Debugging with pdb & IDE Debugger

Debugging is finding and fixing bugs. Python has pdb (Python Debugger) built-in, and IDEs like VS Code provide visual debugging tools.

15 min•By Priygop Team•Updated 2026

Debugging

# Using pdb (Python Debugger)
# import pdb; pdb.set_trace()  # old way
# breakpoint()                  # Python 3.7+ (preferred!)

# pdb commands:
# n  - next line
# s  - step into function
# c  - continue execution
# p  - print variable value
# l  - list source code
# q  - quit debugger
# h  - help

# Demo: debugging a function
def calculate_average(numbers):
    """Bug: doesn't handle empty list."""
    # breakpoint()  # uncomment to debug
    if not numbers:
        return 0
    total = sum(numbers)
    count = len(numbers)
    average = total / count
    return average

# Test cases
print("=== Debugging Demo ===")
print(f"Average [1,2,3]: {calculate_average([1, 2, 3])}")
print(f"Average []: {calculate_average([])}")
print(f"Average [10]: {calculate_average([10])}")

# VS Code debugging tips:
print("\n=== VS Code Debug Tips ===")
print("1. Click left of line number → set breakpoint")
print("2. F5 → start debugging")
print("3. F10 → step over")
print("4. F11 → step into")
print("5. Shift+F11 → step out")
print("6. Watch variables in sidebar")
print("7. Use Debug Console for expressions")

Tip

Use breakpoints with breakpoint() (Python 3.7+). In VS Code, click the gutter to set breakpoints and use F5 to start debugging.

Diagram

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Use console.table for objects. console.time to profile. debugger; for breakpoints. Chrome DevTools > console.log.

Common Mistake

Warning

Using only print() for debugging. Learn to use the debugger - it lets you inspect variables, step through code, and evaluate expressions.

Quick Quiz

Practice Task

Note

(1) Set a breakpoint in your code. (2) Step through line by line. (3) Inspect variable values in the debug console.

Topics in This Module

Debugging

# Using pdb (Python Debugger)
# import pdb; pdb.set_trace()  # old way
# breakpoint()                  # Python 3.7+ (preferred!)

# pdb commands:
# n  - next line
# s  - step into function
# c  - continue execution
# p  - print variable value
# l  - list source code
# q  - quit debugger
# h  - help

# Demo: debugging a function
def calculate_average(numbers):
    """Bug: doesn't handle empty list."""
    # breakpoint()  # uncomment to debug
    if not numbers:
        return 0
    total = sum(numbers)
    count = len(numbers)
    average = total / count
    return average

# Test cases
print("=== Debugging Demo ===")
print(f"Average [1,2,3]: {calculate_average([1, 2, 3])}")
print(f"Average []: {calculate_average([])}")
print(f"Average [10]: {calculate_average([10])}")

# VS Code debugging tips:
print("\n=== VS Code Debug Tips ===")
print("1. Click left of line number → set breakpoint")
print("2. F5 → start debugging")
print("3. F10 → step over")
print("4. F11 → step into")
print("5. Shift+F11 → step out")
print("6. Watch variables in sidebar")
print("7. Use Debug Console for expressions")

Topics in This Module