Dictionaries in Python

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Python dictionary is a data structure that stores the value in key: value pairs. Values in a dictionary can be of any data type and can be duplicated, whereas keys can't be repeated and must be immutable.

Example: Here, The data is stored in key:value pairs in dictionaries, which makes it easier to find values.

d = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
print(d)

{1: 'Geeks', 2: 'For', 3: 'Geeks'}

How to Create a Dictionary

Dictionary can be created by placing a sequence of elements within curly {} braces, separated by a 'comma'.

d1 = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
print(d1)

# create dictionary using dict() constructor
d2 = dict(a = "Geeks", b = "for", c = "Geeks")
print(d2)

{1: 'Geeks', 2: 'For', 3: 'Geeks'}
{'a': 'Geeks', 'b': 'for', 'c': 'Geeks'}

• Dictionary keys are case sensitive: the same name but different cases of Key will be treated distinctly.
• Keys must be immutable: This means keys can be strings, numbers or tuples but not lists.
• Keys must be unique: Duplicate keys are not allowed and any duplicate key will overwrite the previous value.
• Dictionary internally uses Hashing. Hence, operations like search, insert, delete can be performed in Constant Time.

From Python 3.7 Version onward, Python dictionary are Ordered.

Accessing Dictionary Items

We can access a value from a dictionary by using the key within square brackets or get() method.

d = { "name": "Prajjwal", 1: "Python", (1, 2): [1,2,4] }

# Access using key
print(d["name"])

# Access using get()
print(d.get("name"))  

Prajjwal
Prajjwal

Adding and Updating Dictionary Items

We can add new key-value pairs or update existing keys by using assignment.

d = {1: 'Geeks', 2: 'For', 3: 'Geeks'}

# Adding a new key-value pair
d["age"] = 22

# Updating an existing value
d[1] = "Python dict"

print(d)

{1: 'Python dict', 2: 'For', 3: 'Geeks', 'age': 22}

Removing Dictionary Items

We can remove items from dictionary using the following methods:

• del: Removes an item by key.
• pop(): Removes an item by key and returns its value.
• clear(): Empties the dictionary.
• popitem(): Removes and returns the last key-value pair.

d = {1: 'Geeks', 2: 'For', 3: 'Geeks', 'age':22}

# Using del to remove an item
del d["age"]
print(d)

# Using pop() to remove an item and return the value
val = d.pop(1)
print(val)

# Using popitem to removes and returns
# the last key-value pair.
key, val = d.popitem()
print(f"Key: {key}, Value: {val}")

# Clear all items from the dictionary
d.clear()
print(d)

{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Geeks
Key: 3, Value: Geeks
{}

Iterating Through a Dictionary

We can iterate over keys [using keys() method] , values [using values() method] or both [using item() method] with a for loop.

d = {1: 'Geeks', 2: 'For', 'age':22}

# Iterate over keys
for key in d:
    print(key)

# Iterate over values
for value in d.values():
    print(value)

# Iterate over key-value pairs
for key, value in d.items():
    print(f"{key}: {value}")

1
2
age
Geeks
For
22
1: Geeks
2: For
age: 22

Read in detail: Ways to Iterating Over a Dictionary

Nested Dictionaries

Example of Nested Dictionary:

d = {1: 'Geeks', 2: 'For',
        3: {'A': 'Welcome', 'B': 'To', 'C': 'Geeks'}}

print(d)

{1: 'Geeks', 2: 'For', 3: {'A': 'Welcome', 'B': 'To', 'C': 'Geeks'}}

Read in Detail: Python Nested Dictionary

Copying Dictionaries in Python

A copy of a dictionary can be created using either shallow copy or deep copy methods. These methods allow duplicating dictionary objects, but they behave differently when it comes to nested data. Let's dicuss both in detail.

1. Shallow Copy

A shallow copy makes a new dictionary with same outer values as the original. But if the dictionary has nested data (like a list or another dictionary), both copies still share that inner data. So, changes to nested parts will affect other.

It is created using copy.copy() method from Python’s copy module.

Example:

This code shows that in a shallow copy, changes to nested data affect both the original and the copy because the nested parts are shared.

import copy
original = {'name': 'Alice', 'marks': {'math': 90, 'science': 95}}

# Create a shallow copy
shallow = copy.copy(original)

# Modify nested value in the copy
shallow['marks']['math'] = 100

print("Original:", original)
print("Shallow Copy:", shallow)

Original: {'name': 'Alice', 'marks': {'math': 100, 'science': 95}}
Shallow Copy: {'name': 'Alice', 'marks': {'math': 100, 'science': 95}}

Explanation:

• shallow = copy.copy(original): creates a shallow copy, nested 'marks' remains shared.
• shallow['marks']['math'] = 100: updates 'math' in the shared nested dictionary.
• print(original), print(shallow): both show updated 'math' value due to shared data.

2. Deep Copy

A deep copy makes a new dictionary and also creates separate copies of all nested data (like lists or other dictionaries). This means original and copy are completely independent, changes made to nested parts do not affect other.

It is created using copy.deepcopy() method from Python’s copy module.

Example:

This Example shows that a deep copy creates a fully independent copy of both the outer and nested data, so changes in the copy do not affect the original.

import copy
original = {'name': 'Alice', 'marks': {'math': 90, 'science': 95}}

# Create a deep copy
deep = copy.deepcopy(original)

# Modify nested value in the deep copy
deep['marks']['math'] = 100

print("Original:", original)
print("Deep Copy:", deep)

Original: {'name': 'Alice', 'marks': {'math': 90, 'science': 95}}
Deep Copy: {'name': 'Alice', 'marks': {'math': 100, 'science': 95}}

Explanation:

• deep = copy.deepcopy(original): creates a deep copy, nested 'marks' is also copied separately.
• deep['marks']['math'] = 100: updates 'math' in the deep copy’s nested dictionary only.
• print(original), print(deep): original remains unchanged, only deep copy shows the updated 'math' value.

Python Dictionary Operation Programs

• Dictionary Creation Programs.
• Dictionary Add/Append Programs.
• Dictionary Removal Programs.
• Dictionary Access Programs.
• Dictionary Conversion Programs.
• Dictionary Programs involving Lists.
• Dictionary Programs involving String, Tuple and Set.

Python Dictionary Problems

• Length of a Dictionary
• Check if a Key Exists
• Access a Value by Key
• Remove a Key from a Dictionary
• Remove keys with substring values
• Sum All Numeric Values in a Dictionary
• Find Keys with Maximum Value
• Remove Duplicates from a Dictionary
• Filter Dictionary by Key Prefix
• Count Frequency of Elements Using Dictionary
• Check if two arrays are equal or not
• Max distance between two occurrences in array
• 2 Sum - Count Pairs with target sum
• 3 Sum - Count all triplets with target sum
• Count all pairs with absolute difference equal to k
• Remove minimum elements such that no common elements exist in two arrays
• Check If Array Pair Sums Divisible by k
• Longest subarray with sum divisible by K
• Longest Subarray having Majority Elements Greater Than K
• Count distinct elements in every window of size k

Related Dictionary Articles

• Dictionary Comprehension
• Dictionary Methods
• Python Dictionary Exercise
• Python Dictionary Quiz

Dictionary in Python

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