Python Lists, Decisions, Loops and Functions

Before you start

This guide is the reading for Course 2 of the Python for AI Pathway. It takes about 20 minutes and builds on Course 1, Python First Steps. Open a Google Colab notebook and try each example as you read.

You'll learn the four tools behind almost every program: lists to hold many values, dictionaries to label them, if to make decisions, loops to repeat, and functions to reuse your work.

All names and examples here are made up.

Chapter 1: Lists: many values in one place

A list holds several values, in order, inside square brackets:

marks = [72, 85, 64, 91, 58]
print(marks)
print(len(marks))

Output:

[72, 85, 64, 91, 58]
5

len() gives the number of items. Each item has a position called an index, and counting starts at 0:

marks = [72, 85, 64, 91, 58]
print(marks[0])
print(marks[-1])

Output:

72
58

marks[0] is the first item; marks[-1] is the last. You can add items with append:

cities = ["Delhi", "Chennai"]
cities.append("Kolkata")
print(cities)

Output:

['Delhi', 'Chennai', 'Kolkata']

Python also has quick helpers for lists of numbers: sum(), max(), min():

marks = [72, 85, 64, 91, 58]
print(sum(marks), max(marks), min(marks))

Output:

370 91 58

Chapter 2: Dictionaries: values with labels

A dictionary stores pairs of a key and a value, inside curly brackets. It's like a form with labelled boxes:

student = {"name": "Ravi", "class": 10, "city": "Indore"}
print(student["name"])
student["marks"] = 88
print(student)

Output:

Ravi
{'name': 'Ravi', 'class': 10, 'city': 'Indore', 'marks': 88}

You look a value up by its key, such as student["name"], and add a new pair by assigning to a new key. Lists and dictionaries are how most data is held in Python, and in AI tools too.

Chapter 3: Making decisions with if

if runs code only when a condition is True. elif ("else if") checks another condition, and else covers everything left:

marks = 72
if marks >= 75:
    print("Distinction")
elif marks >= 40:
    print("Pass")
else:
    print("Needs another try")

Output:

Pass

Two rules matter:

• The line with if, elif or else ends with a colon :.
• The code that belongs to it is indented (moved right, usually 4 spaces). Python uses indentation to know what's inside the if.

You can combine conditions with and, or and not:

age = 19
has_id = True
print(age >= 18 and has_id)

Output:

True

Chapter 4: Repeating with loops

A for loop repeats code for every item in a list:

expenses = [1200, 450, 3000]
for amount in expenses:
    print("Spent", amount)

Output:

Spent 1200
Spent 450
Spent 3000

range(n) gives the numbers 0 up to n−1, which is handy for repeating a fixed number of times:

for i in range(3):
    print("Round", i)

Output:

Round 0
Round 1
Round 2

Loops and if together are powerful. Count how many students passed:

marks = [72, 85, 34, 91, 28]
passed = 0
for m in marks:
    if m >= 40:
        passed = passed + 1
print(passed, "students passed")

Output:

3 students passed

A while loop repeats as long as a condition stays True. Use it carefully: if the condition never becomes False, the loop runs for ever.

Chapter 5: Functions: reusable steps

A function is a named set of steps you can use again and again. You define it with def, and it can return a result:

def grade(marks):
    if marks >= 75:
        return "Distinction"
    elif marks >= 40:
        return "Pass"
    return "Fail"

print(grade(80))
print(grade(52))
print(grade(12))

Output:

Distinction
Pass
Fail

marks here is a parameter: the input the function works on. Write a function once, then call it with any value. You've already used built-in functions: print, len, sum.

Functions can take several parameters, and give them default values:

def total_with_gst(price, rate=18):
    return price + price * rate / 100

print(total_with_gst(1000))
print(total_with_gst(1000, 5))

Output:

1180.0
1050.0

Chapter 6: Putting it together

A small, realistic program: a monthly budget check.

budget = {"rent": 12000, "food": 6000, "travel": 2000}
spent = {"rent": 12000, "food": 7400, "travel": 1500}

def check(category):
    over = spent[category] - budget[category]
    if over > 0:
        return f"{category}: over by Rs {over}"
    return f"{category}: within budget"

for category in budget:
    print(check(category))

Output:

rent: within budget
food: over by Rs 1400
travel: within budget

It uses dictionaries for the data, a function for the rule, a loop to go through every category, and if to decide. That's the shape of most real programs, including the data work in Course 3.

Chapter 7: Good habits

• Use clear names: monthly_rent beats x.
• Keep functions small: one function, one job.
• Comment the "why", not the obvious.
• Test with small examples first, then with real data.
• Watch the indentation: mixing it up is the most common beginner error in Python.
• Never paste passwords or keys into code you share or upload. Course 5 shows the safe way.

Key words

• List: an ordered collection, [1, 2, 3]; the first index is 0.
• Index: an item's position in a list.
• Dictionary: key–value pairs, {"name": "Ravi"}.
• if / elif / else: run code depending on a condition.
• Indentation: spaces at the start of a line that show which code belongs together.
• for loop: repeat code for each item.
• range(n): the numbers 0 to n−1.
• while loop: repeat while a condition is True.
• Function / def / return: a named, reusable set of steps that gives back a result.
• Parameter: an input a function takes.