What Is a Dictionary?
A dictionary stores data as key-value pairs, giving you fast lookups by key instead of by position. Dictionaries are written with curly braces {} using key: value syntax.
student = {
"name": "Aman",
"course": "Data Science",
"fee_paid": 12000
}
Accessing Values
print(student["name"]) # Aman
print(student.get("course")) # Data Science
print(student.get("phone", "N/A")) # N/A — safe default if key doesn't exist
Tip: use .get() instead of square brackets when a key might not exist — it avoids a KeyError.
Adding and Updating Values
student["phone"] = "9818523125" # add new key
student["fee_paid"] = 24000 # update existing key
Removing Items
student.pop("phone") # removes key and returns its value
del student["fee_paid"] # removes key
student.clear() # empties the whole dictionary
Looping Through a Dictionary
student = {"name": "Aman", "course": "Python"}
for key in student:
print(key, "→", student[key])
for key, value in student.items():
print(key, "→", value)
for value in student.values():
print(value)
Checking If a Key Exists
if "course" in student:
print("Course found:", student["course"])
Dictionary Comprehension
squares = {x: x * x for x in range(1, 6)}
print(squares) # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
Nested Dictionaries
students = {
"STU-0001": {"name": "Aman", "course": "Python"},
"STU-0002": {"name": "Riya", "course": "Data Science"}
}
print(students["STU-0001"]["name"]) # Aman
Key Takeaways
- Dictionaries map unique keys to values and preserve insertion order (Python 3.7+)
- Use
.get()for safe access with a fallback default .items(),.keys(), and.values()are the go-to methods for looping- Keys must be immutable types (strings, numbers, tuples) — lists cannot be used as keys
