Debugging Techniques
Find Bugs, Understand Errors, Fix Code with Confidence.

Reproduce  →  Observe  →  Find the cause  →  Fix  →  Verify

1. Run the same input that shows the problem.
2. Read the error message or compare the actual result with the expected result.
3. Form a small hypothesis and inspect the relevant line or value.
4. Change only what is needed.
5. Run the program again and check for new problems.

# Syntax error
if cena > 100
    print("Skupo")

# Runtime error
cena = 100
kolicina = 0
print(cena / kolicina)

# Logic error
cena = 100
popust = 20
nova_cena = cena + cena * popust / 100
print(nova_cena)

The first example has a SyntaxError because the if line needs a colon. The second raises ZeroDivisionError while running. The third program runs, but adding a discount makes the price higher: that is a logic error, so there may be no traceback at all.

Traceback (most recent call last):
  File "main.py", line 5, in <module>
    rezultat = 100 / 0
ZeroDivisionError: division by zero

Traceback (most recent call last) introduces the call history. The file name and line number show where Python noticed the exception. The final line names the exception and its message. In a larger program, an earlier stack frame may contain the decision that caused the bad value, so the traceback is a useful clue rather than a complete explanation.

cena = 200
popust = 10

iznos_popusta = cena * popust
print("DEBUG:", iznos_popusta)

nova_cena = cena - iznos_popusta
print(nova_cena)

# Correct calculation:
iznos_popusta = cena * popust / 100

The temporary DEBUG print reveals that the discount amount is 2000 instead of 20. Dividing the percentage by 100 fixes the formula. Remove temporary DEBUG prints after the investigation so they do not become confusing output later.

1. Open the Python file and choose the correct interpreter.
2. Click left of a line number to add a breakpoint.
3. Start Run and Debug or press F5.
4. If asked, choose Python File debugging with the Python extension installed.
5. The program stops before it executes the breakpoint line.

A breakpoint pauses the program at a chosen place so you can inspect its state. VS Code layouts and keyboard shortcuts can differ by operating system or settings, so the debugger toolbar buttons are always a good alternative.

def cena_sa_popustom(cena: float, popust: float) -> float:
    iznos_popusta = cena * popust / 100
    nova_cena = cena - iznos_popusta
    return nova_cena

print(cena_sa_popustom(200, 10))

Put a breakpoint on the first line inside the function. In the Variables panel, first inspect cena and popust. Step Over executes one line without entering a called function; Step Into enters one when possible; Step Out continues until the current function returns; Continue runs to the next stop. Watch how iznos_popusta and nova_cena appear as execution moves forward.