pep3134 0.1.4

__traceback__ in Python 2

Tracebacks are very convenient data structure to work with but really irritating and magical if you want to deal with it using pure Python, without patching code or hacking interpreter internals. If you want to see some magic, please checkout, let’s say, Jinja sources. Armin is rather good but I am trying to escape magic if possible.

I cannot keep the same tracebacks to any exceptions even if I want because it requires to do some work on interpreter internals. But anyway this method will return you something.

The rule of thumb is: if it returns an object, it is the proper object you expect. If it returns None then no luck. Moreover: __traceback__ implemented as property so sometimes it raises traceback but afterwards it returns None on the same object. Unfortunately I do not know a good way how to deal with it.

But I can you give some guarantees:

1. __traceback__ on implicit (__context__) and explicit causes (__cause__) always correct.

2. __traceback__ in the associated except clause is always correct.

3. Sometimes it works in other cases but do not rely on that.

This works like this because of _fixed_ sys.exc_info() behavior. Let’s check one example.

import sys

def example():
    try:
        raise KeyError("WOW SUCH ERROR")
    except KeyError:
        first = sys.exc_info()

    second = sys.exc_info()
    return first, second

first, second = example()
assert first == second

It works as a charm in Python2 but raises AssertionError in Python3. So it is not possible to keep tracebacks in the same way in both Python2 and Python3. Sad story.

So if we will rewrite given example with PEP3134

import sys
import pep3134

def example():
    error = -1
    try:
        pep3134.raise_(KeyError("WOW SUCH ERROR"))
    except KeyError as err:
        error = err
        first = sys.exc_info()
        assert error.__traceback__ is first[2]

    second = sys.exc_info()
    assert error.__traceback__ is not second[2]  # works in Python 2 only

example()

This is the only pitfall. Causes, as I mentioned, work well.

PEP3134 library

This library gives you 3 functions you can use. Only 3 so no need to have full documentation on any external source.

pep3134.raise_

Works with the same signature as raise clause in both Python 2 and Python 3. Just a reminder:

raise exc_type, [exc_value, [exc_traceback]]

Raises exceptions on the same problems.

pep3134.reraise

Works in the same way as raise clause without any arguments does in Python 2.

pep3134.raise_from

Works absolutely in the same way as raise ... from ... clause does in Python 3.