print statements

You can trace your code by putting a print statements in the problematic places

PROs:

Easy to use

CONs:

You have to remove them in production ready code

Too much useless typing

Using pdb - The Python Debugger

PROs:

you can use it on remote servers running Python

CONs:

requires time to master

IDEs

PyCharm Debugger offers an easy to use, sophisticated debugger.

More on Debugging Python Code @jetbrains.com

VS Code also offers a very good visual debugger, capable of setting breakpoints, inspect data, and use the debug console as you run your program step by step. Debug a number of different type of Python applications, including multi-threaded, web, and remote applications.

Overview

pdb is a built-in module, that defines an interactive source code debugger for Python programs.

It supports setting (conditional) breakpoints and single stepping at the source line level, inspection of stack frames, source code listing, and evaluation of arbitrary Python code in the context of any stack frame. It also supports post-mortem debugging and can be called under program control.

pdb - enter debug mode

To enter into the debug mode from a normally running program, you can insert:

					import pdb; pdb.set_trace()
				

at the location you want to break into the debugger. This is useful to hard-code a breakpoint at a given point in a program

pdb.py can also be invoked as a script to debug other scripts. For example:

					python3 -m pdb myscript.py
				

Example

			def sum_numbers(num_range):
			  suma = 0
			  for i in num_range:
			    suma =+ i
			    #enter debug mode and set a breakpoint
			    import pdb; pdb.set_trace()

			  return suma

			def main():
			  print( sum_numbers(range(1,10)) )

			if __name__ == '__main__':
			  main()
		

			$ python3 debug_demo.py
			> /tmp/examples/debug_demo.py(3)sum_numbers()
			-> for i in num_range:
			(Pdb)
		

Basic Commands

Basic Commands

Start debug mode and get help - example

			$ python3 -m pdb debug_demo.py
			(Pdb) ?

			Documented commands (type help <topic>):
			========================================
			EOF    c          d        h         list      q        rv       undisplay
			a      cl         debug    help      ll        quit     s        unt
			alias  clear      disable  ignore    longlist  r        source   until
			args   commands   display  interact  n         restart  step     up
			b      condition  down     j         next      return   tbreak   w
			break  cont       enable   jump      p         retval   u        whatis
			bt     continue   exit     l         pp        run      unalias  where

			Miscellaneous help topics:
			==========================
			pdb  exec
		

help for given command - example

			(Pdb) help break
			b(reak) [ ([filename:]lineno | function) [, condition] ]
			        Without argument, list all breaks.

			        With a line number argument, set a break at this line in the
			        current file.  With a function name, set a break at the first
			        executable line of that function.  If a second argument is
			        present, it is a string specifying an expression which must
			        evaluate to true before the breakpoint is honored.

			        The line number may be prefixed with a filename and a colon,
			        to specify a breakpoint in another file (probably one that
			        hasn't been loaded yet).  The file is searched for on
			        sys.path; the .py suffix may be omitted.
			(Pdb)
		

long list - example

			(Pdb) ll
			  1  ->	def sum_numbers(num_range):
			  2  	  suma = 0
			  3  	  for i in num_range:
			  4  	    suma =+ i
			  5  	    # import pdb; pdb.set_trace()
			  6
			  7  	  return suma
			  8
			  9  	def main():
			 10  	  print( sum_numbers(range(1,10)) )
			 11
			 12  	if __name__ == '__main__':
			 13  	  main()
			(Pdb)
		

Set breakpoint on given line number - example

			(Pdb) b 4
			Breakpoint 1 at /tmp/examples/debug_demo.py:4
			(Pdb)
		

Continue execution to next breakpoint - example

			(Pdb) c
			> /tmp/examples/debug_demo.py(4)sum_numbers()
			-> suma =+ i
			(Pdb)
		

View current value of a variable:

			(Pdb) p suma
			3
			(Pdb) p suma, i
			(3, 4)
		

Specify a list of commands for breakpoint - example

			# list breakpoints:
			(Pdb) b
			Num Type         Disp Enb   Where
			1   breakpoint   keep yes   at /tmp/examples/debug_demo.py:4

			#set commands to be executed on breakpoint 1:
			(Pdb) commands 1
			(com) p suma, i
			(com) end

			# continue to breakpoint and look that commands was executed
			(Pdb) c
			(0, 1)
		

Resources

Become a pdb power-user @medium

pdb @python3 docs

Resources

Overview

The Python extension supports debugging of a number of types of Python applications, including the following general capabilities

Watch window

Evaluating expressions

Locals

Arguments

Expanding children

Breakpoints

Conditional breakpoints

Pausing (breaking into) running programs

Custom startup directory

Start Debugger

You can start VS Code debugger by:

Pressing F5

Navigate to Debug Panel by clicking the icon , and from the top of Debug Panel click on Start Debugging

Debug Configuration

When you start VS Code Debugger for the first time, it will prompt you to select a configuration

Debug Configuration

To debug a simple app you can just select Python Environment from the Debug Prompt:

Debug Configuration

But creating a launch configuration file is beneficial because it allows you to configure and save debugging setup details.

VS Code keeps debugging configuration information in a launch.json file located in a .vscode folder in your workspace (project root folder) or in your user settings or workspace settings.

Debug Configuration

To create a launch.json file, open your project folder in VS Code (File > Open Folder) and then click on the Configure gear icon on the Debug view top bar.

Debug Configuration

The VS Code will generate a common launch.json file for you, which you can easy modify or extend.

Debug Configuration

More info for launch.json

Launch configurations @code.visualstudio.com

Debug actions

Command	Keyboard Shortcut
Continue / Pause	F5
Step Over	F10
Step Into	F11
Step Out	Shift+F11
Restart	Ctrl+Shift+F5
Stop	Shift+F5

Breakpoints

Breakpoints can be toggled by clicking on the editor margin

Finer breakpoint control (enable/disable/reapply) can be done in the Debug view's BREAKPOINTS section

Breakpoints in the editor margin are normally shown as red filled circles.

Disabled breakpoints have a filled gray circle.

More "Break" points

Clicking with right mouse button on the editor margin allows to add "Normal Breakpoint", "Conditional Breakpoint", "Log Point"

Data inspection

Variables can be inspected in the VARIABLES section of the Debug view or by hovering over their source in the editor.

Variables and expression evaluation is relative to the selected stack frame in the CALL STACK section.

Data inspection

Variable's context menu

Right clicking on a variable in the VARIABLES sections allows to

Set/Copy Variable Value, Add it to WATCH

Conditional breakpoints

Conditional Breakpoints can be set by

Expression condition - breakpoint will be hit whenever the expression evaluates to true

Hit count - controls how many times a breakpoint needs to be hit before it will 'break' execution.

Remote Debugging

VS Code allows remote debugging

Remote debugging @code.visualstudio.com

Remote Debugging Steps

pipenv install ptvsd==3.0.0

Resources