Python Dictionaries – The Complete Guide for Beginners

Last updated 2 weeks, 2 days ago | 38 views 75     5

Tags:- Python

In Python, a dictionary is a powerful, flexible, and widely-used data structure that allows you to store data in key-value pairs.

In this tutorial, you’ll learn:

  • What dictionaries are

  • How to create and access them

  • Common methods and operations

  • Nesting dictionaries

  • Tips and pitfalls


What Is a Dictionary?

A dictionary in Python:

  • Stores data in key-value pairs

  • Is unordered (in versions <3.7)

  • Is mutable

  • Keys must be unique and immutable

  • Values can be of any type

Example:

person = {
    "name": "Alice",
    "age": 30,
    "is_active": True
}

✅ Creating Dictionaries

Using Curly Braces

user = {"username": "john_doe", "email": "[email protected]"}

Using the dict() Constructor

user = dict(username="john_doe", email="[email protected]")

Creating Empty Dictionary

empty_dict = {}

Accessing Dictionary Values

print(user["username"])  # john_doe

Using get() to Avoid KeyError

print(user.get("email"))          # [email protected]
print(user.get("phone", "N/A"))   # N/A

✏️ Modifying Dictionaries

Add or Update a Key

user["age"] = 25  # Adds if not exists, updates if exists

Remove Items

del user["email"]             # Deletes key
user.pop("age")               # Removes and returns value
user.clear()                  # Empties the dictionary

Looping Through Dictionaries

Loop Through Keys

for key in user:
    print(key, user[key])

Loop Through Values

for value in user.values():
    print(value)

Loop Through Key-Value Pairs

for key, value in user.items():
    print(f"{key}: {value}")

Common Dictionary Methods

Method Description Example
get(key) Get value by key d.get("name")
keys() Returns list of keys d.keys()
values() Returns list of values d.values()
items() Returns list of (key, value) pairs d.items()
pop(key) Removes specified key d.pop("age")
clear() Empties the dictionary d.clear()
update(dict2) Merges another dictionary d.update({"city": "London"})
copy() Returns a shallow copy d.copy()

Nested Dictionaries

Dictionaries can contain other dictionaries.

student = {
    "name": "Tom",
    "grades": {
        "math": 90,
        "science": 85
    }
}

print(student["grades"]["science"])  # 85

Dictionary Comprehensions

You can create dictionaries using a concise syntax:

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

Full Example: Contact Book

contacts = {}

# Add new contact
contacts["Alice"] = {"phone": "123-456-7890", "email": "[email protected]"}
contacts["Bob"] = {"phone": "555-555-5555", "email": "[email protected]"}

# Access
print("Alice's email:", contacts["Alice"]["email"])

# Update
contacts["Bob"]["phone"] = "000-000-0000"

# Remove
del contacts["Alice"]

# Display
for name, info in contacts.items():
    print(f"{name}: {info}")

Tips for Working with Dictionaries

  • Use get() to prevent crashes from missing keys.

  • Use dictionary comprehension for dynamic dictionary creation.

  • Keys must be immutable (e.g., strings, numbers, tuples).

  • Use update() to merge two dictionaries.


⚠️ Common Pitfalls

Pitfall Why It Happens Fix
Accessing missing key Raises KeyError Use get() with default
Using mutable key (e.g., list) Not allowed; keys must be hashable Use tuples instead
Modifying while iterating Can cause unexpected behavior Use .copy() or store keys first
Confusing dict() syntax dict(name="John"){"name": "John"} Be mindful of keyword args

Summary

Feature Description
Data type dict
Key type Immutable (str, int, tuple)
Value type Any (can be nested)
Order preserved? ✅ Yes (from Python 3.7+)
Use cases Fast lookup, mappings, configurations

✅ Practice Exercise

Task: Create a program that counts the frequency of each word in a sentence.

sentence = "python is fun and python is easy"
words = sentence.split()
word_count = {}

for word in words:
    word_count[word] = word_count.get(word, 0) + 1

print(word_count)

Output:

{'python': 2, 'is': 2, 'fun': 1, 'and': 1, 'easy': 1}

What’s Next?

After mastering dictionaries, dive into:

  • JSON and dictionary parsing

  • Handling nested data structures

  • Real-world examples like API responses and config files

 

Tips and Tricks


What is pass in Python?

Python | Pass Statement

The pass statement is used as a placeholder for future code. It represents a null operation in Python. It is generally used for the purpose of filling up empty blocks of code which may execute during runtime but has yet to be written.

 

def myfunction():
    pass

 


How can you generate random numbers?

