Joblib version Travis build status AppVeyor build status Codecov coverage

The homepage of joblib with user documentation is located on:

Getting the latest code

To get the latest code using git, simply type:

git clone git://

If you don’t have git installed, you can download a zip or tarball of the latest code:


You can use pip to install joblib:

pip install joblib

from any directory or

python install

from the source directory.

Joblib has no other mandatory dependency than Python (supported versions are 2.7+ and 3.3+). Numpy (at least version 1.6.1) is an optional dependency for array manipulation.

Workflow to contribute

To contribute to joblib, first create an account on github. Once this is done, fork the joblib repository to have you own repository, clone it using ‘git clone’ on the computers where you want to work. Make your changes in your clone, push them to your github account, test them on several computers, and when you are happy with them, send a pull request to the main repository.

Running the test suite

To run the test suite, you need the pytest (version >= 3) and coverage modules. Run the test suite using:

pytest joblib

from the root of the project.

Building the docs

To build the docs you need to have setuptools and sphinx (>=0.5) installed. Run the command:

python build_sphinx

The docs are built in the build/sphinx/html directory.

Making a source tarball

To create a source tarball, eg for packaging or distributing, run the following command:

python sdist

The tarball will be created in the dist directory. This command will compile the docs, and the resulting tarball can be installed with no extra dependencies than the Python standard library. You will need setuptool and sphinx.

Making a release and uploading it to PyPI

This command is only run by project manager, to make a release, and upload in to PyPI:

python sdist bdist_wheel upload_docs --upload-dir build/sphinx/html
twine upload dist/*

Updating the changelog

Changes are listed in the CHANGES.rst file. They must be manually updated but, the following git command may be used to generate the lines:

git log --abbrev-commit --date=short --no-merges --sparse


joblib is BSD-licenced (3 clause):

This software is OSI Certified Open Source Software. OSI Certified is a certification mark of the Open Source Initiative.

Copyright (c) 2009-2011, joblib developpers All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name of Gael Varoquaux. nor the names of other joblib contributors may be used to endorse or promote products derived from this software without specific prior written permission.

This software is provided by the copyright holders and contributors “as is” and any express or implied warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed. In no event shall the copyright owner or contributors be liable for any direct, indirect, incidental, special, exemplary, or consequential damages (including, but not limited to, procurement of substitute goods or services; loss of use, data, or profits; or business interruption) however caused and on any theory of liability, whether in contract, strict liability, or tort (including negligence or otherwise) arising in any way out of the use of this software, even if advised of the possibility of such damage.

Latest changes


Alexandre Abadie

Remove support for python 2.6

Alexandre Abadie

Remove deprecated format_signature, format_call and load_output functions from Memory API.

Loïc Estève

Add initial implementation of LRU cache cleaning. You can specify the size limit of a Memory object via the bytes_limit parameter and then need to clean explicitly the cache via the Memory.reduce_size method.

Olivier Grisel

Make the multiprocessing backend work even when the name of the main thread is not the Python default. Thanks to Roman Yurchak for the suggestion.

Karan Desai

pytest is used to run the tests instead of nosetests. python setup.pytest or python nosetests do not work anymore, run pytest joblib instead.

Loïc Estève

An instance of joblib.ParallelBackendBase can be passed into the parallel argument in joblib.Parallel.

Loïc Estève

Fix handling of memmap objects with offsets greater than mmap.ALLOCATIONGRANULARITY in joblib.Parrallel. See for more details.

Loïc Estève

Fix performance regression in joblib.Parallel with n_jobs=1. See for more details.

Loïc Estève

Fix race condition when a function cached with joblib.Memory.cache was used inside a joblib.Parallel. See for more details.

Release 0.10.3

Loïc Estève

Fix tests when multiprocessing is disabled via the JOBLIB_MULTIPROCESSING environment variable.


Remove warnings in nested Parallel objects when the inner Parallel has n_jobs=1. See for more details.

Release 0.10.2

Loïc Estève

FIX a bug in stack formatting when the error happens in a compiled extension. See for more details.

Vincent Latrouite

FIX a bug in the constructor of BinaryZlibFile that would throw an exception when passing unicode filename (Python 2 only). See for more details.

Olivier Grisel

Expose joblib.parallel.ParallelBackendBase and joblib.parallel.AutoBatchingMixin in the public API to make them officially re-usable by backend implementers.

Release 0.10.0

Alexandre Abadie

ENH: joblib.dump/load now accept file-like objects besides filenames. for more details.

Niels Zeilemaker and Olivier Grisel

Refactored joblib.Parallel to enable the registration of custom computational backends. Note the API to register custom backends is considered experimental and subject to change without deprecation.

