iPython debugger not providing any insight

Question

Let's say I have the following function:

def simple_func():
    a = 4
    b = 5
    c = a * b
    print c

Here's what I get when I run %debug simple_func():

NOTE: Enter 'c' at the ipdb>  prompt to continue execution.
None
> <string>(1)<module>()

ipdb>

If I enter n the debugger spits 20 back out at me and returns None.

This is a simplified version of what's happening across functions, interpreters, machines, etc. What's going on? Why can't I get any of my debuggers to do what I want, when all I need is to do some very simple line-by-line stepping through?

Answer 1

It doesn't look like debug works with a function that's simply defined in the ipython session. It needs to be imported from a file (that is, the --breakpoint parameter takes a file name and line).

If I create a file test.py

In [9]: cat test.py
def simple_func():
    a = 4
    b = 5
    c = a * b
    print(c)

I can do:

In [10]: import test

In [11]: %debug --breakpoint test.py:1 test.simple_func()
Breakpoint 1 at /home/paul/mypy/test.py:1
NOTE: Enter 'c' at the ipdb>  prompt to continue execution.
> /home/paul/mypy/test.py(2)simple_func()
1     1 def simple_func():
----> 2     a = 4
      3     b = 5
      4     c = a * b
      5     print(c)

ipdb> n
> /home/paul/mypy/test.py(3)simple_func()
1     1 def simple_func():
      2     a = 4
----> 3     b = 5
      4     c = a * b
      5     print(c)

ipdb> n
> /home/paul/mypy/test.py(4)simple_func()
      2     a = 4
      3     b = 5
----> 4     c = a * b
      5     print(c)
      6 

ipdb> a,b
(4, 5)
ipdb> n
> /home/paul/mypy/test.py(5)simple_func()
      2     a = 4
      3     b = 5
      4     c = a * b
----> 5     print(c)
      6 

ipdb> c
20
ipdb> c
20
ipdb> q

There may be other ways of using this, but this seems to be simplest, most straight forward one. I rarely use the debugger. Instead I test code snippets interactively in Ipython, and sprinkle my scripts with debugging prints.