pep3134 0.1.4

Backport of PEP 3134 (with PEP 415 and PEP 409) to Python 2 as close as possible

This library is intended to give you an ability to use exception chaining and embedded tracebacks with both Python 2 and Python 3 (>= 3.3 only). Exception Chaining and Embedded Tracebacks are also well known as PEP3134 that’s why I have such geeky name for that library.

No, it is not. Geeky name is kinda PEP3134 (feat. PEP409, PEP415 Remix) but I think it is an overkill.

If you want to get more about exception chaining and tracebacks please refer to the documentation for Python 3 with modifications done in Python 3.3.

Short excerpt for those who still sit with Python 2 as me.

1. Exceptions have new attributes: __traceback__, __context__, __suppress_context__ and __cause__.

2. Exceptions have new syntax for explicit chaining: raise CustomError("Cannot read settings") from IOError("Cannot open /etc/settings").

3. Exceptions always have their own tracebacks attached in __traceback__ attribute.

4. If exception was raised without explicit cause, it has its own context (say, from sys.exc_info()) in __context__ attribute. In this case __cause__ keeps None.

5. If exception was raised by implicit cause, then __suppress_context__ is False.

6. If exception was raised with explicit cause (raise ... from ...) then __cause__ has a cause, __suppress_context__ is True and __context__ is (suddenly) None.

So this is pretty convenient to have chaining if you want to build human-readable error messages afterwards, right?

This library helps you to keep the same __context__, __cause__ and __suppress_context__ behavior with both Python 2 and Python 3.

I did not mentioned __traceback__. This is a reason

__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.