3.12 Debugging

Debugging is the systematic process of locating, analyzing, and fixing errors (bugs) in a program so that it functions as intended. Every programmer encounters bugs — learning to debug efficiently is a core programming skill.

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Types of Errors (Bugs)

1. Syntax Error

A syntax error occurs when code violates the grammatical rules of Python. The interpreter catches these errors before the program runs.

Common causes:

• Missing colon (:) after if, for, while, or def
• Misspelled keywords
• Unmatched parentheses or brackets

Example:

if x > 5   # SyntaxError: expected ':'
    print(x)

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2. Runtime Error

A runtime error occurs during program execution, often causing the program to crash. Python raises a specific exception for each type.

Common Python runtime errors:

Example:

x = 10 / 0   # ZeroDivisionError: division by zero

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3. Logical Error

A logical error occurs when the program runs without crashing but produces incorrect results due to a flaw in the programmer's reasoning.

• These are the hardest to detect because Python cannot flag them.
• Only careful testing and checking of output reveals them.

Example:

# Intended to calculate area of rectangle
area = length + width   # Bug: should be length * width

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Ways of Debugging in Python

1. Reading Error Messages

When Python raises an exception, it prints a traceback showing:

• The type of error (e.g., ZeroDivisionError)
• The line number where the error occurred
• A description of the problem

Always read the traceback carefully — it points directly to the bug.

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2. Print Statement Debugging

Insert print() statements at key points to display variable values and track program flow.

def calculate_area(l, w):
    print("l =", l, "w =", w)   # Debug print
    area = l * w
    print("area =", area)        # Debug print
    return area

Remove or comment out debug prints once the bug is fixed.

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3. Using the Python IDLE Debugger

Python's IDLE includes a built-in debugger accessible via Debug → Debugger in the menu.

Key debugger features:

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4. Using try / except to Handle Runtime Errors

Wrap code that may cause a runtime error in a try/except block to catch and handle the error gracefully.

try:
    x = int(input("Enter a number: "))
    result = 100 / x
    print(result)
except ZeroDivisionError:
    print("Error: Cannot divide by zero.")
except ValueError:
    print("Error: Please enter a valid integer.")

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Summary Table