Python Didn't Crash. The Result Was Still Wrong.

#python #debugging #beginners #programming

One of the most dangerous bugs isn't a SyntaxError.

It isn't an exception.

It doesn't even produce a traceback.

Sometimes Python executes your entire program successfully...

…and gives you the wrong answer.

Consider this:

def calcular_total(precios, descuento):
subtotal = sum(precios)
total = subtotal - descuento
return total

precios = [120000, 80000]
descuento = 0.10

print(calcular_total(precios, descuento))

Expected:

180000

Actual:

199999.9

Python isn't broken.

Our assumption is.

And that's where debugging becomes much more interesting.

A mistake I made when learning to debug was asking:

“What should I print?”

A better question is:

“What do I need to observe to prove or reject my hypothesis?”

That changes debugging from guessing into an experiment:

Reproduce → Hypothesis → Measure → Evidence → Conclusion

For example:

Are the inputs correct?
Is subtotal correct?
Is descuento what we expect?
At which exact operation does the state become wrong?

Once you know that:

subtotal = 200000 ✓
descuento = 0.10 ✓

the search space becomes tiny.

The suspicious operation is now:

subtotal - descuento

That's the real goal of debugging:

Turn “something is wrong” into one small, testable question.

I wrote a practical guide showing this process with print(), type(), repr(), breakpoint() and step-by-step state inspection.

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