Learn how to use Python's built-in debugger

1. Debugging with PDB
Chrissy Wainwright
Debugging with PDB - Chrissy Wainwright

2. Who Am I?
Director of Engineering for Six Feet Up
Vice President of the Plone Foundation
Co-organizer of FayettePy
Pythonista
Debugging with PDB - Chrissy Wainwright

3. What is PDB?
Python Debugger
Interactive debugging tool
Debugging with PDB - Chrissy Wainwright

4. How to invoke it?
Set a trace directly in the code:
import pdb; pdb.set_trace()
# or in Python 3.7+
breakpoint()
Or start a script with pdb directly:
$ python -m pdb example.py
It is that easy!
Debugging with PDB - Chrissy Wainwright

5. Now what?
Common commands
Navigating the stack
Debugging with PDB - Chrissy Wainwright

6. Help
Type help or ? for short to know what
commands are available
Further help on commands help help or help
list
Debugging with PDB - Chrissy Wainwright

7. Show the current
context
Type list or l to see the current position in
the code
Type longlist or ll to see the whole function
Debugging with PDB - Chrissy Wainwright

8. Go to the next line
Type next or n to go to the next line
Debugging with PDB - Chrissy Wainwright

9. Examine variables
Type the variable name, hit enter!
Or type print or p to see what a variable is set
to
Use pp to pretty print the result for easier
reading
Debugging with PDB - Chrissy Wainwright

10. Examine available
variables
dir(variable)
args
dir()
Debugging with PDB - Chrissy Wainwright

11. Repeat the last
command
Just type return
Debugging with PDB - Chrissy Wainwright

12. Continue execution
Type continue or c to go to the next
breakpoint or finish execution
Debugging with PDB - Chrissy Wainwright

13. Step into a function
Type step or s to step into a function for
further inspection
Debugging with PDB - Chrissy Wainwright

14. Skip to the end of a
function
Type return or r to go to the end of a function
Debugging with PDB - Chrissy Wainwright

15. Getting out
Type quit or q to exit pdb and stop execution
Debugging with PDB - Chrissy Wainwright

16. Breakpoints
Type break or b to see a list of breakpoints
Then b and a line number to set one
To clear out a breakpoint clear or cl and a
breakpoint number
Debugging with PDB - Chrissy Wainwright

17. Conditional
breakpoints
Breakpoints can take an expression to know
when to break:
b 89, t_billable == 't'
Debugging with PDB - Chrissy Wainwright

18. See where you are in
the stack
Type where or w to see the current stack
You can debug at any point in the stack
Debugging with PDB - Chrissy Wainwright

19. Traverse the stack
Type up or u to go up the stack
Type down or d to go back down the stack
Debugging with PDB - Chrissy Wainwright

20. Interact
Type interact
Allows for functions, multiple lines
Ctrl + D to quit
Debugging with PDB - Chrissy Wainwright

21. Other options
Work on top of PDB to offer better interfaces,
tab completion, etc.
pdb++ (pdbpp)
pudb
bpdb (bpython)
Debugging with PDB - Chrissy Wainwright

22. Links
Python Module of the Week: PDB
In Depth PDB - Nathan Yergler
Debugging with PDB - Chrissy Wainwright