Dictionaries in Python programming language

What are Dictionaries?
• Key-value pair data structures
• Unordered, mutable, and dynamic
• Also known as associative arrays or hash maps in other languages

Dictionary Basics
• Created using curly braces {}
• Each item has a key and value, separated by colon :
• Keys must be unique and immutable
student = {
"name": "John Smith",
"age": 20,
"major": "Computer Science"
}

Multiple ways to create dictionaries
# Empty dictionary
dict1 = {}
dict2 = dict()
# Using dict() constructor
dict3 = dict(name="John", age=20)
# From list of tuples
pairs = [("apple", 1), ("banana", 2)]
dict4 = dict(pairs)

Accessing Values
• Using square bracket notation [ ]
• Using get() method (safer)
student = {"name": "John", "age": 20}
# Using brackets
print(student["name"]) # Output: John
# Using get()
print(student.get("age")) # Output: 20
print(student.get("grade", "N/A")) # Default value if key doesn't exist

Modifying Dictionaries
# Adding new key-value pair
student["grade"] = "A"
# Updating existing value
student["age"] = 21
# Updating multiple values
student.update({"age": 22, "major": "Physics"})

Several methods to remove items
student = {"name": "John", "age": 20, "grade": "A"}
# Remove specific key
del student["grade"]
# Pop item with specified key
student[“score”] = student.pop("grade")
# Remove and return last item
last_item = student.popitem()
# Clear all items
student.clear()

Common dictionary methods
• keys() - Returns list of all keys
• values() - Returns list of all values
• items() - Returns list of key-value pairs
print(student.keys())
print(student.values())
print(student.items())

Creating dictionaries using
comprehension
# Square numbers
squares = {x: x**2 for x in range(5)}
# Result: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
# Filtering with conditions
even_squares = {x: x**2 for x in range(5) if x % 2 == 0}
# Result: {0: 0, 2: 4, 4: 16}

Dictionaries can contain other
dictionaries
students = {
"John": {
"age": 20,
"grades": {"math": "A", "physics": "B"}
},
"Mary": {
"age": 19,
"grades": {"math": "B", "physics": "A"}
}
}

Common Use Cases
• Search
• Counting occurrences
• Configuration settings
• Caching results
• Graph representations
• JSON-like data structures

Dictionary Performance
• O(1) average case for insertions, deletions, and lookups
• Hash table implementation
• Comparison with lists:
• Lists: O(n) for search
• Dictionaries: O(1) for search

Memory Usage
• Higher memory usage than lists
• Each key-value pair stores:
• Hash of the key
• Key itself
• Value
• Additional metadata

Best Practices
• Use meaningful keys
• Check for key existence before access
• Use get() with default values
• Consider using collections.defaultdict
• Keep keys immutable

Common Pitfalls
# Mutable keys (will raise TypeError)
bad_dict = {[1, 2]: "value"} # Error!
# Key doesn't exist
student["grade"] # KeyError if 'grade' doesn't exist
# Modifying while iterating
for key in d: d[key+1] = 0 # Bad practice!

Special Dictionary Types From
collections
from collections import defaultdict, OrderedDict, Counter
# defaultdict - provides default values
d = defaultdict(list)
d["new_key"].append(1) # No KeyError!
# OrderedDict - maintains insertion order (pre-Python 3.7)
od = OrderedDict()
# Counter - counts occurrences
c = Counter(['a', 'b', 'a', 'c', 'a'])

Dictionary Views
• Dynamic views of dictionary entries
d = {"a": 1, "b": 2}
keys = d.keys()
values = d.values()
items = d.items()
# Views update automatically
d["c"] = 3
print(keys) # dict_keys(['a', 'b', 'c'])

Type Hints with Dictionaries
from typing import Dict, List, Union
# Simple dictionary
grades: Dict[str, float] = {"math": 95.5, "physics": 89.0}
# Complex dictionary
student: Dict[str, Union[str, int, List[str]]] = {
"name": "John",
"age": 20,
"courses": ["math", "physics"]
}

Set-like operations with dictionary
views
d1 = {"a": 1, "b": 2}
d2 = {"b": 3, "c": 4}
# Keys operations
keys1 = d1.keys()
keys2 = d2.keys()
# Union, intersection, difference
print(keys1 | keys2) # Union
print(keys1 & keys2) # Intersection
print(keys1 - keys2) # Difference

Real-world Example
def word_frequency(text: str) -> Dict[str, int]:
"""Count word frequencies in text."""
words = text.lower().split()
frequency = {}
for word in words:
frequency[word] = frequency.get(word, 0) + 1
return frequency
text = "the quick brown fox jumps over the lazy dog"
print(word_frequency(text))

Practice Exercises
• Create a phone book dictionary
• Implement a cache using dictionary
• Count character frequencies in a string
• Create a nested dictionary for a school database
• Implement a simple graph using dictionary
• Python Documentation
• Real Python Tutorials
• Python Cookbook

Dictionaries in Python programming language

More Related Content

Similar to Dictionaries in Python programming language (20)

More from ssuserbad56d (11)

Recently uploaded (20)

Dictionaries in Python programming language

Editor's Notes