tblib 1.0.0

Traceback fiddling library.

Traceback fiddling library. For now allows you to pickle tracebacks and raise exceptions with pickled tracebacks in different processes. This allows better error handling when running code over multiple processes (imagine multiprocessing, billiard, futures, celery etc).

Requirements

OS:

Any

Runtime:

Python 2.6, 2.7, 3.2, 3.3, 3.4 and PyPy.

Pickling tracebacks

Note: The traceback objects that come out are stripped of some attributes (like variables). But you’ll be able to raise exceptions with those tracebacks or print them - that should cover 99% of the usecases.

>>> from tblib import pickling_support
>>> pickling_support.install()
>>> import pickle, sys
>>> def inner_0():
...     raise Exception('fail')
...
>>> def inner_1():
...     inner_0()
...
>>> def inner_2():
...     inner_1()
...
>>> try:
...     inner_2()
... except:
...     s1 = pickle.dumps(sys.exc_info())
...
>>> len(s1) > 1
True
>>> try:
...     inner_2()
... except:
...     s2 = pickle.dumps(sys.exc_info(), protocol=pickle.HIGHEST_PROTOCOL)
...
>>> len(s2) > 1
True

>>> try:
...     import cPickle
... except ImportError:
...     import pickle as cPickle
>>> try:
...     inner_2()
... except:
...     s3 = cPickle.dumps(sys.exc_info(), protocol=pickle.HIGHEST_PROTOCOL)
...
>>> len(s3) > 1
True

Unpickling

>>> pickle.loads(s1)
(<...Exception'>, Exception('fail',), <traceback object at ...>)

>>> pickle.loads(s2)
(<...Exception'>, Exception('fail',), <traceback object at ...>)

>>> pickle.loads(s3)
(<...Exception'>, Exception('fail',), <traceback object at ...>)

Raising

>>> from six import reraise
>>> reraise(*pickle.loads(s1))
Traceback (most recent call last):
  ...
  File "<doctest README.rst[14]>", line 1, in <module>
    reraise(*pickle.loads(s2))
  File "<doctest README.rst[8]>", line 2, in <module>
    inner_2()
  File "<doctest README.rst[5]>", line 2, in inner_2
    inner_1()
  File "<doctest README.rst[4]>", line 2, in inner_1
    inner_0()
  File "<doctest README.rst[3]>", line 2, in inner_0
    raise Exception('fail')
Exception: fail
>>> reraise(*pickle.loads(s2))
Traceback (most recent call last):
  ...
  File "<doctest README.rst[14]>", line 1, in <module>
    reraise(*pickle.loads(s2))
  File "<doctest README.rst[8]>", line 2, in <module>
    inner_2()
  File "<doctest README.rst[5]>", line 2, in inner_2
    inner_1()
  File "<doctest README.rst[4]>", line 2, in inner_1
    inner_0()
  File "<doctest README.rst[3]>", line 2, in inner_0
    raise Exception('fail')
Exception: fail
>>> reraise(*pickle.loads(s3))
Traceback (most recent call last):
  ...
  File "<doctest README.rst[14]>", line 1, in <module>
    reraise(*pickle.loads(s2))
  File "<doctest README.rst[8]>", line 2, in <module>
    inner_2()
  File "<doctest README.rst[5]>", line 2, in inner_2
    inner_1()
  File "<doctest README.rst[4]>", line 2, in inner_1
    inner_0()
  File "<doctest README.rst[3]>", line 2, in inner_0
    raise Exception('fail')
Exception: fail

What if we have a local stack, does it show correctly ?

Yes it does:

>>> exc_info = pickle.loads(s3)
>>> def local_0():
...     reraise(*exc_info)
...
>>> def local_1():
...     local_0()
...
>>> def local_2():
...     local_1()
...
>>> local_2()
Traceback (most recent call last):
  File "...doctest.py", line ..., in __run
    compileflags, 1) in test.globs
  File "<doctest README.rst[24]>", line 1, in <module>
    local_2()
  File "<doctest README.rst[23]>", line 2, in local_2
    local_1()
  File "<doctest README.rst[22]>", line 2, in local_1
    local_0()
  File "<doctest README.rst[21]>", line 2, in local_0
    reraise(*exc_info)
  File "<doctest README.rst[11]>", line 2, in <module>
    inner_2()
  File "<doctest README.rst[5]>", line 2, in inner_2
    inner_1()
  File "<doctest README.rst[4]>", line 2, in inner_1
    inner_0()
  File "<doctest README.rst[3]>", line 2, in inner_0
    raise Exception('fail')
Exception: fail

The tblib.Traceback object

It is used by the pickling_support. You can use it too if you want more flexibility:

>>> from tblib import Traceback
>>> try:
...     inner_2()
... except:
...     et, ev, tb = sys.exc_info()
...     tb = Traceback(tb)
...     reraise(et, ev, tb.as_traceback())
...
Traceback (most recent call last):
  ...
  File "<doctest README.rst[21]>", line 6, in <module>
    reraise(et, ev, tb.as_traceback())
  File "<doctest README.rst[21]>", line 2, in <module>
    inner_2()
  File "<doctest README.rst[5]>", line 2, in inner_2
    inner_1()
  File "<doctest README.rst[4]>", line 2, in inner_1
    inner_0()
  File "<doctest README.rst[3]>", line 2, in inner_0
    raise Exception('fail')
Exception: fail

You can use the to_dict method and the from_dict classmethod to convert a Traceback into and from a dictionary serializable by the stdlib json.JSONDecoder:

