
Python Dictionaries – The Complete Guide for Beginners
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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:
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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:
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Stores data in key-value pairs
-
Is unordered (in versions <3.7)
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Is mutable
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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.
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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:
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JSON and dictionary parsing
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Handling nested data structures
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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=3Hence, 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