What is "debugging"?

Debugging is

the methodical process of finding and eliminating bugs (defects) in a software programme.

Debugging Methods

• static code analysis

• (unit) testing

• code reviews

• lexical analysis

• interactive debugging

Interactive Debugging

all debugging, which involves a human, interacting with the computer (or its output).

Mostly used to track down known bugs.

Process

The process (after a bug was reported) usually goes as follows:

1. Developer tries to reproduce the bug
2. defines minimal set of conditions, environment, state and inputs to trigger the bug
3. setup conditions/trigger bug
4. analyse (try to find the bug)
5. make a change
6. rinse, repeat

Debugging Tools

• (post mortem) logs analysis

• code execution walk-through

• REPLs

• history bisecting

• in process debuggers

Log Anaylsis

Code Walk through

dry line-by-line execution of code in your head or on a piece of paper

L2: With items  = ["a", "b", "a"]

L3: found = {}

L4: FOR: key = "a"

     L5: key not in found -> skip to L7

     L7: found= {"a": None}

     L8: len(found) < 10 -> skip to L4

     .....

REPL

(Read-Eval-Print-Loop)

History Bisecting

`git bisect` to find the error

interactive Debugger

get inside your process

interactive Debugger

The base idea of an interactive debugger is to allow you to stop execution (using breakpoints) of your program at any time, jump into an interactive session and allow you to examine the state at that time and continue step by step to observe what is going on or resume execute.

FF Developer Tools

in-browser Javascript debugging

Python Debugger

import pdb; pdb.set_trace()

Pry+Byebug for Ruby

require 'pry'; binding.pry

NodeJS debugger

debugger; node debug script.js