[Python-3000] PEP 344: attaching tracebacks to exceptions + pickle

[Collin Winter]

In 2.x, exceptions are pickle-able; tracebacks, however, are not. If
tracebacks are attached to exceptions in Python 3 (as specified in PEP
344), exceptions will no longer be pickle-able.

There are of course options:

1. Discard an exception's __traceback__ attribute when pickling.

2. Include __traceback__ in the pickle, but in a more limited form.
That is: the hard parts of exceptions (frames, code objects) aren't
fully needed to, say, print out a traceback. The following is all you
need:

class Code(object):
    def __init__(self, code):
        self.co_filename = code.co_filename
        self.co_name = code.co_name

class Frame(object):
    def __init__(self, frame):
        self.f_globals = {"__file__": frame.f_globals["__file__"]}
        self.f_code = Code(frame.f_code)

class Traceback(object):
    def __init__(self, tb):
        self.tb_frame = Frame(tb.tb_frame)
        self.tb_lineno = tb.tb_lineno
        if tb.tb_next is None:
            self.tb_next = None
        else:
            self.tb_next = Traceback(tb.tb_next)

When pickling an exception, __traceback__ could be replaced with the
limited Traceback implementation above.

3. Reject the proposal to attach tracebacks to exceptions. (This is my
least favorite.)

Thoughts?

Collin Winter