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Python Dictionaries

A dictionary in Python is a powerful data structure used to store data in key–value pairs. Dictionaries are optimized for fast lookup, making them one of the most important and widely used data types in Python.
This topic covers dictionaries from basics to practical usage.

What Is a Dictionary?

A dictionary:
  • Stores data as key : value pairs
  • Is mutable
  • Does not allow duplicate keys
  • Allows mixed data types
  • Provides fast access using keys
python
student = {
    "name": "Jayesh",
    "age": 25,
    "course": "Python"
}

print(student)

Dictionary vs List vs Tuple vs Set

FeatureListTupleSetDictionary
OrderedYesYesNoYes (Python 3.7+)
MutableYesNoYesYes
DuplicatesYesYesNoKeys: No
AccessIndexIndexNoKey
StructureValuesValuesValuesKey–Value

Creating Dictionaries

Using Curly Braces {}

python
person = {"name": "Amit", "age": 30}
print(person)

Using dict() Constructor

python
person = dict(name="Amit", age=30)
print(person)
python
pairs = [("a", 1), ("b", 2)]
d = dict(pairs)
print(d)

Access Dictionary Items

Using Key

python
print(person["name"])

Using get() (Safe Access)

python
print(person.get("age"))
print(person.get("salary"))  # None

Dictionary Keys and Values

Keys Must Be Immutable

python
d = {(1, 2): "value"}
print(d)
Invalid:
python
# d = {[1, 2]: "value"}  # TypeError

Values Can Be Any Data Type

python
data = {
    "id": 1,
    "skills": ["Python", "AWS"],
    "active": True
}

Change Dictionary Items

python
person["age"] = 31
print(person)

Add New Items

python
person["city"] = "Pune"
print(person)

Remove Dictionary Items

pop()

python
person.pop("age")
print(person)

del

python
del person["city"]

clear()

python
person.clear()

Loop Through Dictionary

Loop Through Keys

python
for key in person:
    print(key)

Loop Through Values

python
for value in person.values():
    print(value)

Loop Through Key–Value Pairs

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

Check If Key Exists

python
if "name" in person:
    print("Key exists")

Dictionary Length

python
print(len(person))

Nested Dictionaries

python
students = {
    "student1": {"name": "A", "age": 20},
    "student2": {"name": "B", "age": 22}
}

print(students["student1"]["name"])

Copy a Dictionary

python
copy_dict = person.copy()
print(copy_dict)

Merge Dictionaries

python
a = {"x": 1}
b = {"y": 2}

c = a | b
print(c)
python
a.update(b)
print(a)

Dictionary Comprehension

python
numbers = [1, 2, 3]
squares = {x: x*x for x in numbers}
print(squares)

When to Use Dictionaries

Use dictionaries when:
  • Data has a meaningful key
  • Fast lookup is required
  • Data is structured
  • Representing real-world objects

Common Mistakes

Accessing Missing Key Directly

python
# print(person["salary"])  # KeyError
Use get() instead.

Summary

  • Dictionaries store key–value pairs
  • Keys must be immutable and unique
  • Fast lookup performance
  • Mutable and dynamic
  • Support nesting and comprehension
  • Essential for real-world Python applications