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Parallel task graph framework.

Project description

About TaskGraph

TaskGraph is a library that was developed to help manage complicated computational software pipelines consisting of long running individual tasks. Many of these tasks could be executed in parallel, almost all of them wrote results to disk, and many times results could be reused from part of the pipeline. TaskGraph manages all of this for you. With it you can schedule tasks with dependencies, avoid recomputing results that have already been computed, and allot multiple CPU cores to execute tasks in parallel if desired.

TaskGraph Dependencies

Task Graph is written in pure Python, but if the psutils package is installed the distributed multiprocessing processes will be niced.

Example Use

Install TaskGraph with

pip install taskgraph


import os
import pickle
import logging

import taskgraph


def _create_list_on_disk(value, length, target_path):
    """Create a numpy array on disk filled with value of `size`."""
    target_list = [value] * length
    pickle.dump(target_list, open(target_path, 'wb'))

def _sum_lists_from_disk(list_a_path, list_b_path, target_path):
    """Read two lists, add them and save result."""
    list_a = pickle.load(open(list_a_path, 'rb'))
    list_b = pickle.load(open(list_b_path, 'rb'))
    target_list = []
    for a, b in zip(list_a, list_b):
    pickle.dump(target_list, open(target_path, 'wb'))

# create a taskgraph that uses 4 multiprocessing subprocesses when possible
if __name__ == '__main__':
    workspace_dir = 'workspace'
    task_graph = taskgraph.TaskGraph(workspace_dir, 4)
    target_a_path = os.path.join(workspace_dir, 'a.dat')
    target_b_path = os.path.join(workspace_dir, 'b.dat')
    result_path = os.path.join(workspace_dir, 'result.dat')
    result_2_path = os.path.join(workspace_dir, 'result2.dat')
    value_a = 5
    value_b = 10
    list_len = 10
    task_a = task_graph.add_task(
        args=(value_a, list_len, target_a_path),
    task_b = task_graph.add_task(
        args=(value_b, list_len, target_b_path),
    sum_task = task_graph.add_task(
        args=(target_a_path, target_b_path, result_path),
        dependent_task_list=[task_a, task_b])


    # expect that result is a list `list_len` long with `value_a+value_b` in it
    result = pickle.load(open(result_path, 'rb'))


  • Taskgraph’s default method of checking whether a file has changed (hash_algorithm='sizetimestamp') uses the filesystem’s modification timestamp, interpreted in integer nanoseconds. This check is only as accurate as the filesystem’s timestamp. For example:

    • FAT and FAT32 timestamps have a 2-second modification timestamp resolution
    • exFAT has a 10 millisecond timestamp resolution
    • NTFS has a 100 nanosecond timestamp resolution
    • HFS+ has a 1 second timestamp resolution
    • APFS has a 1 nanosecond timestamp resolution
    • ext3 has a 1 second timestamp resolution
    • ext4 has a 1 nanosecond timestamp resolution

    If you suspect timestamp resolution to be an issue on your filesystem, you may wish to store your files on a filesystem with more accurate timestamps or else consider using a different hash_algorithm.

Running Tests

Taskgraph includes a tox configuration for automating builds across multiple python versions and whether psutil is installed. To execute all tests on all platforms, run:

$ tox

Alternatively, if you’re only trying to run tests on a single configuration (say, python 3.7 without psutil), you’d run:

$ tox -e py37

Or if you’d like to run the tests for the combination of Python 3.7 with psutil, you’d run:

$ tox -e py37-psutil

If you don’t have multiple python installations already available on your system, an easy way to accomplish this is to use tox-conda ( which will use conda environments to manage the versions of python available:

$ pip install tox-conda
$ tox

TaskGraph Release History

0.11.0 (2021-10-12)

