To help you get started, we've collected the most common ways that tblib is being used within popular public projects.

browsertron / pytest-parallel / pytest_parallel / __init__.py View on Github

def run(self):
        pickling_support.install()
        while True:
            try:
                index = self.queue.get()
                if index == 'stop':
                    self.queue.task_done()
                    break
            except ConnectionRefusedError:
                time.sleep(.1)
                continue
            item = self.session.items[index]
            try:
                run_test(self.session, item, None)
            except BaseException:
                import pickle
                import sys

View more ways to use tblib

Overview

Serialization library for Exceptions and Tracebacks.

• Free software: BSD license

It 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).
• Create traceback objects from strings (the from_string method). No pickling is used.
• Serialize tracebacks to/from plain dicts (the from_dict and to_dict methods). No pickling is used.
• Raise the tracebacks created from the aforementioned sources.
• Pickle an Exception together with its traceback and exception chain (raise ... from ...) (Python 3 only)

Again, note that using the pickle support is completely optional. You are solely responsible for security problems should you decide to use the pickle support.

Installation

pip install tblib

Documentation

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 tracebacks

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

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

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

Raising

>>> from six import reraise
>>> reraise(*pickle.loads(s1))
Traceback (most recent call last):
  ...
  File "", line 1, in 
    reraise(*pickle.loads(s2))
  File "", line 2, in 
    inner_2()
  File "", line 2, in inner_2
    inner_1()
  File "", line 2, in inner_1
    inner_0()
  File "", line 2, in inner_0
    raise Exception('fail')
Exception: fail
>>> reraise(*pickle.loads(s2))
Traceback (most recent call last):
  ...
  File "", line 1, in 
    reraise(*pickle.loads(s2))
  File "", line 2, in 
    inner_2()
  File "", line 2, in inner_2
    inner_1()
  File "", line 2, in inner_1
    inner_0()
  File "", line 2, in inner_0
    raise Exception('fail')
Exception: fail
>>> reraise(*pickle.loads(s3))
Traceback (most recent call last):
  ...
  File "", line 1, in 
    reraise(*pickle.loads(s2))
  File "", line 2, in 
    inner_2()
  File "", line 2, in inner_2
    inner_1()
  File "", line 2, in inner_1
    inner_0()
  File "", line 2, in inner_0
    raise Exception('fail')
Exception: fail

Pickling Exceptions together with their traceback and chain (Python 3 only)

>>> try:  # doctest: +SKIP
...     try:
...         1 / 0
...     except Exception as e:
...         raise Exception("foo") from e
... except Exception as e:
...     s = pickle.dumps(e)
>>> raise pickle.loads(s)  # doctest: +SKIP
Traceback (most recent call last):
  File "", line 3, in 
    1 / 0
ZeroDivisionError: division by zero

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

Traceback (most recent call last):
  File "", line 1, in 
    raise pickle.loads(s)
  File "", line 5, in 
    raise Exception("foo") from e
Exception: foo

BaseException subclasses defined after calling pickling_support.install() will not retain their traceback and exception chain pickling. To cover custom Exceptions, there are three options:

1. Use @pickling_support.install as a decorator for each custom Exception

   > python > >>> from tblib import pickling_support > >>> # Declare all imports of your package's dependencies > >>> import numpy # doctest: +SKIP > > >>> pickling_support.install() # install for all modules imported so far > > >>> @pickling_support.install > ... class CustomError(Exception): > ... pass >

   Eventual subclasses of CustomError will need to be decorated again.

2. Invoke pickling_support.install() after all modules have been imported and all Exception subclasses have been declared

   > python > >>> # Declare all imports of your package's dependencies > >>> import numpy # doctest: +SKIP > >>> from tblib import pickling_support > > >>> # Declare your own custom Exceptions > >>> class CustomError(Exception): > ... pass > > >>> # Finally, install tblib > >>> pickling_support.install() >

3. Selectively install tblib for Exception instances just before they are pickled

   > python > pickling_support.install(, [Exception instance], ...) >

   The above will install tblib pickling for all listed exceptions as well as any other exceptions in their exception chains.

