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Lesson 3 of 5 18 min read

Lists, dictionaries and functions

Store collections of data in lists and dictionaries, and organise your code into reusable functions.

In this lesson you will

  • Create, index, slice and modify lists
  • Store and look up structured data with dictionaries
  • Define functions with parameters, default values and return values

Lists

A list is an ordered, changeable collection:

Python
skills = ["Python", "SQL", "Git"]
print(skills[0])      # Python  (indexes start at 0)
print(skills[-1])     # Git     (negative indexes count from the end)

skills.append("Docker")
skills.remove("SQL")
print(skills)         # ['Python', 'Git', 'Docker']
print(len(skills))    # 3
print(skills[0:2])    # ['Python', 'Git']  (slice: start included, end excluded)

Loop over a list directly, or use enumerate when you also need the position:

Python
for position, skill in enumerate(skills, start=1):
    print(position, skill)

List comprehensions build a new list in one readable line:

Python
marks = [45, 82, 67, 91]
passed = [m for m in marks if m >= 50]   # [82, 67, 91]
doubled = [m * 2 for m in marks]         # [90, 164, 134, 182]

Dictionaries

A dictionary stores key–value pairs, perfect for describing one thing with named fields:

Python
student = {
    "name": "Arjun",
    "year": 2,
    "skills": ["Python", "Git"],
}

print(student["name"])              # Arjun
student["year"] = 3                 # update a value
student["city"] = "Kochi"           # add a new key
print(student.get("email", "none")) # none  (default when the key is missing)

for key, value in student.items():
    print(key, "→", value)
The student dictionary after the updates: each key points to a value

student (contains "name": "Arjun", "year": 3, "skills": ["Python", "Git"], "city": "Kochi")

Using student["email"] for a missing key raises a KeyError; .get() lets you provide a fallback instead.

A list of dictionaries is a common way to represent table-like data:

Python
courses = [
    {"title": "Python Basics", "hours": 10},
    {"title": "SQL Essentials", "hours": 8},
]
total_hours = sum(c["hours"] for c in courses)   # 18

Functions

Define a function with def. Use return to send a result back:

Python
def average(numbers):
    if not numbers:          # an empty list is "falsy"
        return 0
    return sum(numbers) / len(numbers)

def greet(name, greeting="Hello"):   # greeting has a default value
    return f"{greeting}, {name}!"

print(average([70, 80, 90]))         # 80.0
print(greet("Asha"))                 # Hello, Asha!
print(greet("Asha", "Welcome"))      # Welcome, Asha!

A function without a return statement returns the special value None.

Walkthrough: a word-frequency counter

Python
def word_counts(text):
    counts = {}
    for word in text.lower().split():
        counts[word] = counts.get(word, 0) + 1
    return counts

result = word_counts("Learn build rise learn build learn")
print(result)   # {'learn': 3, 'build': 2, 'rise': 1}

Step by step: lower() makes the text lowercase, split() breaks it into words, and for each word the dictionary count goes up by one, starting from 0 when the word is new. Counting things with a dictionary is a pattern you'll use constantly in data work.

Hands-on exercise

Create a list of five dictionaries, each describing a student with a name and a marks value. Write a function top_student(students) that returns the name of the student with the highest marks, and a function class_average(students) that returns the average. Print both results. Bonus: use a list comprehension to build a list of the names of students who scored 75 or more.

Knowledge check

3 questions • 70% to pass • Practice only, no certificate

  1. 1.What is printed?
    CODE
    colors = ["red", "green", "blue"]
print(colors[1:])
  2. 2.Which expression safely reads the key "email" and returns None if it is missing?
  3. 3.What does a Python function return if it has no return statement?
0 of 3 answered

Key concepts

  • Lists are ordered and changeable; index from 0, use negative indexes from the end, and slice with [start:end].
  • Dictionaries map keys to values; use .get() to supply a default for missing keys.
  • Functions are defined with def, can have default parameter values and return None if they don't return anything else.
  • Lists of dictionaries are a simple way to represent table-like data.