0408 | Debugging

Debugging Theory

General

• Accept that bugs happen all the time
• Process of finding, isolating and fixing bugs
  • Something is not working as expected, how do I fix it?
• Fixing syntax errors is a form of debugging
• print("here"), print("here 2"), print("here 3") is a form of debugging too
• Breakpoints in a debugger help for larger, complex programs

Breakpoints

• freeze or break an application at a certain point
  • Inspect or change variables, memory or registers
• Conditional breakpoints only break when condition is met
• Breakpoints help identify problem sources and track down bugs
• Testing identifies problems, debugging fixes problems
  • SQLi is a bug, which needs to be found and fixed

Terminology

• Static analysis | read the program without executing
• Source code analysis | automatically check for known bugs
• Tracing or Print Debugging | print("1"), print("2"), print("3")
• Remote Debugging | debug from a different remote system
• Post-mortem Debugging | debug based on crash and error logs

Notes

• Debugging is hard and tracing root causes can be complex
  • Can be easy to find the effect, but not the cause
  • Can be hard to reproduce the bug or issue
  • Interdepencies can cause a 'fix' to result in a new bug
  • Can take longer to debug than to create a bug
• Print can be useful strategy, but there are limitations

Debugging a Scrip

• pdb | Python Debugger

  • comes with the standard python distribution
  • no need to install anything else

• demo-example | pdb_demo.py | (play around with commenting/uncommenting as you see fit)

  print("-"*80)
  ## --------------- Loading PDB --------------- ##
  
  breakpoint()
  
  test = "test"
  print(test)
  
  
  # NOTE -- by default, python will import pdb and call pdb.set_trace()
  # -- set_trace hardcodes a breakpoint where the method was called
  
  # # adding a breakpoint
  # # NOTE -- runnig it -- once you run it -- breakpoint will be hit
  # breakpoint()
  
  # NOTE -- loading the debugger in a different way
  # -- use `python -m pdb pdb_demo.py` 
  # -- will load the debugger before even starting the program  
  
  
  ## --------------- PDB CONTROL commands --------------- ##
  
  # PDB CONTROL commands
  # -- you will have access to the program within the debugger
  
  # PDB CONTROL commands -- exiting
  # -- use `quit` or `exit` to exit it
  # PDB CONTROL commands -- restarting the program
  # -- use `run` to restart the program at any time
  
  # PDB CONTROL commands -- continue execution
  # -- use `c` / `cont` / `continue` to continue execution 
  # -- (till the next breakpoint is found)
  
  # PDB CONTROL commands -- stepping through the code -- next
  # -- stepping through the code -- `n` or `next` 
  # -- until the next line and stays within the current function
  
  # PDB CONTROL commands -- stepping through the code -- step
  # -- stepping through the code -- `s` or `step` -- (step into) 
  # -- execute the current line and will stop an a foreign function if one is called
  
  # PDB CONTROL commands -- help
  # -- use `help` to display all the documented commands in pdb
  # -- getting more info about a command -- `help c`
  
  # PDB CONTROL commands -- breakpoint
  # -- use `b` or `break` to list all the break points
  # -- with a line number argument --> set a break at the line
  # -- `b 3`
  
  # PDB CONTROL commands -- list the source code of the current file
  # -- use `l` or `list`
  # -- viewing only the firs 4 lines -- `l 1,4`
  # -- use `ll` or `longlist` to list the source code for the whole file
  
  # PDB CONTROL commands -- delete/disable a breakpoint
  # -- use `disable` with the bpnumber -- `disable 1`
  
  
  ## --------------- DEBUGGING with PDB --------------- ##
  
  # breakpoint()
  # once run -- use `p test` to view the value of the variable test at this point of execution
  # -- or `print(test)` -- you are passing an expression to be evaluated to python
  # -- or modifying the variable -- `test="123"` and `print(test)`
  # -- or introduce a new variable -- `new="456"` and `print(new)`
  
  # NOTE -- breakpoint numbers
  # -- whenever you add a breakpoint, the debugger assings a number to it
  # -- starting with 1 and incrementing for each additional breakpoint which is added
  
  # NOTE -- jumping during execution
  # -- change the flow of the program during run time
  # -- `j` or `jump` to skip or jump to certain areas within your code
  # -- like to jump to line 6 -- `j 6`
  # CAREFUL -- you can not jump into a function before the arguments are defined
  # -- or into the middle of the try-except block
  
  
  ## --------------- DEBUGGING with PDB -- loop example --------------- ##
  for i in range(10):
      print(i)
  
  
  print("-"*80)
  ## --------------- END --------------- ##