   For example, one could write a wrapper to be used with ProcessPoolExecutor, Dask.distributed, or similar libraries:

   >>> from tblib import pickling_support >>> def wrapper(func, *args, **kwargs): ... try: ... return func(*args, **kwargs) ... except Exception as e: ... pickling_support.install(e) ... raise

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 "", line 1, in 
    local_2()
  File "", line 2, in local_2
    local_1()
  File "", line 2, in local_1
    local_0()
  File "", line 2, in local_0
    reraise(*exc_info)
  File "", line 2, in 
    inner_2()
  File "", line 2, in inner_2
    inner_1()
  File "", line 2, in inner_1
    inner_0()
  File "", line 2, in inner_0
    raise Exception('fail')
Exception: fail

It also supports more contrived scenarios

Like tracebacks with syntax errors:

>>> from tblib import Traceback
>>> from examples import bad_syntax
>>> try:
...     bad_syntax()
... except:
...     et, ev, tb = sys.exc_info()
...     tb = Traceback(tb)
...
>>> reraise(et, ev, tb.as_traceback())
Traceback (most recent call last):
  ...
  File "", line 1, in 
    reraise(et, ev, tb.as_traceback())
  File "", line 2, in 
    bad_syntax()
  File "...tests...examples.py", line 18, in bad_syntax
    import badsyntax
  File "...tests...badsyntax.py", line 5
    is very bad
     ^
SyntaxError: invalid syntax

Or other import failures:

>>> from examples import bad_module
>>> try:
...     bad_module()
... except:
...     et, ev, tb = sys.exc_info()
...     tb = Traceback(tb)
...
>>> reraise(et, ev, tb.as_traceback())
Traceback (most recent call last):
  ...
  File "", line 1, in 
    reraise(et, ev, tb.as_traceback())
  File "", line 2, in 
    bad_module()
  File "...tests...examples.py", line 23, in bad_module
    import badmodule
  File "...tests...badmodule.py", line 3, in 
    raise Exception("boom!")
Exception: boom!

Or a traceback that's caused by exceeding the recursion limit (here we're forcing the type and value to have consistency across platforms):

>>> def f(): f()
>>> try:
...    f()
... except RuntimeError:
...    et, ev, tb = sys.exc_info()
...    tb = Traceback(tb)
...
>>> reraise(RuntimeError, RuntimeError("maximum recursion depth exceeded"), tb.as_traceback())
Traceback (most recent call last):
  ...
  File "", line 1, in f
    def f(): f()
  File "", line 1, in f
    def f(): f()
  File "", line 1, in f
    def f(): f()
  ...
RuntimeError: maximum recursion depth exceeded

Reference

tblib.Traceback

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 "", line 6, in 
    reraise(et, ev, tb.as_traceback())
  File "", line 2, in 
    inner_2()
  File "", line 2, in inner_2
    inner_1()
  File "", line 2, in inner_1
    inner_0()
  File "", line 2, in inner_0
    raise Exception('fail')
Exception: fail

tblib.Traceback.to_dict

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 pprint import pprint
>>> try:
...     inner_2()
... except:
...     et, ev, tb = sys.exc_info()
...     tb = Traceback(tb)
...     tb_dict = tb.to_dict()
...     pprint(tb_dict)
{'tb_frame': {'f_code': {'co_filename': '',
                         'co_name': ''},
              'f_globals': {'__name__': '__main__'},
              'f_lineno': 5},
 'tb_lineno': 2,
 'tb_next': {'tb_frame': {'f_code': {'co_filename': ...,
                                     'co_name': 'inner_2'},
                          'f_globals': {'__name__': '__main__'},
                          'f_lineno': 2},
             'tb_lineno': 2,
             'tb_next': {'tb_frame': {'f_code': {'co_filename': ...,
                                                 'co_name': 'inner_1'},
                                      'f_globals': {'__name__': '__main__'},
                                      'f_lineno': 2},
                         'tb_lineno': 2,
                         'tb_next': {'tb_frame': {'f_code': {'co_filename': ...,
                                                             'co_name': 'inner_0'},
                                                  'f_globals': {'__name__': '__main__'},
                                                  'f_lineno': 2},
                                     'tb_lineno': 2,
                                     'tb_next': None}}}}

