Python Dictionaries

Dictionaries (dict) in Python are unordered, mutable collections of key-value pairs. They are optimized for fast data lookup and are widely used in real-world applications like JSON data handling, database operations, and more.



What is a Dictionary?:


Example: 

student = {"name": "Alice", "age": 21, "courses": ["Math", "CS"]}
print(student)  
# Output: {'name': 'Alice', 'age': 21, 'courses': ['Math', 'CS']}


1. Creating Dictionaries

# Using curly braces
person = {"name": "John", "age": 30, "city": "New York"}

# Using dict() constructor
person2 = dict(name="Jane", age=25, city="London")


2. Accessing Values

print(person["name"])       # Output: John
print(person.get("age"))    # Output: 30
print(person.get("salary", "Not found"))  # Default value


3. Updating Values

person["age"] = 35
person["salary"] = 50000  # Adds new key


4. Deleting Items

del person["city"]          # Deletes 'city'
person.pop("salary")        # Removes 'salary'
person.clear()              # Removes all items


5. Looping Through a Dictionary

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

for key, value in person.items():
    print(key, ":", value)

print(person.keys())        # dict_keys(['name', 'age'])
print(person.values())      # dict_values(['John', 35])
print(person.items())       # dict_items([('name', 'John'), ('age', 35)])


6. Dictionary Methods :


Method Description
get(key) Returns value for key or None
keys( ) Returns all keys
values( ) Returns all values
items( ) Returns key-value pairs
update(dict) Updates one dict with another
pop(key) Removes key and returns value
clear( ) Empties the dictionary
copy( ) Shallow copy of the dictionary
setdefault(k,v) Returns key value or sets default


7. Nested Dictionaries

students = {
    "101": {"name": "Alice", "marks": 85},
    "102": {"name": "Bob", "marks": 90}
}
print(students["101"]["name"])  # Output: Alice


8. Dictionary Comprehension

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


9. Merging Dictionaries

a = {"x": 1, "y": 2}
b = {"y": 3, "z": 4}
c = {**a, **b}  # Merges a and b, b's values override
print(c)       # Output: {'x': 1, 'y': 3, 'z': 4}


10. Use Case: Counting Elements

text = "banana"
count = {}
for char in text:
    count[char] = count.get(char, 0) + 1
print(count)
# Output: {'b': 1, 'a': 3, 'n': 2}


11. Immutable Keys Only

d = {("a", "b"): 1}  # Tuple as key


12. Dictionary vs List


Feature Dictionary List
Access Key-based (dict[key]) Index-based (list[index])
Uniqueness Keys must be unique Duplicates allowed
Lookup speed Fast Slower (in large data)


Common Python Dictionary Questions

Here are some of the most common and important dictionary questions in Python, suitable for interviews, exams, and practice:



A dictionary is an unordered collection of key-value pairs. Keys must be unique and immutable.

my_dict = {"name": "Alice", "age": 25}
print(my_dict["name"])
print(my_dict.get("age"))

d["missing"] → raises KeyError

d.get("missing") → returns None (or default value if provided)

my_dict["city"] = "Delhi"
my_dict["age"] = 30
del my_dict["city"]
my_dict.pop("age")
for k, v in my_dict.items():
    print(k, v)

Used to safely access keys with a default return value.

my_dict.get("salary", "Not Found")

No, keys must be immutable (e.g., int, str, tuple).

d1 = {"a": 1}
d2 = {"b": 2}
d3 = {**d1, **d2}
new_dict = my_dict.copy()
squares = {x: x*x for x in range(5)}
text = "banana"
count = {}
for char in text:
    count[char] = count.get(char, 0) + 1

keys() → returns keys

values() → returns values

items() → returns key-value pairs

No, duplicate keys overwrite the previous value.

d = {"a": 1, "a": 2}  # Final dict: {"a": 2}

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