Teaching Python to Make Decisions

Same deal as last time: open session2.py, keep it next to this article, type everything yourself.

Comparison Operators - Asking Yes or No Questions

Every decision starts with a question that has exactly two possible answers: True or False. That's what comparison operators give you.

age = 20
score = 75

print(age == 20)     # True - age IS 20
print(age == 25)     # False - age is NOT 25
print(score > 70)    # True
print(score <= 74)   # False - 75 is not <= 74

True
False
True
False

I flag one thing before anyone even runs this: == asks a question, = stores a value. Mixing them up is the single most common typo in this entire course - everyone does it at least once, usually while I'm mid-sentence explaining something else.

String comparisons work the same way, but with a twist:

track = "Data Science"
print(track == "Data Science")   # True
print(track == "data science")   # False - case sensitive!
print("Amina" < "Brian")          # True - alphabetical order

True
False
True

'Data Science' and 'data science' are completely different strings to Python. If you want to compare without caring about case, lowercase both sides first: track.lower() == "data science".

if - Making a Decision

An if statement checks a condition. If it's True, the indented block runs. If it's False, Python skips it - nothing happens, no error, it just moves on. Think of it like a gate: no ID, no drama, you just don't get let in.

score = int(input("Enter your score: "))
if score >= 50:
    print("You passed!")
print("Thank you for taking the test.")

Enter your score: 75
You passed!
Thank you for taking the test.

The line "Thank you for taking the test." isn't indented, so it always runs - pass or fail. Only the indented line is conditional. Run it again with a score of 30 and watch "You passed!" disappear while the thank-you stays.

else and elif - More Than One Path

else gives you the opposite path - exactly one of the two blocks runs, never both, never neither:

score = int(input("Enter your score: "))
if score >= 50:
    print("PASS - Well done!")
else:
    print("FAIL - Please try again.")
print(f"Your score was: {score}")

Enter your score: 45
FAIL - Please try again.
Your score was: 45

elif - short for "else if" - lets you chain more than two paths:

score = int(input("Enter score: "))
if score >= 80:
    print("Grade A - Excellent!")
elif score >= 70:
    print("Grade B - Good")
elif score >= 60:
    print("Grade C - Average")
elif score >= 50:
    print("Grade D - Below Average")
else:
    print("Grade F - Failed")

Enter score: 73
Grade B - Good

Here's the part that trips people up every single cohort: Python checks top to bottom and stops at the first True condition. Order the boundaries wrong - say, checking score >= 50 before score >= 80 - and every passing score gets swallowed by the first, loosest condition before Python ever reaches the accurate one. Always put the most specific condition first.

Logical Operators - Checking More Than One Thing

Sometimes one question isn't enough. and needs both conditions True. or needs at least one. not flips True to False and back.

age = int(input("Age: "))
has_id = input("Do you have an ID? (yes/no): ")

if age >= 18 and has_id == "yes":
    print("Access granted - welcome!")
else:
    print("Access denied.")

Age: 20
Do you have an ID? (yes/no): yes
Access granted - welcome!

Age 20 with no ID, denied. Age 15 with ID, denied. Only both together get you through - that's and. Swap it for or and only one condition needs to hold, like a discount that applies if you're a student or a senior, not both.

Nested if - A Decision Inside a Decision

A nested if is exactly what it sounds like - an if inside another if, used when the second question only makes sense once the first has already been answered YES.

username = input("Username: ")
if username == "admin":
    password = input("Password: ")
    if password == "kenya2025":
        print("Welcome, admin!")
    else:
        print("Wrong password.")
else:
    print("User not found.")

Username: admin
Password: kenya2025
Welcome, admin!

Notice the password question never even appears if the username is wrong - there's no point asking. That's the whole reason to reach for nested if instead of elif: elif is for the same question with different possible answers (track: DS or DE); nested if is for a question that only exists because of how the previous one was answered.

Putting It Together - NHIF Deduction Calculator

By this point everyone's used every idea from today at least once. So we close with something real - an NHIF-style deduction calculator using actual government bracket logic, simplified for learning:

print("=== NHIF DEDUCTION CALCULATOR ===")
print()

name = input("Employee name: ")
salary = int(input("Gross monthly salary (Ksh): "))

if salary < 6000:
    nhif = 150
elif salary < 8000:
    nhif = 300
elif salary < 12000:
    nhif = 400
elif salary < 15000:
    nhif = 500
elif salary < 25000:
    nhif = 750
elif salary < 50000:
    nhif = 850
else:
    nhif = 950

net = salary - nhif

print()
print(f"Employee: {name}")
print(f"Gross: Ksh {salary:,}")
print(f"NHIF: Ksh {nhif:,}")
print(f"Net: Ksh {net:,}")

=== NHIF DEDUCTION CALCULATOR ===
Employee name: Brian Otieno
Gross monthly salary (Ksh): 35000

Employee: Brian Otieno
Gross: Ksh 35,000
NHIF: Ksh 850
Net: Ksh 34,150

Someone always asks, at this exact point, "wait - this is basically how payroll software works?" Yes. That's exactly what it is. It's just brackets and comparisons, stacked.

Try It Yourself

Easy - Matatu ticket pricing. Ask for a distance in km, then use if/elif/else to charge Ksh 30 for 0–5km, Ksh 50 for 6–15km, Ksh 80 for 16–30km, and Ksh 120 above that. Test with 3, 10, 25, and 50 km - you should get 30, 50, 80, 120.

Medium - M-Pesa daily limit checker. Ask for an account type (basic/premium) and an amount to send. Basic accounts cap at Ksh 70,000, premium at Ksh 300,000 - approve or reject based on which type and whether the amount fits the limit.

Challenge - Bank loan eligibility, using nested if. Ask if the user has a bank account. No → stop there. Yes → ask how many months they've been a customer. Under 6 months → not eligible yet. 6+ months → ask their monthly salary. Under Ksh 20,000 → doesn't qualify. Ksh 20,000+ → qualifies. Each question only appears because the one before it passed - that's the nested-if pattern in action.

What I Noticed Teaching This Round

• The = vs == mix-up happened to almost everyone within the first ten minutes - and once it happens once, it basically never happens again
• Nobody guessed correctly that elif order could silently break a grading program until I showed them Example 6 breaking live - that one lands better as a demonstration than an explanation

Next Week

Somebody's going to ask "what if I need to do this 100 times?" before I even bring it up - it happens every cohort. Loops answer that:

for i in range(5):
    print(f"Student {i + 1}")

Printing five things without five print() lines is a small thing to see for the first time, but it changes how people think about repetition entirely. That's next week.

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