tblib.Traceback.from_dict

Building on the previous example:

>>> tb_json = json.dumps(tb_dict)
>>> tb = Traceback.from_dict(json.loads(tb_json))
>>> reraise(et, ev, tb.as_traceback())
Traceback (most recent call last):
  ...
  File "", line 6, in 
    reraise(et, ev, tb.as_traceback())
  File "", line 2, in 
    inner_2()
  File "", line 2, in inner_2
    inner_1()
  File "", line 2, in inner_1
    inner_0()
  File "", line 2, in inner_0
    raise Exception('fail')
Exception: fail

tblib.Traceback.from_string

>>> tb = Traceback.from_string("""
... File "skipped.py", line 123, in func_123
... Traceback (most recent call last):
...   File "tests/examples.py", line 2, in func_a
...     func_b()
...   File "tests/examples.py", line 6, in func_b
...     func_c()
...   File "tests/examples.py", line 10, in func_c
...     func_d()
...   File "tests/examples.py", line 14, in func_d
... Doesn't: matter
... """)
>>> reraise(et, ev, tb.as_traceback())
Traceback (most recent call last):
  ...
  File "", line 6, in 
    reraise(et, ev, tb.as_traceback())
  File "...examples.py", line 2, in func_a
    func_b()
  File "...examples.py", line 6, in func_b
    func_c()
  File "...examples.py", line 10, in func_c
    func_d()
  File "...examples.py", line 14, in func_d
    raise Exception("Guessing time !")
Exception: fail

If you use the strict=False option then parsing is a bit more lax:

>>> tb = Traceback.from_string("""
... File "bogus.py", line 123, in bogus
... Traceback (most recent call last):
...  File "tests/examples.py", line 2, in func_a
...   func_b()
...    File "tests/examples.py", line 6, in func_b
...     func_c()
...    File "tests/examples.py", line 10, in func_c
...   func_d()
...  File "tests/examples.py", line 14, in func_d
... Doesn't: matter
... """, strict=False)
>>> reraise(et, ev, tb.as_traceback())
Traceback (most recent call last):
  ...
  File "", line 6, in 
    reraise(et, ev, tb.as_traceback())
  File "bogus.py", line 123, in bogus
  File "...examples.py", line 2, in func_a
    func_b()
  File "...examples.py", line 6, in func_b
    func_c()
  File "...examples.py", line 10, in func_c
    func_d()
  File "...examples.py", line 14, in func_d
    raise Exception("Guessing time !")
Exception: fail

tblib.decorators.return_error

>>> from tblib.decorators import return_error
>>> inner_2r = return_error(inner_2)
>>> e = inner_2r()
>>> e

>>> e.reraise()
Traceback (most recent call last):
  ...
  File "", line 1, in 
    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 "", line 2, in inner_2
    inner_1()
  File "", line 2, in inner_1
    inner_0()
  File "", line 2, in inner_0
    raise Exception('fail')
Exception: fail

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

# Note that Python 3.4 and later will show the remote traceback (but as a string sadly) so we skip testing this.
>>> import traceback
>>> from multiprocessing import Pool
>>> from examples import func_a
>>> pool = Pool()  # doctest: +SKIP
>>> try:  # doctest: +SKIP
...     for i in pool.map(func_a, range(5)):
...         print(i)
... except:
...     print(traceback.format_exc())
...
Traceback (most recent call last):
  File "", line 2, in 
    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 !

