🐞 Python Tutorial — Debugging with pdb

Introduction 🌟

pdb (Python Debugger) is Python’s built-in interactive debugging tool. It allows you to pause program execution, inspect variables, step through code, set breakpoints, and fix bugs efficiently.

Note

💡 pdb = Python Debugger
💡 Works in terminal, scripts, or inside code
💡 Supports stepping, breakpoints, stack inspection

1. Starting pdb from Code 🧱

Use pdb.set_trace() to pause execution anywhere.

set_trace_example.py

import pdb

x = 10
y = 20

pdb.set_trace()   # debugger starts here

z = x + y
print(z)

✔ Execution stops → pdb prompt appears

2. Starting pdb from Terminal 🔧

Run a script under the debugger:

Code Snippet

python -m pdb myscript.py

✔ Starts debugging before the script runs

3. Essential pdb Commands 📋

CommandDescription
l (list)Show current code context
n (next)Step to next line (same function)
s (step)Step into function calls
c (continue)Run until next breakpoint
b (break)Set breakpoint
cl (clear)Clear breakpoints
p (print)Print variable value
w (where)Show stack trace
q (quit)Exit debugger

4. Setting Breakpoints 🎯

Break at a specific line

Code Snippet

b 12
Break in a function

Code Snippet

b my_function
Break with a condition

Code Snippet

b 25, x > 10

✔ Useful for loops or debugging states

5. Inspecting Variables 🔍

print_vars.py

p x        # print value of x
p locals() # show local variables

✔ Helps understand what's happening inside your program

6. Stepping Through Code 🐾

  • 🔸 n → next line
  • 🔸 s → step inside function
  • 🔸 r → run until function returns

7. Full Example — Debugging a Buggy Program 🐛

bug_example.py

import pdb

def divide(a, b):
    pdb.set_trace()   # start debugger
    return a / b

result = divide(10, 0)
print(result)

✔ pdb shows an exception
You can inspect a and b
Change values on the fly

8. Debugging Inside Loops 🔁

loop_debug.py

import pdb

for i in range(5):
    if i == 3:
        pdb.set_trace()
    print(i)

✔ Automatically pauses at specific iteration

9. Using post-mortem Debugging ⚠️

Debug code after it crashes.

post_mortem.py

try:
    x = 10 / 0
except Exception:
    import pdb
    pdb.post_mortem()

✔ Opens debugger at the line where exception occurred

10. Debugging with breakpoint() (Python 3.7+) 🚀

breakpoint_example.py

a = 5
breakpoint()  # same as pdb.set_trace()
print(a)

✔ Cleaner & recommended in modern Python

11. Non-Interactive Debugging — Logging Mode 📝

You can log debugging output without stopping execution:

trace_output.py

import pdb

pdb.traceback()

12. Debugging Tips & Tricks 💡

  • ✔ Use p locals() to see all variables
  • ✔ Use conditional breakpoints for complex bugs
  • ✔ Use until to skip long loops
  • ✔ Avoid adding too many set_trace() calls → use breakpoints
  • ✔ Use where to inspect call stack
  • ✔ Integrate pdb with VSCode/PyCharm for GUI debugging

Cheat Sheet 📘

CommandMeaning
lList code
nNext line
sStep into function
cContinue execution
bSet breakpoint
clClear breakpoints
p varPrint variable
wShow call stack
qQuit debugger

Conclusion 🎉

>>“Debugging is not a punishment — it’s the skill that turns good developers into great ones.” ✨

You now fully understand Debugging with pdb in Python! Want the next topic? Try Unit Testing, Traceback, Logging, or Profiling with cProfile. Just tell me! 😊