  • Testing against python 3.10 in github actions and officially noting support for 3.10 in
  • Testing against python 3.9 in github actions and noting support in
  • Fixed an issue where exceptions raised during execution where the task completed before TaskGraph.join() was called would not be raised. Now, if a task raises an exception, its exception will always be raised when either Task.join() and TaskGraph.join() is called.
  • Fixed an issue where tasks with hash_algorithm='sizetimestamp' would, under certain conditions, fail to re-execute when they should. This only occurred when a graph writing the same amount of , but possibly different, data is executed successively, with less than about 1.5 seconds between task executions.
  • After many years with the Natural Capital Project, Rich Sharp has stepped down from the Project and as the maintainer of taskgraph. James Douglass is taking his place, and this change is now reflected in
  • Fixes an issue that causes an EOFError or BrokenPipeError to occur when the TaskGraph terminates.
  • Updated the taskgraph example in the README for the latest API changes and to clarify the need for if __name__ == '__main__':
  • Fixed an issue that could cause the TaskGraph object to hang if duplicate Task objects were created.
  • Fixed an issue that was causing TaskGraph to ignore a changed hash_algorithm if the TaskGraph was created on one run, was deconstructed, then restarted. If the user chose a different hash, TaskGraph would use the hash that the target file was originally hashed under rather than the new algorithm.
  • Removed copy_duplicate_artifact and hardlink_allowed parameters and functionality from TaskGraph. This is to address a design error that TaskGraph is not well suited for caching file results to avoid recomputation. Rather than add additional complexity around the limitations of this feature it is being removed to guide a design toward a standalone cache library if needed.

0.10.3 (2021-01-29)

  • Fixed issue that could cause combinatorial memory usage leading to poor runtime or MemoryError if a dictionary were passed that had thousands of elements.
  • Fixed issue that would cause TaskGraph to not recognize a directory that was meant to be ignored and in some cases cause Task to unnecessarily reexecute.

0.10.2 (2020-12-11)

  • Fixed an issue that would raise an exception when __del__ was deconstructing a taskgraph object and a thread join() would cause a deadlock.

0.10.1 (2020-12-11)

  • Fixed an issue that would ignore the state of a transient_run flag if a previous Task run had run it with that flag set to False.
  • Removed a limit on the number of times TaskGraph can attempt to update its database up to 5 minutes of continuous failures. This is to address expected issues when many parallel threads may compete for an update. Relevant information about why the database update fails is logged.
  • Fixed an issue where the logging queue would always report an exception even if the logging thread shut down correctly.

0.10.0 (2020-08-25)

  • Fixed several race conditions that could cause the TaskGraph object to hang on an otherwise ordinary termination.
  • Changed logging level to “INFO” on cases where the taskgraph was not precalculated since it’s an expected path of execution in TaskGraph.
  • Adding a hardlink_allowed parameter to add_task that allows the attempt to hardlink a file in a case where a copy_artifact=True may permit one. This will save on disk space as well as computation time if large files are not needed to copy.
  • Adding a store_result flag to add_task that conditionally stores the func result in the database for later .get. This was added to guard against return types that were not picklable and would otherwise cause an exception when being executed normally.
  • Fixed issue that would cause the logger thread to continue reporting status after all tasks were complete and the graph was closed.

0.9.1 (2020-06-04)

  • Fixed issue that would cause an infinite loop if a TaskGraph object were created with a database from an incompatible previous version. Behavior now is to log the issue, delete the old database, and create a new compatible one.
  • Fixed issue that would cause some rare infinite loops if TaskGraph were to fail due to some kinds of task exceptions.
  • Adding open source BSD-3-Clause license.

0.9.0 (2020-03-05)

  • Updating primary repository URL to GitHub.
  • Adding support for Python 3.8.
  • Removing the EncapsulatedOp abstract class. In practice the development loop that encouraged the use of EncapsulatedOp is flawed and can lead to design errors.
  • Removing unnecessary internal locks which will improve runtime performance of processing many small Tasks.
  • Refactor to support separate TaskGraph objects that use the same database.
  • Removed the n_retries parameter from add_task. Users are recommended to handle retries within functions themselves.
  • Added a hash_target_files flag to add_task that when set to False, causes TaskGraph to only note the existence of target files after execution or as part of an evaluation to determine if the Task was precalculated. This is useful for operations that initialize a file but subsequent runs of the program modify it such as a new database or a downloaded file.
  • Fixed an issue on the monitor execution thread that caused shutdown of a TaskGraph object to be delayed up to the amount of delay in the monitor reporting update.
  • Added a .get() function for Task objects that returns the result of the respective func call. This value is cached in the TaskGraph database and hence can be used to avoid repeated execution. Note the addition of this function changes the functionality of calling add_task with no target path list. In previous versions the Task would execute once per TaskGraph instance, now successive Task objects with the same execution signature will use cached results.
  • To support the addition of the .get() function a transient_run parameter is added to add_task that causes TaskGraph to avoid recording a completed Task even if the execution hash would have been identical to a previously completed run where the target artifacts still existed.

