Amelio Vazquez-Reina Amelio Vazquez-Reina - 6 months ago 69
Python Question

Step-by-step debugging with IPython

From what I have read, there are two ways to debug code in Python:


  • With a traditional debugger such as
    pdb
    or
    ipdb
    . This supports commands such as
    c
    for
    continue
    ,
    n
    for
    step-over
    ,
    s
    for
    step-into
    etc.), but you don't have direct access to an IPython shell which can be extremely useful for object inspection.

  • Using IPython by embedding an IPython shell in your code. You can do
    from ipython import embed
    , and then use
    embed()
    in your code. When your program/script hits an
    embed()
    statement, you are dropped into an IPython shell. This allows the full inspection of objects and testing of Python code using all the IPython goodies. However, when using
    embed()
    you can't step-by-step through the code anymore with handy keyboard shortcuts.



Is there any way to combine the best of both worlds? I.e.


  1. Be able to step-by-step through your code with handy pdb/ipdb keyboard shortcuts.

  2. At any such step (e.g. on a given statement), have access to a full-fledged IPython shell.



IPython debugging as in MATLAB:



An example of this type of "enhanced debugging" can be found in MATLAB, where the user always has full access to the MATLAB engine/shell, and she can still step-by-step through her code, define conditional breakpoints, etc. From what I have discussed with other users, this is the debugging feature that people miss the most when moving from MATLAB to IPython.

IPython debugging in Emacs and other editors:



I don't want to make the question too specific, but I work mostly in Emacs, so I wonder if there is any way to bring this functionality into it. Ideally, Emacs (or the editor) would allow the programmer to set breakpoints anywhere on the code and communicate with the interpreter or debugger to have it stop in the location of your choice, and bring to a full IPython interpreter on that location.

Answer

(Update on May 28, 2016) Using RealGUD in Emacs

For anyone in Emacs, this thread shows how to accomplish everything described in the OP (and more) using

  1. a new important debugger in Emacs called RealGUD which can operate with any debugger (including ipdb).
  2. The Emacs package isend-mode.

The combination of these two packages is extremely powerful and allows one to recreate exactly the behavior described in the OP and do even more.


Original answer:

After having tried many different methods for debugging Python, including everything mentioned in this thread, one of my preferred ways of debugging Python with IPython is with embedded shells.

Defining a custom embedded IPython shell:

Add the following on a script to your PYTHONPATH, so that the method ipsh() becomes available.

import inspect

# First import the embed function
from IPython.terminal.embed import InteractiveShellEmbed
from IPython.config.loader import Config

# Configure the prompt so that I know I am in a nested (embedded) shell
cfg = Config()
prompt_config = cfg.PromptManager
prompt_config.in_template = 'N.In <\\#>: '
prompt_config.in2_template = '   .\\D.: '
prompt_config.out_template = 'N.Out<\\#>: '

# Messages displayed when I drop into and exit the shell.
banner_msg = ("\n**Nested Interpreter:\n"
"Hit Ctrl-D to exit interpreter and continue program.\n"
"Note that if you use %kill_embedded, you can fully deactivate\n"
"This embedded instance so it will never turn on again")   
exit_msg = '**Leaving Nested interpreter'

# Wrap it in a function that gives me more context:
def ipsh():
    ipshell = InteractiveShellEmbed(config=cfg, banner1=banner_msg, exit_msg=exit_msg)

    frame = inspect.currentframe().f_back
    msg   = 'Stopped at {0.f_code.co_filename} at line {0.f_lineno}'.format(frame)

    # Go back one level! 
    # This is needed because the call to ipshell is inside the function ipsh()
    ipshell(msg,stack_depth=2)

Then, whenever I want to debug something in my code, I place ipsh() right at the location where I need to do object inspection, etc. For example, say I want to debug my_function below

Using it:

def my_function(b):
  a = b
  ipsh() # <- This will embed a full-fledged IPython interpreter
  a = 4

and then I invoke my_function(2) in one of the following ways:

  1. Either by running a Python program that invokes this function from a Unix shell
  2. Or by invoking it directly from IPython

Regardless of how I invoke it, the interpreter stops at the line that says ipsh(). Once you are done, you can do Ctrl-D and Python will resume execution (with any variable updates that you made). Note that, if you run the code from a regular IPython the IPython shell (case 2 above), the new IPython shell will be nested inside the one from which you invoked it, which is perfectly fine, but it's good to be aware of. Eitherway, once the interpreter stops on the location of ipsh, I can inspect the value of a (which be 2), see what functions and objects are defined, etc.

The problem:

The solution above can be used to have Python stop anywhere you want in your code, and then drop you into a fully-fledged IPython interpreter. Unfortunately it does not let you add or remove breakpoints once you invoke the script, which is highly frustrating. In my opinion, this is the only thing that is preventing IPython from becoming a great debugging tool for Python.

The best you can do for now:

A workaround is to place ipsh() a priori at the different locations where you want the Python interpreter to launch an IPython shell (i.e. a breakpoint). You can then "jump" between different pre-defined, hard-coded "breakpoints" with Ctrl-D, which would exit the current embedded IPython shell and stop again whenever the interpreter hits the next call to ipsh().

If you go this route, one way to exit "debugging mode" and ignore all subsequent breakpoints, is to use ipshell.dummy_mode = True which will make Python ignore any subsequent instantiations of the ipshell object that we created above.