Can I force debugging python on AssertionError?

Question

Suppose I have a python program where assert has been used to define how things should be, and I would like to capture anomalies with the read-eval-loop rather than having AssertionError be thrown.

Granted, I could have

if (reality!=expectation):
    print("assertion failed");
    import pdb; pdb.set_trace();

but that's far more ugly in the code than a plain assert(reality==expectation).

I could have pdb.set_trace() called in an except: block at top-level, but then I'd have lost all the context of the failure, right ? (I mean, stacktrace could be recovered from the exception object, but not argument values, etc.)

Is there anything like a --magic command-line flag that could turn the python3 interpreter into what I need ?

Answer 1

Mainly taken from this great snippet:

import sys

def info(type, value, tb):
    if hasattr(sys, 'ps1') or not sys.stderr.isatty() or type != AssertionError:
        # we are in interactive mode or we don't have a tty-like
        # device, so we call the default hook
        sys.__excepthook__(type, value, tb)
    else:
        import traceback, pdb
        # we are NOT in interactive mode, print the exception...
        traceback.print_exception(type, value, tb)
        print
        # ...then start the debugger in post-mortem mode.
        pdb.pm()

sys.excepthook = info

When you initialize your code with this, all AssertionErrors should invoke pdb.

Answer 2

Have a look at the nose project. You can use it with the --pdb option to drop into the debugger on errors.