08.3 - Debugging Techniques

Theory 20 min Intermediate

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Reading a Traceback

When an exception occurs, Python prints a traceback showing the call stack:

Traceback (most recent call last):
  File "app.py", line 15, in main          ← outermost call
    result = process_data(raw_input)
  File "app.py", line 8, in process_data   ← called by main
    return int(data.strip())
ValueError: invalid literal for int() with base 10: 'abc'
         ↑              ↑
     exception type   message

Read from bottom to top: the last line shows the error; the stack shows how you got there.

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breakpoint() — Built-in Debugger

Python 3.7+ has a built-in breakpoint() function:

def calculate(a, b):
    result = a * 2
    breakpoint()   # execution pauses here → opens pdb
    return result + b

calculate(5, 3)

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pdb Commands

When the debugger pauses execution:

Command	Short	Action
help	h	Show all commands
next	n	Execute next line (step over)
step	s	Step into function call
return	r	Run until current function returns
continue	c	Continue until next breakpoint
quit	q	Quit debugger
print x	p x	Print value of x
list	l	Show source code around current line
where	w	Show call stack
up/down	u/d	Move up/down the call stack

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VS Code Debugger

1. Click the gutter to set a breakpoint (red dot)
2. Press F5 and select "Python File"
3. Use the toolbar: Continue (F5), Step Over (F10), Step Into (F11), Step Out (Shift+F11)
4. Inspect variables in the Variables panel

launch.json for custom debug configurations:

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Python: Current File",
      "type": "python",
      "request": "launch",
      "program": "${file}",
      "console": "integratedTerminal",
      "justMyCode": false,
      "env": {"DEBUG": "1"}
    }
  ]
}

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assert Statements

Assertions catch programming errors (not user input errors):

def process(items):
    assert isinstance(items, list), f"Expected list, got {type(items).__name__}"
    assert len(items) > 0, "Items cannot be empty"
    return [item.upper() for item in items]

# Python can disable assertions with -O flag
# Never use assert for security or input validation!

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Useful Debugging Techniques

# 1. Print with context
print(f"DEBUG: {variable_name=}")   # prints "variable_name=value"

# 2. Check object state
import pprint
pprint.pprint(complex_dict)   # pretty-printed dict

# 3. Inspect types
print(type(obj).__mro__)   # class hierarchy
print(dir(obj))             # all attributes

# 4. Measure performance
import cProfile
cProfile.run("my_function()")

# 5. Memory usage
import tracemalloc
tracemalloc.start()
# ... your code ...
snapshot = tracemalloc.take_snapshot()

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Key Vocabulary

Term	Definition
Traceback	Stack trace showing the sequence of function calls that led to an exception
breakpoint()	Built-in function to pause execution and enter the debugger
pdb	Python Debugger — interactive source code debugger
Step over	Execute next line without entering function bodies
Step into	Enter the function being called
assert	Statement that raises AssertionError if condition is False
Call stack	The sequence of active function calls at a point in execution

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Summary

• Read tracebacks bottom-to-top: the last line shows the error, the stack shows the path
• breakpoint() (Python 3.7+) pauses execution and opens pdb interactively
• Key pdb commands: n (next), s (step into), c (continue), p x (print x), q (quit)
• VS Code debugger provides a visual interface with the same capabilities
• Use assert to check programming invariants — never for user input validation
• f"{var=}" is a quick debug print that shows both variable name and value

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📄️ 08.1 - Exceptions

Handle Python exceptions gracefully with try/except/else/finally, understand the exception hierarchy, and raise exceptions correctly

📄️ 08.2 - Custom Exceptions & Logging

Create custom exception hierarchies and replace print() debugging with Python's professional logging module

📄️ 08.3 - Debugging

Debug Python programs using breakpoints, pdb, VS Code debugger, assertions, and traceback analysis

📄️ Lab - Module 08

Build a CLI contact manager with custom exceptions, professional logging, and input validation

📄️ Quiz - Module 08

30 questions on exceptions, try/except, custom errors, logging, and debugging