Mastering Python Dictionaries: Creation, Manipulation, and Advanced Techniques

Python Dictionary

This comprehensive guide explains Python dictionaries—how to create them, access, add, delete elements, and leverage built‑in methods.

Video: Python Dictionaries to Store key/value Pairs

Python dictionaries are unordered collections of key/value pairs. Each entry associates a unique, immutable key with a value that can be of any type.

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Creating a Python Dictionary

Defining a dictionary is straightforward: place comma‑separated key/value pairs inside curly braces {}. Keys must be immutable (strings, numbers, or tuples of immutable elements) and unique; values may be any data type and can repeat.

# empty dictionary
my_dict = {}

# dictionary with integer keys
my_dict = {1: 'apple', 2: 'ball'}

# dictionary with mixed keys
my_dict = {'name': 'John', 1: [2, 4, 3]}

# using dict()
my_dict = dict({1:'apple', 2:'ball'})

# from sequence of pairs
my_dict = dict([(1,'apple'), (2,'ball')])

All of the above approaches create the same dictionary structure.

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Accessing Elements from a Dictionary

Dictionary values are retrieved via their keys, either with square brackets [] or the get() method. Using brackets raises a KeyError if the key is absent; get() returns None by default.

# get vs [] for retrieving elements
my_dict = {'name': 'Jack', 'age': 26}

# Output: Jack
print(my_dict['name'])

# Output: 26
print(my_dict.get('age'))

# Trying to access a missing key returns None
print(my_dict.get('address'))

# KeyError
print(my_dict['address'])

Output

Jack
26
None
Traceback (most recent call last):
  File "<string>", line 15, in <module>
    print(my_dict['address'])
KeyError: 'address'

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Changing and Adding Dictionary Elements

Dictionaries are mutable. Assigning to an existing key updates its value; assigning to a new key adds a fresh entry.

# Changing and adding dictionary elements
my_dict = {'name': 'Jack', 'age': 26}

# update value
my_dict['age'] = 27

# Output: {'age': 27, 'name': 'Jack'}
print(my_dict)

# add item
my_dict['address'] = 'Downtown'

# Output: {'address': 'Downtown', 'age': 27, 'name': 'Jack'}
print(my_dict)

Output

{'name': 'Jack', 'age': 27}
{'name': 'Jack', 'age': 27, 'address': 'Downtown'}

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Removing Elements from a Dictionary

Use pop() to delete a specific key and retrieve its value. popitem() removes an arbitrary pair, while clear() wipes the entire dictionary. The del keyword can remove individual entries or the entire variable.

# Removing elements from a dictionary

# create a dictionary
squares = {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# remove a particular item, returns its value
# Output: 16
print(squares.pop(4))

# Output: {1: 1, 2: 4, 3: 9, 5: 25}
print(squares)

# remove an arbitrary item, return (key,value)
# Output: (5, 25)
print(squares.popitem())

# Output: {1: 1, 2: 4, 3: 9}
print(squares)

# remove all items
squares.clear()

# Output: {}
print(squares)

# delete the dictionary itself
del squares

# Throws Error
print(squares)

Output

16
{1: 1, 2: 4, 3: 9, 5: 25}
(5, 25)
{1: 1, 2: 4, 3: 9}
{}
Traceback (most recent call last):
  File "<string>", line 30, in <module>
    print(squares)
NameError: name 'squares' is not defined

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Python Dictionary Methods

The table below lists common dictionary methods and their purposes. Many are demonstrated in the examples that follow.

Method	Description
clear()	Removes all items from the dictionary.
copy()	Returns a shallow copy of the dictionary.
fromkeys(seq[, v])	Creates a new dictionary from seq with every key set to v (default None).
get(key[,d])	Returns the value for key; if absent, returns d (default None).
items()	Returns a view of the dictionary’s (key, value) pairs.
keys()	Returns a view of the dictionary’s keys.
pop(key[,d])	Deletes key and returns its value or d; raises KeyError if key is missing and d isn’t supplied.
popitem()	Removes and returns an arbitrary (key, value) pair; raises KeyError if the dictionary is empty.
setdefault(key[,d])	Returns the value for key; if absent, inserts key with value d (default None) and returns d.
update([other])	Inserts or updates key/value pairs from other, overwriting existing keys.
values()	Returns a view of the dictionary’s values.

Example usage:

# Dictionary Methods
marks = {}.fromkeys(['Math', 'English', 'Science'], 0)

# Output: {'English': 0, 'Math': 0, 'Science': 0}
print(marks)

for item in marks.items():
    print(item)

# Output: ['English', 'Math', 'Science']
print(list(sorted(marks.keys())))

Output

{'Math': 0, 'English': 0, 'Science': 0}
('Math', 0)
('English', 0)
('Science', 0)
['English', 'Math', 'Science']

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Python Dictionary Comprehension

Dictionary comprehension offers a concise syntax for building dictionaries from iterables. It follows the pattern {key: value for element in iterable}.

# Dictionary Comprehension
squares = {x: x*x for x in range(6)}

print(squares)

Output

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Equivalent imperative version:

squares = {}
for x in range(6):
    squares[x] = x*x
print(squares)

Output

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Comprehensions can include multiple loops and if filters. Example: only odd squares.

# Dictionary Comprehension with if conditional
odd_squares = {x: x*x for x in range(11) if x % 2 == 1}

print(odd_squares)

Output

{1: 1, 3: 9, 5: 25, 7: 49, 9: 81}

For more on dictionary comprehensions, see Python Dictionary Comprehension.

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Other Dictionary Operations

Dictionary Membership Test

Use the in operator to check if a key exists; it does not inspect values.

# Membership Test for Dictionary Keys
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}

# Output: True
print(1 in squares)

# Output: True
print(2 not in squares)

# membership tests for key only not value
# Output: False
print(49 in squares)

Output

True
True
False

Iterating Through a Dictionary

Iterate over keys directly, then access values.

# Iterating through a Dictionary
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
for i in squares:
    print(squares[i])

Output

1
9
25
49
81

Dictionary Built‑in Functions

Standard functions such as all(), any(), len(), and sorted() are frequently applied to dictionaries.

Function	Description
all()	True if every key evaluates to True (or if the dictionary is empty).
any()	True if any key evaluates to True; False for an empty dictionary.
len()	Number of key/value pairs.
cmp()	Compares two dictionaries (removed in Python 3).
sorted()	Returns a sorted list of keys.

Examples:

# Dictionary Built‑in Functions
squares = {0: 0, 1: 1, 3: 9, 5: 25, 7: 49, 9: 81}

# Output: False
print(all(squares))

# Output: True
print(any(squares))

# Output: 6
print(len(squares))

# Output: [0, 1, 3, 5, 7, 9]
print(sorted(squares))

Output

False
True
6
[0, 1, 3, 5, 7, 9]