys.
Add a city key to student1 and assign a value.
Loop through all students and print their details in a clean, formatted way.
Add another field, such as "marks" or "course", and display it in your loop.