Data Structures · Unit 3

Dictionaries

Key → value pairs

Unit 3 · Data Structures

Choose the right container for your data

A data structure organizes values so a program can find and work with them. Explore four built-in Python structures and use each lab to see how its rules affect real data.

Lists

Ordered and changeable

Tuples

Ordered and fixed

Sets

Unique values

Dictionaries

Key → value lookup

Keyed lookup · changeable · insertion order preserved

Dictionaries: look up a value by its key

A dictionary maps each unique key to a value. Instead of remembering that the course is at position 1, a program can ask for record["course"]. Keys must be unique and hashable.

Think of a student record: the field name is the key, and the stored detail is its value. A label such as “semester” is clearer than an unexplained number in a sequence.

Key–value record lab

Search a field, change its value, or remove it. The record below updates immediately.

studentMira

coursePython

semester2

Key

Enter the field name to search in the record.

Look up a key, update its value, or remove the pair.

record[key]Get a value; missing keys raise KeyError.

record.get(key)Look up safely; returns None if missing.

record[key] = valueAdd a key or replace its value.

Run the dictionary operation in Python

Starting up Python…

student_record = {
    "student": "Mira",
    "course": "Python",
    "semester": "2"
}
print("Value:", student_record.get("course", "Key not found"))

The editor is ready while Python loads in the background.

Output

Your program's output will appear here.

Common Python mistakes

NameError: name 'mark' is not defined

NameError: A name was used before Python could find a variable or function with that name.

IndexError: list index out of range

IndexError: The requested position is outside the items in this list or sequence.

KeyError: 'email'

KeyError: That key does not exist in this dictionary.

TypeError: can only concatenate str (not "int") to str

TypeError: This operation does not work with the kind of value it received.

ZeroDivisionError: division by zero

ZeroDivisionError: A number cannot be divided by zero.

SyntaxError: invalid syntax

SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.

IndentationError: expected an indented block

IndentationError: The spaces at the start of a line do not match Python's block structure.

Avoid common dictionary mistakes

A missing key can raise KeyError: use get() when the key may not exist.
Keys are unique: assigning to an existing key replaces its old value; it does not create a second pair.
Use square brackets for lookup: record["student"] retrieves a value; braces create the dictionary.

Dictionaries check

1. How do you retrieve the value stored under the key 'course'?

2. What does assigning a value to an existing dictionary key do?

Compare the four structures

Ask two questions: does order matter, and should the collection change after creation?

Structure	Ordered?	Changeable?	Access values by
list	Yes	Yes	Position (index)
tuple	Yes	No	Position (index)
set	No guaranteed order	Yes	Membership, not index
dict	Insertion order is preserved	Yes	Key

Use a list for a sequence that changes, a tuple for a fixed record, a set for unique membership, and a dictionary for named lookups.

Sets Built-in Functions