raise_from does not behave exactly like raise from

[mbr]

Here's a bit of sample code from [https://www.python.org/dev/peps/pep-3134/](PEP 3134):

class DatabaseError(Exception):
    pass

class FileDatabase(object):
    def __init__(self, filename):
        try:
            self.file = open(filename)
        except IOError as exc:
            raise DatabaseError('failed to open') from exc

fd = FileDatabase('dne')

This, of course, does not work on Python 2. Using Python 3 I get the following output:

Traceback (most recent call last):
  File "pepex2.py", line 7, in __init__
    self.file = open(filename)
FileNotFoundError: [Errno 2] No such file or directory: 'dne'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "pepex2.py", line 11, in <module>
    fd = FileDatabase('dne')
  File "pepex2.py", line 9, in __init__
    raise DatabaseError('failed to open') from exc
__main__.DatabaseError: failed to open

When using raise_from for Python2 compatibility

from future.utils import raise_from
# [...]
            raise_from(DatabaseError('failed to open'), exc)

this output (still with Python3) changes to:

FileNotFoundError: [Errno 2] No such file or directory

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "pepex2.py", line 13, in <module>
    fd = FileDatabase('dne')
  File "pepex2.py", line 11, in __init__
    raise_from(DatabaseError('failed to open'), exc)
  File "/home/marc/.virtualenvs/scratch_future3/lib/python3.3/site-packages/future/utils/__init__.py", line 382, in raise_from
    exec(execstr, myglobals, mylocals)
  File "<string>", line 1, in <module>
__main__.DatabaseError: failed to open

Note that the traceback and exception information are missing from the FileNotFoundError.

[ankostis]

Just confirming that ver 0.15.0 still is buggy.

And the consequences are not neglible since the root of the error is lost.

[edschofield]

Thanks for the feedback, Kostis!

I think we've fixed now in v0.16.0 thanks to a new raise_from implementation on Py3. Could you please give it a whirl and let me know if it all looks good to you?

[ankostis]

My apologies, I cannot answer your questions - you see, lately have started moving away from py2/3 code towards py3-only code. The compatibility debt was too great to curry.

I'm very grateful for your efforts on this library.

[Stelminator]

I have a potentially related problem with raise_from, specifically the behavior of __context__ on python2:

native python3

try:
    try:
        try:
            raise Exception('foo')
        except Exception as foo:
            raise Exception('bar') from foo
    except Exception as bar:
        raise bar from Exception('quux')
except Exception as bar_quux:
    last = bar_quux

print(last.args[0] == 'bar')
print(last.__cause__.args[0] == 'quux')
print(last.__context__.args[0] == 'foo')
print(last.__context__.args[0])

output:

True
True
True
foo

using raise_from

from future.utils import raise_from
try:
    try:
        try:
            raise Exception('foo')
        except Exception as foo:
            raise_from(Exception('bar'), foo)
    except Exception as bar:
        raise_from(bar, Exception('quux'))
except Exception as bar_quux:
    last = bar_quux

print(last.args[0] == 'bar')
print(last.__cause__.args[0] == 'quux')
print(last.__context__.args[0] == 'foo')
print(last.__context__.args[0])

python3 output:

True
True
True
foo

python2 output:

True
True
False
bar

so basically, the __context__ is being set when it already has been. In this case the bar exception is being set as its own __context__ on python2.

I think I have an appropriate fix in: Stelminator@42cee74

which is just:

-        e.__context__ = sys.exc_info()[1]
+        if not hasattr(e, '__context__'):
+            e.__context__ = sys.exc_info()[1]

[Stelminator]

I suppose that's still not quite complete:

try:
    try:
        raise Exception('foo')
    except Exception as foo:
        try:
            raise Exception('bar') from foo
        except Exception as bar:
            print(bar.__context__)
            raise foo from bar
except Exception as foo:
    wat = foo
print(wat.__context__.__context__)

foo
None

#python2 with above fix
try:
    try:
        raise Exception('foo')
    except Exception as foo:
        try:
            raise_from(Exception('bar'), foo)
        except Exception as bar:
            print(bar.__context__)
            raise_from(foo, bar)
except Exception as foo:
    wat = foo
print(wat.__context__.__context__)

foo
foo

Edit: I this actually might be a bug in python3. I don't think __context__ is ever supposed to get reset to None (unless done explicitly).

[Stelminator]

crosslink to my report to python bug tracker: http://bugs.python.org/issue31213

[JoshRosen]

Cross-link to what looks (at first glance) to be a similar problem in six's Python 2 version of raise_from: benjaminp/six#193