0.8.5 (2019-09-11)

  • Dropped support for Python 2.7.
  • Fixed an issue where paths in ignore_paths were not getting ignored in the case of copy_duplicate_artifact=True.
  • Fixed an issue where the “percent completed” in the logging monitor would sometimes exceed 100%. This occurred when a duplicate task was added to the TaskGraph object.
  • Fixed an issue where a relative path set as a target path would always cause TaskGraph to raise an exception after the task was complete.
  • Fixed an issue where kwargs that were unhashable were not considered when determining if a Task should be re-run.
  • Fixed an issue where files with almost identical modified times and sizes would hash equal in cases even when the filenames were different.

0.8.4 (2019-05-23)

  • Fixed an exception that occurred when two tasks were constructed that targeted the same file but one path was relative and the other was absolute.

0.8.3 (2019-02-26)

  • Fixed an issue that would cause TaskGraph to raise an IOError if an add_task call was marked for copy_duplicate_artifact but the base target file was missing.
  • Fixed an issue that would prevent the source distribution from installing.
  • Taskgraph is now tested against python versions 2.7, 3.6 and 3.7.

0.8.2 (2019-01-31)

  • Adjusted logging levels so most chatty information is lowered to debug and oddness in __del__ shutdown are degraded from error to debug so as not to cause alarm.

0.8.1 (2019-01-09)

  • Fixed an issue that would cause a deadlock if two tasks were added that had the same function signature except different target paths.

0.8.0 (2019-01-07)

  • Fixed a race condition that would sometimes cause an exception when multiple threads attempted to read or write to the completed Task Database.
  • Fixed an issue that could cause an exception in __del__ to print to stderr during Python interpreter shutdown.
  • Added a hash_algorithm parameter to add_task that is a string of either ‘sizetimestamp’ or anything in hashlib.algorithms_available. This option tells TaskGraph how to fingerprint input and target files to determine the need for recomputation.
  • Added a copy_duplicate_artifact parameter to add_task that when True tells TaskGraph to copy duplicate target results to a new target so long as all the parameters and base/target files fingerprint to the same value. This can save significant computation time when use in scenarios where there are small changes in a workflow, but otherwise significant changes in filenames. This often occurs when putting timestamps or other suffixes on files that otherwise have identical content.

0.7.2 (2018-11-21)

  • TaskGraph now stores all task completion information in a single SQLite database stored in its cache directory. In previous versions TaskGraph would write a small text file for each task in a highly branching directory tree. This structure made removal of those directory trees computationally difficult.
  • Fixed an issue that would cause TaskGraph to reexecute if the target path was included in the argument list and that path was not normalized to the operating system’s path style.
  • Fixed a deadlock in some cases where Tasks failed while other tasks checked for pre-execution clauses.

0.7.0 (2018-10-22)

  • Fixed an issue where very long strings might be interpreted as paths and Windows crashes because the path is too long.
  • Fixed a deadlock issue where a Task might raise an unhandled exception as a new task was added to the TaskGraph.
  • Fixed the occasional BrokenPipeError that could occur when a Task encountered an unhandled exception.
  • Added an n_retries parameter to add_task that lets TaskGraph attempt to reexecute a failing Task up to n_retries times before terminating the TaskGraph.
  • Removed the delayed_start option.

0.6.1 (2018-08-14)

  • Resolving an issue with duplicate logging being printed to stdout when n_workers > 0. Logging is now only handled in the process that contains the TaskGraph instance.
  • Updated main logging message to indicate which tasks, by task name, are currently active and how many tasks are ready to execute but can’t because there is not an open worker.
  • Attempted to fix an issue where processes in the process pool were not terminating on a Linux system by aggressively joining all threads and processes when possible.
  • Fixed an issue that would cause tasks that had been previously calculated to prematurely trigger children tasks even if the parent tasks of the current task needed to be reexecuted.

