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edk's notes

ignoring exceptions in Python

Sometimes Python programs have unhandled exceptions. Usually these make your program exit, but wouldn't it be nice if you could fix your environment in a Python shell and try again? I thought so, so I've written a very silly exception hook which gives you the option to retry the current line, or skip past it and continue executing from the next one.

It's implemented using a bytecode-patcher. Anyway, for an example, given this broken program:

def do_something(foo):
    z = 1 + 2 +3
    print '1 + 2 + 3 = %d' % z

    return foo + bar

x = do_something(17)
print x

After the NameError is raised when we try to access bar, we see this prompt:

$ python fail.py
1 + 2 + 3 = 6
[reckless] intercepting an exception:
Traceback (most recent call last):
  File "fail.py", line 8, in <module>
    x = do_something(17)
  File "fail.py", line 6, in do_something
    return foo + bar
NameError: global name 'bar' is not defined
 c : continue execution (insane)
 d : debugger (bpdb)
 e : exit
[n]: do nothing
 r : retry (insane)
 s : shell (bpython)
what now?

We can enter s and fix the environment by defining bar:

>>> bar = 9

^D to exit the shell, and then enter r at the prompt:

[reckless] resuming execution
[reckless] reached end of program

It's on PyPI. If you're interested in the gory details of the bytecode patching, I suggest you download it and have a look at execute.py (it's not pretty).