Main theme of the presentation

• errors / bugs
• debugging

Why?

• because most tutorials shows you how to write code that works
• in real life you spend more time dealing with bugs than writing a new code...

Outline

• Python Errors

• Traceback

• Python debugger

• Avoiding bugs and discussion

Format

• Presentation

• Live Coding

• Hands-on Exercises / Pair Coding

• Discussion

Feel free to interrupt me at any time!!

What is debugging?

• investigating why the program is not working properly
• fixing the issues

When does the program not work properly?

• returns an error
• returns results that differ from what you expected
• keep running and doesn't return anything

Errors in Python

• Syntax Errors

• Exception Errors

  • FileNotFoundError
  • AssertionError
  • ZeroDivisionError

Syntax Errors

Exception Errors

Python Traceback

• show a Python call stack at the point the exception was raised

Python call stack

pdb - the Python debugger

• interactive source code debugger

• part of the Python's standard library

• allows to stop and exammine running code

• two main ways of using pdb:

  • start the program with the debugger:
    python -m pdb code.py

  • stop the program and invoke the debugger:
    import pdb; pdb.set_trace() or breakpoint() (in Python 3.7+)

using the interactive debugger

after inoking the debugger using breakpoint() in the code:

• q - quit the debugger

• c(ontinue) - continue execution until the next breakpoint

pdb commands: continue and step

• n(ext) - continue execution until the next line in the current function

• s(tep) - execute the current line, steps inside a called function

• c(ontinue) - continue execution until the next breakpoint

• b <line number, name of function> - adding new breakpoint

• <enter> - repeating the last command

• <upper arrow> - search the commands history

pdb commands: explore the code and variables

• p - evaluate the expression and print its value

• pp - like p, but uses the pprint module

• l(ist) - list source code around the current line

• ll - list all source code for the current function or frame

• a(rgs) - print the argument list of the current function

pdb commands: check the stack trace

• w(here) - print a stack trace, with the most recent frame at the bottom

• u(p) - move the current frame one level up

• d(own) - move the current frame one level down

Check full description of up / down and more commands here

pdb: variables listing

• locals() - display all the local variables

• globals() - display all the global variables

• display var (pdb command) - track the changes of variable var

pdb: additional checks

• Python functions

  • dir(), dir(obj)
  • type(obj)

• inspect module

  • provides functions to get information about live objects such as modules, classes, functions, tracebacks, etc.
  • inspect.getdoc - getting the documentation
  • inspect.signature - checking function signature

Can pdb be even better?

• pdb++

  • a drop-in replacement for pdb

  • adds new features to make debugging experience nicer

    • syntax highlighting of code
    • TAB completion of Python expressions

  • can be installed with pip: pip install pdbpp

• check debugging option in your favorite IDE

How to avoid bugs?

• don't change too many things at the same time

• use Version Control System (e.g. Git) to track your changes

• write good test suits and use Continuous Integration

• test your code with various versions of python and libraries

• use try / except in your code to handle possible errors

• try to write clean code and follow Python convention (PEP8), use pycodestyle, black, etc., to check and fix your style

Final thoughts on debugging

• try to understand the root of your problems

  • e.g. perhaps the directory that is passed to your function should never be empty? perhaps you have some connection issues?

• don't change unrelated things when debugging

• you should combine debugging with reading the code

• debugging can take a lot of time - take breaks, but make notes

• try to add tests that cover the issue next time

Additional Resources

• RealPython: Python debugging with PDB

• Adventures in Python Debugging