What Confused Me While Building My Vending Machine in Python

Introduction

Building a vending machine using OOP was exciting, but also frustrating. Here are some of the most real issues I ran into — things that made me stop and wonder, “Why is this not working?” or “What does that even mean?”

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Common Confusions I Faced

1. Accessing Variables from __init__

I didn’t understand why I had to use self.variable inside other methods. I kept forgetting it and getting errors.

class Example:
    def __init__(self):
        self.money = 0

    def add_money(self):
        self.money += 100  # NOT just money += 100

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2. Using One Method Inside Another

Calling a method inside the same class was confusing. I thought just writing the method name was enough, but no — you need self..

def run(self):
    self.check_stock()  # not just check_stock()

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3. Too Many If-Statements

When I had a lot of menu choices or checks, my code got messy. I learned about:

• Using dictionaries instead of if-chains
• Breaking big methods into smaller ones
• Early returns to exit logic early

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4. Reading from List of Dictionaries

for item in self.goodsList:
    print(item["brand"])  # Not item.brand

I kept trying to use dot notation instead of indexing with ["key"].

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5. Getting Valid User Input

I struggled to validate numeric input within a range:

if x.isdigit() and 1 <= int(x) <= 3:
    # valid

I learned to chain the condition and convert safely.

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6. if __name__ == "__main__"

This line confused me. It turns out it just checks if this file is being run directly — useful for testing.

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7. “Why is everything running from one line?”

Calling a.moneyOutput() runs several other functions. That’s because methods inside methods were being called — like a chain reaction.

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8. Using == but Nothing Happens

I wrote x == 3 and expected it to “do something”. But == just compares — I had to put it inside an if-statement to make it do something.

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9. Why pass Looks Grayed Out

My editor made pass look faded, which made me think something was wrong. Turns out it’s just a placeholder, and the editor is reminding me it’s incomplete.

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10. Why Didn’t print() Work?

Sometimes I used return in a function but forgot to print() the result. I assumed the result would just appear — wrong.

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11. Redundant Function Calls

I was accidentally calling chooseNumber() more than once because I didn’t reuse the return value. Fixing it meant storing the result in a variable and reusing it.

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What I learned

These issues taught me to:

• Slow down and trace the flow of function calls
• Think carefully about variable scope and data flow
• Understand how small misunderstandings cause bugs that feel like magic

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What I want to do next

I want to:

• Practice using self properly until it becomes second nature
• Build small examples that isolate just one of these confusions
• Teach someone else these mistakes so they learn faster

This might not be my most polished project — but it’s one of the most honest ones I’ve done.