Debugging Competent¶

The built-in debugger (pdb)¶

Pause execution and inspect/step through live state with breakpoint(). Reach for it when print-debugging isn't enough — complex control flow or state you need to poke at.

# Add breakpoint in your code
def buggy_function(data):
    breakpoint()                  # Python 3.7+ — drops into pdb
    result = process(data)
    return result

Key pdb commands¶

Logging (better than print)¶

Record diagnostics with levels you can filter — the right tool for anything beyond a throwaway script, and essential in production where you can't watch stdout.

import logging

logging.basicConfig(
    level=logging.DEBUG,
    format="%(asctime)s [%(levelname)s] %(message)s"
)

logger = logging.getLogger(__name__)

logger.debug("Variable x = %s", 42)
logger.info("Processing started")
logger.warning("Disk space low")
logger.error("Failed to connect")
logger.critical("System crash!")

Use %s placeholders (not f-strings) so the string is only formatted if the level is active.

Post-mortem: inspect a crash after it happens¶

Post-mortem: inspect a crash after it happens in Debugging — what it is and when to use it.

import pdb

def boom():
    x = 10
    return x / 0

try:
    boom()
except ZeroDivisionError:
    # pdb.post_mortem()  # drops into the frame where it crashed
    pass

pdb.post_mortem() opens the debugger at the exact frame that raised — you can inspect locals (x here) without re-running.

Reading tracebacks programmatically¶

Reading tracebacks programmatically in Debugging — what it is and when to use it.

import traceback

try:
    [][5]
except IndexError:
    tb = traceback.format_exc()
    print("IndexError" in tb)   # True

traceback.format_exc() returns the full traceback as a string — handy for logging.

Quick inspection helpers¶

Quick inspection helpers in Debugging — what it is and when to use it.

# What attributes/methods does an object have?
print([m for m in dir("x") if not m.startswith("_")][:3])   # ['capitalize', 'casefold', 'center']

# Where is this object in memory / what type?
print(type(42).__name__)   # int

# Introspect a function's signature
import inspect
def f(a, b=2): ...
print(str(inspect.signature(f)))   # (a, b=2)

Debugging checklist¶

Debugging checklist in Debugging — what it is and when to use it.

Practice exercises¶

1. Use breakpoint() to step through a sorting algorithm and inspect variables at each iteration.
2. Set up logging with different levels for a multi-module program.
3. Debug a function by examining the call stack with w in pdb.