Alexandre Abadie

Joblib pickle format change: joblib.dump always create a single pickle file and joblib.dump/ never do any memory copy when writing/reading pickle files. Reading pickle files generated with joblib versions prior to 0.10 will be supported for a limited amount of time, we advise to regenerate them from scratch when convenient. joblib.dump and joblib.load also support pickle files compressed using various strategies: zlib, gzip, bz2, lzma and xz. Note that lzma and xz are only available with python >= 3.3. for more details.

Antony Lee

ENH: joblib.dump/load now accept pathlib.Path objects as filenames. for more details.

Olivier Grisel

Workaround for “WindowsError: [Error 5] Access is denied” when trying to terminate a multiprocessing pool under Windows:

Release 0.9.4

Olivier Grisel

FIX a race condition that could cause a joblib.Parallel to hang when collecting the result of a job that triggers an exception.

Olivier Grisel

FIX a bug that caused joblib.Parallel to wrongly reuse previously memmapped arrays instead of creating new temporary files. for more details.

Loïc Estève

FIX for raising non inheritable exceptions in a Parallel call. See for more details.

Alexandre Abadie

FIX joblib.hash error with mixed types sets and dicts containing mixed types keys when using Python 3. see

Loïc Estève

FIX joblib.dump/load for big numpy arrays with dtype=object. See for more details.

Loïc Estève

FIX joblib.Parallel hanging when used with an exhausted iterator. See for more details.

Release 0.9.3

Olivier Grisel

Revert back to the fork start method (instead of forkserver) as the latter was found to cause crashes in interactive Python sessions.

Release 0.9.2

Loïc Estève

Joblib hashing now uses the default pickle protocol (2 for Python 2 and 3 for Python 3). This makes it very unlikely to get the same hash for a given object under Python 2 and Python 3.

In particular, for Python 3 users, this means that the output of joblib.hash changes when switching from joblib 0.8.4 to 0.9.2 . We strive to ensure that the output of joblib.hash does not change needlessly in future versions of joblib but this is not officially guaranteed.

Loïc Estève

Joblib pickles generated with Python 2 can not be loaded with Python 3 and the same applies for joblib pickles generated with Python 3 and loaded with Python 2.

During the beta period 0.9.0b2 to 0.9.0b4, we experimented with a joblib serialization that aimed to make pickles serialized with Python 3 loadable under Python 2. Unfortunately this serialization strategy proved to be too fragile as far as the long-term maintenance was concerned (For example see That means that joblib pickles generated with joblib 0.9.0bN can not be loaded under joblib 0.9.2. Joblib beta testers, who are the only ones likely to be affected by this, are advised to delete their joblib cache when they upgrade from 0.9.0bN to 0.9.2.

Arthur Mensch

Fixed a bug with joblib.hash that used to return unstable values for strings and numpy.dtype instances depending on interning states.

Olivier Grisel

Make joblib use the ‘forkserver’ start method by default under Python 3.4+ to avoid causing crash with 3rd party libraries (such as Apple vecLib / Accelerate or the GCC OpenMP runtime) that use an internal thread pool that is not not reinitialized when a fork system call happens.

Olivier Grisel

New context manager based API (with block) to re-use the same pool of workers across consecutive parallel calls.

Vlad Niculae and Olivier Grisel

Automated batching of fast tasks into longer running jobs to hide multiprocessing dispatching overhead when possible.

Olivier Grisel

FIX make it possible to call joblib.load(filename, mmap_mode='r') on pickled objects that include a mix of arrays of both memory memmapable dtypes and object dtype.

Release 0.8.4

2014-11-20 Olivier Grisel

OPTIM use the C-optimized pickler under Python 3

This makes it possible to efficiently process parallel jobs that deal with numerous Python objects such as large dictionaries.

Release 0.8.3

2014-08-19 Olivier Grisel

FIX disable memmapping for object arrays

2014-08-07 Lars Buitinck

MAINT NumPy 1.10-safe version comparisons

2014-07-11 Olivier Grisel

FIX #146: Heisen test failure caused by thread-unsafe Python lists

This fix uses a queue.Queue datastructure in the failing test. This datastructure is thread-safe thanks to an internal Lock. This Lock instance not picklable hence cause the picklability check of delayed to check fail.

When using the threading backend, picklability is no longer required, hence this PRs give the user the ability to disable it on a case by case basis.

Release 0.8.2

2014-06-30 Olivier Grisel

BUG: use mmap_mode=’r’ by default in Parallel and MemmapingPool

The former default of mmap_mode=’c’ (copy-on-write) caused problematic use of the paging file under Windows.

