class joblib.Parallel(n_jobs=1, backend=None, verbose=0, timeout=None, pre_dispatch='2 * n_jobs', batch_size='auto', temp_folder=None, max_nbytes='1M', mmap_mode='r')

Helper class for readable parallel mapping.


n_jobs: int, default: 1

The maximum number of concurrently running jobs, such as the number of Python worker processes when backend=”multiprocessing” or the size of the thread-pool when backend=”threading”. If -1 all CPUs are used. If 1 is given, no parallel computing code is used at all, which is useful for debugging. For n_jobs below -1, (n_cpus + 1 + n_jobs) are used. Thus for n_jobs = -2, all CPUs but one are used.

backend: str or None, default: ‘multiprocessing’

Specify the parallelization backend implementation. Supported backends are:

  • “multiprocessing” used by default, can induce some communication and memory overhead when exchanging input and output data with the worker Python processes.
  • “threading” is a very low-overhead backend but it suffers from the Python Global Interpreter Lock if the called function relies a lot on Python objects. “threading” is mostly useful when the execution bottleneck is a compiled extension that explicitly releases the GIL (for instance a Cython loop wrapped in a “with nogil” block or an expensive call to a library such as NumPy).
  • finally, you can register backends by calling register_parallel_backend. This will allow you to implement a backend of your liking.

verbose: int, optional

The verbosity level: if non zero, progress messages are printed. Above 50, the output is sent to stdout. The frequency of the messages increases with the verbosity level. If it more than 10, all iterations are reported.

timeout: float, optional

Timeout limit for each task to complete. If any task takes longer a TimeOutError will be raised. Only applied when n_jobs != 1

pre_dispatch: {‘all’, integer, or expression, as in ‘3*n_jobs’}

The number of batches (of tasks) to be pre-dispatched. Default is ‘2*n_jobs’. When batch_size=”auto” this is reasonable default and the multiprocessing workers should never starve.

batch_size: int or ‘auto’, default: ‘auto’

The number of atomic tasks to dispatch at once to each worker. When individual evaluations are very fast, multiprocessing can be slower than sequential computation because of the overhead. Batching fast computations together can mitigate this. The 'auto' strategy keeps track of the time it takes for a batch to complete, and dynamically adjusts the batch size to keep the time on the order of half a second, using a heuristic. The initial batch size is 1. batch_size="auto" with backend="threading" will dispatch batches of a single task at a time as the threading backend has very little overhead and using larger batch size has not proved to bring any gain in that case.

temp_folder: str, optional

Folder to be used by the pool for memmaping large arrays for sharing memory with worker processes. If None, this will try in order: - a folder pointed by the JOBLIB_TEMP_FOLDER environment variable, - /dev/shm if the folder exists and is writable: this is a RAMdisk

filesystem available by default on modern Linux distributions,

  • the default system temporary folder that can be overridden with TMP, TMPDIR or TEMP environment variables, typically /tmp under Unix operating systems.

Only active when backend=”multiprocessing”.

max_nbytes int, str, or None, optional, 1M by default

Threshold on the size of arrays passed to the workers that triggers automated memory mapping in temp_folder. Can be an int in Bytes, or a human-readable string, e.g., ‘1M’ for 1 megabyte. Use None to disable memmaping of large arrays. Only active when backend=”multiprocessing”.

mmap_mode: {None, ‘r+’, ‘r’, ‘w+’, ‘c’}

Memmapping mode for numpy arrays passed to workers. See ‘max_nbytes’ parameter documentation for more details.


This object uses the multiprocessing module to compute in parallel the application of a function to many different arguments. The main functionality it brings in addition to using the raw multiprocessing API are (see examples for details):

  • More readable code, in particular since it avoids constructing list of arguments.

  • Easier debugging:
    • informative tracebacks even when the error happens on the client side
    • using ‘n_jobs=1’ enables to turn off parallel computing for debugging without changing the codepath
    • early capture of pickling errors
  • An optional progress meter.