Python | Generate random numbers

Python provides a module called random using which we can generate random numbers. e.g: print(random.random())

 

 

We have to import a random module and call the random() method as shown below:

 import random

 print(random.random())

The random() method generates float values lying between 0 and 1 randomly.


To generate customized random numbers between specified ranges, we can use the randrange() method
Syntax: randrange(beginning, end, step)
 

import random

print(random.randrange(5,100,2))

 


What is lambda in Python?

Python | Lambda function

A lambda function is a small anonymous function. This function can have any number of parameters but, can have just one statement.
 

 

Syntex: 
lambda arguments : expression
 

a = lambda x,y : x+y

print(a(5, 6))

It also provides a nice way to write closures. With that power, you can do things like this.

def adder(x):
    return lambda y: x + y

add5 = adder(5)

add5(1)    #6

As you can see from the snippet of Python, the function adder takes in an argument x and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.
 


What is swapcase() function in the Python?

Python | swapcase() Function

It is a string's function that converts all uppercase characters into lowercase and vice versa. It automatically ignores all the non-alphabetic characters.
 

string = "IT IS IN LOWERCASE."  

print(string.swapcase())  

 


How to remove whitespaces from a string in Python?

Python | strip() Function | Remove whitespaces from a string 

To remove the whitespaces and trailing spaces from the string, Python provides a strip([str]) built-in function. This function returns a copy of the string after removing whitespaces if present. Otherwise returns the original string.
 

string = "  Python " 
 
print(string.strip())  

 


What is the usage of enumerate() function in Python?

Python | enumerate() Function

The enumerate() function is used to iterate through the sequence and retrieve the index position and its corresponding value at the same time.
 

lst = ["A","B","C"] 
 
print (list(enumerate(lst)))

#[(0, 'A'), (1, 'B'), (2, 'C')]

 


Can you explain the filter(), map(), and reduce() functions?

Python | filter(), map(), and reduce() Functions

  • filter()  function accepts two arguments, a function and an iterable, where each element of the iterable is filtered through the function to test if the item is accepted or not.
    >>> set(filter(lambda x:x>4, range(7)))
    
    # {5, 6}
    
    

     

  • map() function calls the specified function for each item of an iterable and returns a list of result

    >>> set(map(lambda x:x**3, range(7)))
    
    # {0, 1, 64, 8, 216, 27, 125}

     

  • reduce() function reduces a sequence pair-wise, repeatedly until we arrive at a single value..
     

    >>> reduce(lambda x,y:y-x, [1,2,3,4,5])
    
    # 3
    

    Let’s understand this:

    2-1=1
    3-1=2
    4-2=2
    5-2=3

    Hence, 3.

 


What is a namedtuple?

Python | namedtuple

A namedtuple will let us access a tuple’s elements using a name/label. We use the function namedtuple() for this, and import it from collections.

>>> from collections import namedtuple

#format
>>> result=namedtuple('result','Physics Chemistry Maths') 

#declaring the tuple
>>> Chris=result(Physics=86,Chemistry=92,Maths=80) 

>>> Chris.Chemistry
# 92

 


Write a code to add the values of same keys in two different dictionaries and return a new dictionary.

We can use the Counter method from the collections module

from collections import Counter

dict1 = {'a': 5, 'b': 3, 'c': 2}
dict2 = {'a': 2, 'b': 4, 'c': 3}

new_dict = Counter(dict1) + Counter(dict2)


print(new_dict)
# Print: Counter({'a': 7, 'b': 7, 'c': 5})


 


Python In-place swapping of two numbers

 Python | In-place swapping of two numbers

>>> a, b = 10, 20
>>> print(a, b)
10 20

>>> a, b = b, a
>>> print(a, b)
20 10

 


Reversing a String in Python

Python | Reversing a String

>>> x = 'PythonWorld'
>>> print(x[: : -1])
dlroWnohtyP

 


Python join all items of a list to convert into a single string

Python | Join all items of a list to convert into a single string

>>> x = ["Python", "Online", "Training"]
>>> print(" ".join(x))
Python Online Training

 


python return multiple values from functions

Python | Return multiple values from functions

>>> def A():
	return 2, 3, 4

>>> a, b, c = A()

>>> print(a, b, c)
2 3 4

 


Python Print String N times

Python | Print String N times

>>> s = 'Python'
>>> n = 5

>>> print(s * n)
PythonPythonPythonPythonPython

 


Python check the memory usage of an object

Python | Check the memory usage of  an object

>>> import sys
>>> x = 100

>>> print(sys.getsizeof(x))
28