2014-06-27 Olivier Grisel

BUG: fix usage of the /dev/shm folder under Linux

Release 0.8.1

2014-05-29 Gael Varoquaux

BUG: fix crash with high verbosity

Release 0.8.0

2014-05-14 Olivier Grisel

Fix a bug in exception reporting under Python 3

2014-05-10 Olivier Grisel

Fixed a potential segfault when passing non-contiguous memmap instances.

2014-04-22 Gael Varoquaux

ENH: Make memory robust to modification of source files while the interpreter is running. Should lead to less spurious cache flushes and recomputations.

2014-02-24 Philippe Gervais

New Memory.call_and_shelve API to handle memoized results by reference instead of by value.

Release 0.8.0a3

2014-01-10 Olivier Grisel & Gael Varoquaux

FIX #105: Race condition in task iterable consumption when pre_dispatch != ‘all’ that could cause crash with error messages “Pools seems closed” and “ValueError: generator already executing”.

2014-01-12 Olivier Grisel

FIX #72: joblib cannot persist “output_dir” keyword argument.

Release 0.8.0a2

2013-12-23 Olivier Grisel

ENH: set default value of Parallel’s max_nbytes to 100MB

Motivation: avoid introducing disk latency on medium sized parallel workload where memory usage is not an issue.

FIX: properly handle the JOBLIB_MULTIPROCESSING env variable

FIX: timeout test failures under windows

Release 0.8.0a

2013-12-19 Olivier Grisel

FIX: support the new Python 3.4 multiprocessing API

2013-12-05 Olivier Grisel

ENH: make Memory respect mmap_mode at first call too

ENH: add a threading based backend to Parallel

This is low overhead alternative backend to the default multiprocessing backend that is suitable when calling compiled extensions that release the GIL.

Author: Dan Stahlke <> Date: 2013-11-08

FIX: use safe_repr to print arg vals in trace

This fixes a problem in which extremely long (and slow) stack traces would be produced when function parameters are large numpy arrays.

2013-09-10 Olivier Grisel

ENH: limit memory copy with Parallel by leveraging numpy.memmap when possible

Release 0.7.1

2013-07-25 Gael Varoquaux

MISC: capture meaningless argument (n_jobs=0) in Parallel

2013-07-09 Lars Buitinck

ENH Handles tuples, sets and Python 3’s dict_keys type the same as lists. in pre_dispatch

2013-05-23 Martin Luessi

ENH: fix function caching for IPython

Release 0.7.0

This release drops support for Python 2.5 in favor of support for Python 3.0

2013-02-13 Gael Varoquaux

BUG: fix nasty hash collisions

2012-11-19 Gael Varoquaux

ENH: Parallel: Turn of pre-dispatch for already expanded lists

Gael Varoquaux 2012-11-19

ENH: detect recursive sub-process spawning, as when people do not protect the __main__ in scripts under Windows, and raise a useful error.

Gael Varoquaux 2012-11-16

ENH: Full python 3 support

Release 0.6.5

2012-09-15 Yannick Schwartz

BUG: make sure that sets and dictionnaries give reproducible hashes

2012-07-18 Marek Rudnicki

BUG: make sure that object-dtype numpy array hash correctly

2012-07-12 GaelVaroquaux

BUG: Bad default n_jobs for Parallel

Release 0.6.4

2012-05-07 Vlad Niculae

ENH: controlled randomness in tests and doctest fix

2012-02-21 GaelVaroquaux

ENH: add verbosity in memory

2012-02-21 GaelVaroquaux

BUG: non-reproducible hashing: order of kwargs

The ordering of a dictionnary is random. As a result the function hashing was not reproducible. Pretty hard to test

Release 0.6.3

2012-02-14 GaelVaroquaux

BUG: fix joblib Memory pickling

2012-02-11 GaelVaroquaux

BUG: fix hasher with Python 3

2012-02-09 GaelVaroquaux

API: filter_args: *args, **kwargs -> args, kwargs

Release 0.6.2

2012-02-06 Gael Varoquaux

BUG: make sure Memory pickles even if cachedir=None

Release 0.6.1

Bugfix release because of a merge error in release 0.6.0

Release 0.6.0

Beta 3

2012-01-11 Gael Varoquaux

BUG: ensure compatibility with old numpy

DOC: update installation instructions

BUG: file semantic to work under Windows

2012-01-10 Yaroslav Halchenko

BUG: a fix toward 2.5 compatibility

Beta 2

2012-01-07 Gael Varoquaux

ENH: hash: bugware to be able to hash objects defined interactively in IPython

2012-01-07 Gael Varoquaux

ENH: Parallel: warn and not fail for nested loops

ENH: Parallel: n_jobs=-2 now uses all CPUs but one

2012-01-01 Juan Manuel Caicedo Carvajal and Gael Varoquaux

ENH: add verbosity levels in Parallel

Release 0.5.7

2011-12-28 Gael varoquaux

API: zipped -> compress

2011-12-26 Gael varoquaux

ENH: Add a zipped option to Memory

API: Memory no longer accepts save_npy

2011-12-22 Kenneth C. Arnold and Gael varoquaux

BUG: fix numpy_pickle for array subclasses

2011-12-21 Gael varoquaux

ENH: add zip-based pickling

2011-12-19 Fabian Pedregosa

Py3k: compatibility fixes. This makes run fine the tests test_disk and test_parallel

