How to change the message in an exception to add context information.

add_ex_context.py

def process(val):
    try:
        do_something(val)
    except Exception as ex:
        msg = '{}: {}'.format(val, ex.args[0]) if ex.args else str(val)
        ex.args = (msg,) + ex.args[1:]
        raise

See http://stackoverflow.com/questions/17677680/how-can-i-add-context-to-an-exception-in-python