Lesson · ⏱ 8 min read

🔁 Python logic: if/else, loops and functions

Q: You need to apply the same discount rule to 1,000 orders. What do you use?

A function inside a loop. Write the rule once as a function, then loop over the orders and call it for each one.

🏠 Real-life example

A traffic signal makes decisions: if red, stop; else if green, go. That's if/else. A washing machine repeats the same rinse cycle several times. That's a loop. And your mom's chai recipe is written once and reused every morning. That's a function: steps you package once and call whenever you need them.

💡 In simple words

Programs need to make decisions, repeat work and stay organised. 'if / elif / else' runs different code depending on a condition. A 'for' loop repeats code for each item in a collection, and a 'while' loop repeats until a condition becomes false. A function (made with 'def') bundles steps under a name, takes inputs (parameters) and gives back an output with 'return'. Good functions do one clear job, which makes code easier to test and reuse.

🪜 Step by step

Decisions with if / elif / else: if marks >= 90: grade = "A" elif marks >= 75: grade = "B" else: grade = "C". Indentation (4 spaces) shows what's inside each block.
Repeat with for: for city in ["Pune", "Delhi"]: print(city). Use range(5) to repeat 5 times.
Repeat until something changes with while: while balance < target: balance += saving. Make sure the loop can end.
Package logic in a function: def emi(principal, rate, months): ... return value. Call it with different inputs instead of copying code.
Test functions with small known cases: if emi(100000, 0, 10) should be 10000, check it with assert.

🔑 Key terms

Condition: A question with a True/False answer, e.g. price > 100.
Loop: Code that repeats for each item or until a condition changes.
Function: A named, reusable block of code with inputs and an output.

👀 See it in action

python

def bill(amount):
    if amount >= 500:
        return amount * 0.9   # 10% off on big orders
    return amount

orders = [120, 650, 300]
for o in orders:
    print(o, "→", bill(o))
# 120 → 120, 650 → 585.0, 300 → 300

A kirana-store discount rule written once and used for every order.

⚠️ Common mistakes to avoid

Wrong indentation: mixing tabs and spaces or forgetting to indent after a colon causes errors or wrong logic.
Infinite while loops: the condition never becomes false. Always update the variable you check.
Printing inside a function instead of returning. print shows a value; return gives it back so other code can use it.

✅ Quick check

You need to apply the same discount rule to 1,000 orders. What do you use?

Copy-paste the code 1,000 times
A single if statement
A function inside a loop

Show answer

C. A function inside a loop: Write the rule once as a function, then loop over the orders and call it for each one.

🙋 Questions learners ask

When should I use for vs while?

Use for when you know what to go through (a list, a range). Use while when you repeat until a condition changes and don't know how many times.

How big should a function be?

Small enough to describe in one sentence. If you need 'and' twice to describe it, split it into two functions.

📌 Remember

if = decide, loop = repeat, function = reuse.

Learn this as part of a full course

Track your progress, take the quick checks and follow the lessons in the right order. This lesson is part of:

AI Foundations Path · AI Product Engineer · MLOps / AI Platform Engineer · LLM / Agent Engineer · AI Forward Deployed Engineer

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