At some point in programming, this will happen:
You write code.
You run it.
Python does something…
…and it’s not what you expected.
No error message.
No crash.
Just the wrong behavior.
Welcome to logic bugs.
This is not failure.
This is programming.
What a Logic Bug Actually Is
A logic bug happens when:
Your code runs
Python is perfectly happy
But the result is wrong
Python didn’t misunderstand you.
It did exactly what you told it to do.
You just told it the wrong thing.
And that’s okay.
A Classic Example
Look at this code:
age = int(input("How old are you? "))
if age >= 18:
print("Adult")
elif age >= 13:
print("Teen")
else:
print("Child")
Seems fine, right?
Now imagine you accidentally write this instead:
if age >= 13:
print("Teen")
elif age >= 18:
print("Adult")
Suddenly, adults are teens.
Python isn’t broken.
Your order of conditions is.
This is the most common logic bug beginners hit.
The Golden Rule of Debugging Logic
When something behaves strangely, ask:
“What did Python actually check?”
Python evaluates conditions top to bottom and stops at the first true one.
It doesn’t know your intention.
It only knows the order you gave it.
Step 1: Slow It Down
When logic feels confusing, slow everything down.
Add prints.
Yes, really.
print("Checking age...")
print("Age is:", age)
Inside conditions:
if age >= 18:
print("Age >= 18 is TRUE")
print("Adult")
elif age >= 13:
print("Age >= 13 is TRUE")
print("Teen")
This lets you see which path Python takes.
Debugging is not guessing.
It’s observing.
Step 2: Check Your Conditions One by One
Test comparisons separately:
print(age >= 18)
print(age >= 13)
Seeing True or False printed often makes the problem obvious immediately.
Many logic bugs vanish the moment you print the condition.
Step 3: Watch Out for These Common Traps
Trap 1: Wrong Order
Always check the most specific conditions first.
# Good
if score >= 90:
...
elif score >= 70:
...
Not the other way around:
# Bad
if score >= 70:
...
elif score >= 90:
...
Trap 2: Impossible Conditions
This will never be true:
if age < 10 and age > 20:
No number can satisfy both.
When debugging, ask:
“Can this condition ever be true?”
Trap 3: Comparing the Wrong Thing
This happens more than people admit:
if name == "Alex":
But the input was "alex".
Normalize your input:
name = name.strip().lower() #.strip() removes spaces, .lower() converts all characters to lowercase
Then compare.
Trap 4: Forgetting What or Really Means
This is wrong:
if age == 18 or 19:
Python reads that as:
age == 18 → maybe
19 → always true
So the condition is always true.
Correct version:
if age == 18 or age == 19:
Or better:
if age >= 18:
Step 4: Talk Through the Code Like a Human
One of the best debugging tools costs nothing.
Read your code out loud.
Literally.
if has_ticket and age >= 18:
Becomes:
“If the person has a ticket and is at least 18…”
If that sentence doesn’t match what you meant, the code won’t either.
Debugging Is a Skill, Not a Talent
Beginners often think:
“Good programmers don’t make bugs.”
Reality:
Good programmers are just good at finding bugs.
Every confusing moment you survive makes you better at the next one.
Debugging isn’t a setback.
It’s how understanding is built.
What You’ve Learned
You now know how to:
Recognize logic bugs
Understand why Python behaves the way it does
Use
print()to debugCheck condition order
Spot impossible logic
Read code like plain English
This is a huge step toward confidence.
Mini Quiz
Try these:
What is a logic bug?
Why doesn’t Python warn you about logic bugs?
What’s the first thing you should do when code behaves strangely?
Why does condition order matter?
What will this print?
x = 5
if x > 10:
print("Big")
elif x > 3:
print("Medium")
else:
print("Small")