python pdb: resume code execution after exception caught?

Question

If I run a code with the ipython %pdb magic enabled and the code throws an exception, is there any way to tell the code to continue executing afterwards?

e.g., say the exception is a ValueError: x=0 not allowed. Can I, in pdb, set x=1 and allow the code to continue (resume) executing?

Answer 1

I don't think you can resume code post-mortem (i.e. the exception was actually raised, triggering invocation of the debugger). What you can do, is put breakpoints in your code where you have been seeing errors, and that allows you to change values, and continue the program avoiding the error.

Given a script myscript.py:

# myscript.py
from IPython.core.debugger import Tracer

# a callable to invoke the IPython debugger. debug_here() is like pdb.set_trace()
debug_here = Tracer()

def test():
    counter = 0
    while True:
        counter += 1
        if counter % 4 == 0:
             # invoke debugger here, so we can prevent the forbidden condition
            debug_here()
        if counter % 4 == 0:
            raise ValueError("forbidden counter: %s" % counter)

        print counter

test()

Which continually increments a counter, raising an error if it's ever divisible by 4. But we have edited it to drop into the debugger under the error condition, so we might be able to save ourselves.

Run this script from IPython:

In [5]: run myscript
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> /Users/minrk/dev/ip/mine/myscript.py(14)test()
     13             debug_here()
---> 14         if counter % 4 == 0:
     15             raise ValueError("forbidden counter: %s" % counter)

# increment counter to prevent the error from raising:
ipdb> counter += 1
# continue the program:
ipdb> continue
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> /Users/minrk/dev/ip/mine/myscript.py(13)test()
     12              # invoke debugger here, so we can prevent the forbidden condition

---> 13             debug_here()
     14         if counter % 4 == 0:

# if we just let it continue, the error will raise
ipdb> continue
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
IPython/utils/py3compat.pyc in execfile(fname, *where)
    173             else:
    174                 filename = fname
--> 175             __builtin__.execfile(filename, *where)

myscript.py in <module>()
     17         print counter
     18 
---> 19 test()

myscript.py in test()
     11         if counter % 4 == 0:
     12              # invoke debugger here, so we can prevent the forbidden condition

     13             debug_here()
     14         if counter % 4 == 0:
---> 15             raise ValueError("forbidden counter: %s" % counter)

ValueError: forbidden counter: 8

In [6]: