Ipython Debugger

Running the ipython debugger (ipbd) from the command line

This is meant to be an interactive session. Follow the steps below:

• Download demo.py from: here This has been copied from the lib/python3.6/turtle.py, ie the main turtle module.
• Download a text editor like notepad++ or sublime and view the file
• Open a terminal and: pip install ipdb
• Navigate to the directory where demo.py is saved and launch the ipython debugger

python -m ipdb demo.py

> /Users/halkypi/Downloads/demo.py(2)<module>()
      1 # Demos taken from: lib/python3.6/turtle.py
----> 2 from turtle import *
      3 import time

Typing help or h will open the help topics. A number of commands have aliases, for example:

ipdb> h h

h(elp)
        Without argument, print the list of available commands.
        With a command name as argument, print help about that command.
        "help pdb" shows the full pdb documentation.
        "help exec" gives help on the ! command.

ipdb> h c

c(ont(inue))
        Continue execution, only stop when a breakpoint is encountered.

Commands

The help menu lists all the commands:

ipdb> h

Documented commands (type help <topic>):
========================================
EOF    cl         disable  interact  next    psource  rv         unt
a      clear      display  j         p       q        s          until
alias  commands   down     jump      pdef    quit     source     up
args   condition  enable   l         pdoc    r        step       w
b      cont       exit     list      pfile   restart  tbreak     whatis
break  continue   h        ll        pinfo   return   u          where
bt     d          help     longlist  pinfo2  retval   unalias
c      debug      ignore   n         pp      run      undisplay

The commands we need to execute the code are:

• Run the help or h command on l and ll then run them in the console.

ipdb> h l
ipdb> h ll
ipdb> l
ipdb> ll

• Enter c and watch the program continue to the end
• Enter run to restart the program
• Enter h b and read the break info

ipdb> h b

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
to true before the breakpoint is honored.

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.  

• Repeat this for enable, disable and clear
• Set a break point for demo2()
• continue to the break

• Enter n to execute the next line
• Press <Enter> or <Return> again to execute the last command
• Keep pressing enter and watch each line being executed until the for loop on line 72.
• Enter p _ to print the number stored in the _ value

The _ underscore is a throwaway variable because it doesn’t mean anything once the for loop has finished.

• Break out of the loop by entering until 75

Exploring methods

The program enters a while loop on line 76.

• Take a look at the help for pdoc and pinfo which provides information on a method
• Have a look at the docstrings for undobufferentries and undo then enter s to step into the function
• Once the function is called enter ll to view the lib/python3.6/turtle.py(2603)undobufferentries() source code

If we print the value we can see it is a buffer of all the entries in the undobuffer. The function returns 0 when the buffer reaches None, otherwise it returns the count of entries.

• Enter r so the undobufferentries() function returns and step into the undo() function
• Note the example uses a range(8) instead of the number of entries in the buffer

The while loop iterates through entries in the buffer and executes undo() on them until it hits None. The buffer then returns 0 or False which breaks the loop.

• Enter return to step out of undo and enter n to move to the next line. Hold down the <Enter> key to watch it undo() the buffer

Restart and explore

• restart the program
• clear the breakpoint and use n, s and r to step through the program and explore methods
• Browse turtle documentation and note examples come from the docs strings in the source code: https://docs.python.org/3.3/library/turtle.html
• Comment out demo2() in demo.py and run demo1()
• Step through peace.py in the turtledemo module

With these simple tools it is possible to look under the hood, explore code, run examples and become fluent in any python package. All it takes is a curious mind and some interactive exploration!

Next, we’ll add some data wrangling tools to the lab.