tblib 0.1

Traceback fiddling library.

Traceback fiddling library.

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

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

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
>>> 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[31]>", line 2, in <module>
    for i in pool.map(func_a, range(5)):
  File "/usr/lib/.../multiprocessing/pool.py", line ..., in map
    return self.map_async(func, iterable, chunksize).get()
  File "/usr/lib/.../multiprocessing/pool.py", line ..., in get
    raise self._value
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[36]>", 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()