>>> import json
>>> from tblib import Traceback
>>> try:
...     inner_2()
... except:
...     et, ev, tb = sys.exc_info()
...     tb = Traceback(tb)
...     tb_json = json.dumps(tb.to_dict())
...     tb_2 = Traceback.from_dict(json.loads(tb_json))
...     reraise(et, ev, tb.as_traceback())
...
Traceback (most recent call last):
  ...
  File "<doctest README.rst[21]>", line 6, in <module>
    reraise(et, ev, tb.as_traceback())
  File "<doctest README.rst[21]>", line 2, in <module>
    inner_2()
  File "<doctest README.rst[5]>", line 2, in inner_2
    inner_1()
  File "<doctest README.rst[4]>", line 2, in inner_1
    inner_0()
  File "<doctest README.rst[3]>", line 2, in inner_0
    raise Exception('fail')
Exception: fail

Decorators

return_error

>>> from tblib.decorators import return_error
>>> inner_2r = return_error(inner_2)
>>> e = inner_2r()
>>> e
<tblib.decorators.Error object at ...>
>>> e.reraise()
Traceback (most recent call last):
  ...
  File "<doctest README.rst[26]>", line 1, in <module>
    e.reraise()
  File "...tblib...decorators.py", line 19, in reraise
    reraise(self.exc_type, self.exc_value, self.traceback)
  File "...tblib...decorators.py", line 25, in return_exceptions_wrapper
    return func(*args, **kwargs)
  File "<doctest README.rst[5]>", line 2, in inner_2
    inner_1()
  File "<doctest README.rst[4]>", line 2, in inner_1
    inner_0()
  File "<doctest README.rst[3]>", line 2, in inner_0
    raise Exception('fail')
Exception: fail

How’s this useful ? Imagine you’re using multiprocessing like this:

>>> import traceback
>>> from multiprocessing import Pool
>>> from examples import func_a
>>> if sys.version_info[:2] == (3, 4):
...     import multiprocessing.pool
...     # Undo the fix for http://bugs.python.org/issue13831 so that we can see the effects of our change.
...     # because Python 3.4 will show the remote traceback (but as a string sadly)
...     multiprocessing.pool.ExceptionWithTraceback = lambda e, t: e
>>> pool = Pool()
>>> try:
...     for i in pool.map(func_a, range(5)):
...         print(i)
... except:
...     print(traceback.format_exc())
...
Traceback (most recent call last):
  File "<doctest README.rst[...]>", line 2, in <module>
    for i in pool.map(func_a, range(5)):
  File "...multiprocessing...pool.py", line ..., in map
    ...
  File "...multiprocessing...pool.py", line ..., in get
    ...
Exception: Guessing time !
<BLANKLINE>
>>> pool.terminate()

Not very useful is it? Let’s sort this out:

>>> from tblib.decorators import apply_with_return_error, Error
>>> from itertools import repeat
>>> pool = Pool()
>>> try:
...     for i in pool.map(apply_with_return_error, zip(repeat(func_a), range(5))):
...         if isinstance(i, Error):
...             i.reraise()
...         else:
...             print(i)
... except:
...     print(traceback.format_exc())
...
Traceback (most recent call last):
  File "<doctest README.rst[...]>", line 4, in <module>
    i.reraise()
  File "...tblib...decorators.py", line ..., in reraise
    reraise(self.exc_type, self.exc_value, self.traceback)
  File "...tblib...decorators.py", line ..., in return_exceptions_wrapper
    return func(*args, **kwargs)
  File "...tblib...decorators.py", line ..., in apply_with_return_error
    return args[0](*args[1:])
  File "...examples.py", line 2, in func_a
    func_b()
  File "...examples.py", line 5, in func_b
    func_c()
  File "...examples.py", line 8, in func_c
    func_d()
  File "...examples.py", line 11, in func_d
    raise Exception("Guessing time !")
Exception: Guessing time !
<BLANKLINE>
>>> pool.terminate()

Much better !

What if we have a local call stack ?

>>> def local_0():
...     pool = Pool()
...     for i in pool.map(apply_with_return_error, zip(repeat(func_a), range(5))):
...         if isinstance(i, Error):
...             i.reraise()
...         else:
...             print(i)
...
>>> def local_1():
...     local_0()
...
>>> def local_2():
...     local_1()
...
>>> try:
...     local_2()
... except:
...     print(traceback.format_exc())
Traceback (most recent call last):
  File "<doctest README.rst[...]>", line 2, in <module>
    local_2()
  File "<doctest README.rst[...]>", line 2, in local_2
    local_1()
  File "<doctest README.rst[...]>", line 2, in local_1
    local_0()
  File "<doctest README.rst[...]>", line 5, in local_0
    i.reraise()
  File "...tblib...decorators.py", line 19, in reraise
    reraise(self.exc_type, self.exc_value, self.traceback)
  File "...tblib...decorators.py", line 25, in return_exceptions_wrapper
    return func(*args, **kwargs)
  File "...tblib...decorators.py", line 42, in apply_with_return_error
    return args[0](*args[1:])
  File "...tests...examples.py", line 2, in func_a
    func_b()
  File "...tests...examples.py", line 5, in func_b
    func_c()
  File "...tests...examples.py", line 8, in func_c
    func_d()
  File "...tests...examples.py", line 11, in func_d
    raise Exception("Guessing time !")
Exception: Guessing time !
<BLANKLINE>

Credits

• mitsuhiko/jinja2 for figuring a way to create traceback objects.