  • Interruption of multiprocesses jobs with ‘Ctrl-C’

  • Flexible pickling control for the communication to and from the worker processes.

  • Ability to use shared memory efficiently with worker processes for large numpy-based datastructures.


A simple example:

>>> from math import sqrt
>>> from joblib import Parallel, delayed
>>> Parallel(n_jobs=1)(delayed(sqrt)(i**2) for i in range(10))
[0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0]

Reshaping the output when the function has several return values:

>>> from math import modf
>>> from joblib import Parallel, delayed
>>> r = Parallel(n_jobs=1)(delayed(modf)(i/2.) for i in range(10))
>>> res, i = zip(*r)
>>> res
(0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5, 0.0, 0.5)
>>> i
(0.0, 0.0, 1.0, 1.0, 2.0, 2.0, 3.0, 3.0, 4.0, 4.0)

The progress meter: the higher the value of verbose, the more messages:

>>> from time import sleep
>>> from joblib import Parallel, delayed
>>> r = Parallel(n_jobs=2, verbose=5)(delayed(sleep)(.1) for _ in range(10)) 
[Parallel(n_jobs=2)]: Done   1 out of  10 | elapsed:    0.1s remaining:    0.9s
[Parallel(n_jobs=2)]: Done   3 out of  10 | elapsed:    0.2s remaining:    0.5s
[Parallel(n_jobs=2)]: Done   6 out of  10 | elapsed:    0.3s remaining:    0.2s
[Parallel(n_jobs=2)]: Done   9 out of  10 | elapsed:    0.5s remaining:    0.1s
[Parallel(n_jobs=2)]: Done  10 out of  10 | elapsed:    0.5s finished

Traceback example, note how the line of the error is indicated as well as the values of the parameter passed to the function that triggered the exception, even though the traceback happens in the child process:

>>> from heapq import nlargest
>>> from joblib import Parallel, delayed
>>> Parallel(n_jobs=2)(delayed(nlargest)(2, n) for n in (range(4), 'abcde', 3)) 
Sub-process traceback:
TypeError                                          Mon Nov 12 11:37:46 2012
PID: 12934                                    Python 2.7.3: /usr/bin/python
/usr/lib/python2.7/heapq.pyc in nlargest(n=2, iterable=3, key=None)
    419         if n >= size:
    420             return sorted(iterable, key=key, reverse=True)[:n]
    422     # When key is none, use simpler decoration
    423     if key is None:
--> 424         it = izip(iterable, count(0,-1))                    # decorate
    425         result = _nlargest(n, it)
    426         return map(itemgetter(0), result)                   # undecorate
    428     # General case, slowest method

TypeError: izip argument #1 must support iteration

Using pre_dispatch in a producer/consumer situation, where the data is generated on the fly. Note how the producer is first called a 3 times before the parallel loop is initiated, and then called to generate new data on the fly. In this case the total number of iterations cannot be reported in the progress messages:

>>> from math import sqrt
>>> from joblib import Parallel, delayed

>>> def producer():
...     for i in range(6):
...         print('Produced %s' % i)
...         yield i

>>> out = Parallel(n_jobs=2, verbose=100, pre_dispatch='1.5*n_jobs')(
...                         delayed(sqrt)(i) for i in producer()) 
Produced 0
Produced 1
Produced 2
[Parallel(n_jobs=2)]: Done 1 jobs     | elapsed:  0.0s
Produced 3
[Parallel(n_jobs=2)]: Done 2 jobs     | elapsed:  0.0s
Produced 4
[Parallel(n_jobs=2)]: Done 3 jobs     | elapsed:  0.0s
Produced 5
[Parallel(n_jobs=2)]: Done 4 jobs     | elapsed:  0.0s
[Parallel(n_jobs=2)]: Done 5 out of 6 | elapsed:  0.0s remaining: 0.0s
[Parallel(n_jobs=2)]: Done 6 out of 6 | elapsed:  0.0s finished