>>> pool.terminate()  # doctest: +SKIP

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 "", line 4, in 
    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 6, in func_b
    func_c()
  File "...examples.py", line 10, in func_c
    func_d()
  File "...examples.py", line 14, in func_d
    raise Exception("Guessing time !")
Exception: Guessing time !

>>> pool.terminate()

Much better !

What if we have a local call stack ?

>>> def local_0():
...     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)
...     finally:
...         pool.close()
...
>>> def local_1():
...     local_0()
...
>>> def local_2():
...     local_1()
...
>>> try:
...     local_2()
... except:
...     print(traceback.format_exc())
Traceback (most recent call last):
  File "", line 2, in 
    local_2()
  File "", line 2, in local_2
    local_1()
  File "", line 2, in local_1
    local_0()
  File "", line 6, in local_0
    i.reraise()
  File "...tblib...decorators.py", line 20, in reraise
    reraise(self.exc_type, self.exc_value, self.traceback)
  File "...tblib...decorators.py", line 27, in return_exceptions_wrapper
    return func(*args, **kwargs)
  File "...tblib...decorators.py", line 47, 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 6, in func_b
    func_c()
  File "...tests...examples.py", line 10, in func_c
    func_d()
  File "...tests...examples.py", line 14, in func_d
    raise Exception("Guessing time !")
Exception: Guessing time !

Other weird stuff

Clearing traceback works (Python 3.4 and up):

>>> tb = Traceback.from_string("""
... File "skipped.py", line 123, in func_123
... Traceback (most recent call last):
...   File "tests/examples.py", line 2, in func_a
...     func_b()
...   File "tests/examples.py", line 6, in func_b
...     func_c()
...   File "tests/examples.py", line 10, in func_c
...     func_d()
...   File "tests/examples.py", line 14, in func_d
... Doesn't: matter
... """)
>>> import traceback, sys
>>> if sys.version_info > (3, 4):
...     traceback.clear_frames(tb)

Credits

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

Changelog

2.0.0 (2023-06-22)

• Removed support for legacy Pythons (2.7 and 3.6) and added Python 3.11 in the test grid.
• Some cleanups and refactors (mostly from ruff).

1.7.0 (2020-07-24)

• Add more attributes to Frame and Code objects for pytest compatibility. Contributed by Ivanq in #58.

1.6.0 (2019-12-07)

• When pickling an Exception, also pickle its traceback and the Exception chain (raise ... from ...). Contributed by Guido Imperiale in #53.

1.5.0 (2019-10-23)

• Added support for Python 3.8. Contributed by Victor Stinner in #42.
• Removed support for end of life Python 3.4.
• Few CI improvements and fixes.

1.4.0 (2019-05-02)

• Removed support for end of life Python 3.3.
• Fixed tests for Python 3.7. Contributed by Elliott Sales de Andrade in #36.
• Fixed compatibility issue with Twised (twisted.python.failure.Failure expected a co_code attribute).

1.3.2 (2017-04-09)

• Add support for PyPy3.5-5.7.1-beta. Previously AttributeError: 'Frame' object has no attribute 'clear' could be raised. See PyPy issue #2532.

1.3.1 (2017-03-27)

• Fixed handling for tracebacks due to exceeding the recursion limit. Fixes #15.

1.3.0 (2016-03-08)

• Added Traceback.from_string.

1.2.0 (2015-12-18)

• Fixed handling for tracebacks from generators and other internal improvements and optimizations. Contributed by DRayX in #10 and #11.

1.1.0 (2015-07-27)

• Added support for Python 2.6. Contributed by Arcadiy Ivanov in #8.

1.0.0 (2015-03-30)

• Added to_dict method and from_dict classmethod on Tracebacks. Contributed by beckjake in #5.