Release 0.5.6

2011-12-11 Lars Buitinck

ENH: Replace os.path.exists before makedirs with exception check New disk.mkdirp will fail with other errnos than EEXIST.

2011-12-10 Bala Subrahmanyam Varanasi

MISC: pep8 compliant

Release 0.5.5

2011-19-10 Fabian Pedregosa

ENH: Make joblib installable under Python 3.X

Release 0.5.4

2011-09-29 Jon Olav Vik

BUG: Make mangling path to filename work on Windows

2011-09-25 Olivier Grisel

FIX: doctest heisenfailure on execution time

2011-08-24 Ralf Gommers

STY: PEP8 cleanup.

Release 0.5.3

2011-06-25 Gael varoquaux

API: All the usefull symbols in the __init__

Release 0.5.2

2011-06-25 Gael varoquaux

ENH: Add cpu_count

2011-06-06 Gael varoquaux

ENH: Make sure memory hash in a reproducible way

Release 0.5.1

2011-04-12 Gael varoquaux

TEST: Better testing of parallel and pre_dispatch

Yaroslav Halchenko 2011-04-12

DOC: quick pass over docs – trailing spaces/spelling

Yaroslav Halchenko 2011-04-11

ENH: JOBLIB_MULTIPROCESSING env var to disable multiprocessing from the environment

Alexandre Gramfort 2011-04-08

ENH : adding log message to know how long it takes to load from disk the cache

Release 0.5.0

2011-04-01 Gael varoquaux

BUG: pickling MemoizeFunc does not store timestamp

2011-03-31 Nicolas Pinto

TEST: expose hashing bug with cached method

2011-03-26...2011-03-27 Pietro Berkes

BUG: fix error management in rm_subdirs BUG: fix for race condition during tests in mem.clear()

Gael varoquaux 2011-03-22...2011-03-26

TEST: Improve test coverage and robustness

Gael varoquaux 2011-03-19

BUG: hashing functions with only *var **kwargs

Gael varoquaux 2011-02-01... 2011-03-22

BUG: Many fixes to capture interprocess race condition when mem.cache is used by several processes on the same cache.

Fabian Pedregosa 2011-02-28

First work on Py3K compatibility

Gael varoquaux 2011-02-27

ENH: pre_dispatch in parallel: lazy generation of jobs in parallel for to avoid drowning memory.

GaelVaroquaux 2011-02-24

ENH: Add the option of overloading the arguments of the mother ‘Memory’ object in the cache method that is doing the decoration.

Gael varoquaux 2010-11-21

ENH: Add a verbosity level for more verbosity

Release 0.4.6

Gael varoquaux 2010-11-15

ENH: Deal with interruption in parallel

Gael varoquaux 2010-11-13

BUG: Exceptions raised by Parallel when n_job=1 are no longer captured.

Gael varoquaux 2010-11-13

BUG: Capture wrong arguments properly (better error message)

Release 0.4.5

Pietro Berkes 2010-09-04

BUG: Fix Windows peculiarities with path separators and file names BUG: Fix more windows locking bugs

Gael varoquaux 2010-09-03

ENH: Make sure that exceptions raised in Parallel also inherit from the original exception class ENH: Add a shadow set of exceptions

Fabian Pedregosa 2010-09-01

ENH: Clean up the code for parallel. Thanks to Fabian Pedregosa for the patch.

Release 0.4.4

Gael varoquaux 2010-08-23

BUG: Fix Parallel on computers with only one CPU, for n_jobs=-1.

Gael varoquaux 2010-08-02

BUG: Fix for extra setuptools args.

Gael varoquaux 2010-07-29

MISC: Silence tests (and hopefuly Yaroslav :P)

Release 0.4.3

Gael Varoquaux 2010-07-22

BUG: Fix hashing for function with a side effect modifying their input argument. Thanks to Pietro Berkes for reporting the bug and proving the patch.

Release 0.4.2

Gael Varoquaux 2010-07-16

BUG: Make sure that joblib still works with Python2.5. => release 0.4.2

Release 0.4.1