0.6.0 (2018-07-24)

  • Added a delayed_start flag to TaskGraph to allow for delayed execution of taskgraph tasks. If enabled on threaded or multiprocess mode, calls to add_task will not execute tasks until the join method is invoked on taskgraph. This allows for finer control over execution order when tasks are passed non-equivalent priority levels.
  • Fixing an issue where a non-JSON serializeable object would cause add_task to crash. Now TaskGraph is more tolerant of non-JSON serializeable objects and will log warnings when parameters cannot be serialized.
  • TaskGraph constructor has an option to report a ongoing logging message at a set interval. The message reports how many tasks have been committed and completed.
  • Fixed a bug that would cause TaskGraph to needlessly reexecute a task if the only change was the order of the target_path_list or dependent_task_list variables.
  • Fixed a bug that would cause a task to reexecute between runs if input argument was a file that would be generated by a task that had not yet executed.
  • Made a code change that makes it very likely that tasks will be executed in priority order if added to a TaskGraph in delayed execution mode.
  • Refactored internal TaskGraph scheduling to fix a design error that made it likely tasks would be needlessly reexecuted. This also simplified TaskGraph flow control and cause slight performance improvements.
  • Fixed an issue discovered when a scipy.sparse matrix was passed as an argument and add_task crashed on infinite recursion. Type checking of arguments has been simplified and now iteration only occurs on the Python set, dict, list, and tuple types.
  • Fixed an issue where the TaskGraph was not joining the worker process pool on a closed/join TaskGraph, or when the TaskGraph object was being deconstructed. This would occasionally cause a race condition where the TaskGraph may still have a cache .json file open. Discovered through a flaky build test.
  • Added functionality to the TaskGraph object to propagate log messages from workers back to the parent process. This only applies for cases where a TaskGraph instance is started with n_workers > 0.
  • Fixed an issue where a function that was passed as an argument would cause a reexecution on a separate run because the __repr__ of a function includes its pointer address.
  • Adjusted logging levels so that detailed task information is shown on DEBUG but basic status updates are shown in INFO.

0.5.2 (2018-06-20)

  • Fixing an issue where a Task would hang on a join if the number of workers in TaskGraph was -1 and a call to add_task has a non-None passed to target_path_list and the resulting task was \.joined after a second run of the same program.

0.5.1 (2018-06-20)

  • Fixing an issue where TaskGraph would hang on a join if the number of workers was -1 and a call to add_task has None passed to target_path_list.

0.5.0 (2018-05-04)

  • Taskgraph now supports python versions 2 and 3 (tested with python 2.7, 3.6).
  • Fixed an issue with taskgraph.TaskGraph that prevented a multiprocessed graph from executing on POSIX systems when psutil was installed.
  • Adding matrix-based test automation (python 2.7, python 3.6, with/without psutil) via tox.
  • Updating repository path to

0.4.0 (2018-04-18)

  • Auto-versioning now happens via setuptools_scm, replacing previous calls to natcap.versioner.
  • Added an option to TaskGraph constructor to allow negative values in the n_workers argument to indicate that the entire object should run in the main thread. A value of 0 will indicate that no multiprocessing will be used but concurrency will be allowed for non-blocking add_task.
  • Added an abstract class task.EncapsulatedTaskOp that can be used to instance a class that needs scope in order to be used as an operation passed to a process. The advantage of using EncapsulatedTaskOp is that the __name__ hash used by TaskGraph to determine if a task is unique is calculated in the superclass and the subclass need only worry about implementation of __call__.
  • Added a priority optional scalar argument to TaskGraph.add_task to indicates the priority preference of the task to be executed. A higher priority task whose dependencies are satisfied will executed before one with a lower priority.

0.3.0 (2017-11-17)

  • Refactor of core scheduler. Old scheduler used asynchronicity to attempt to test if a Task was complete, occasionally testing all Tasks in potential work queue per task completion. Scheduler now uses bookkeeping to keep track of all dependencies and submits tasks for work only when all dependencies are satisfied.
  • TaskGraph and Task .join methods now have a timeout parameter. Additionally join now also returns False if join terminates because of a timeout.
  • More robust error reporting and shutdown of TaskGraph if any tasks fail during execution using pure threading or multiprocessing.

0.2.7 (2017-11-09)

  • Fixed a critical error from the last hotfix that prevented taskgraph from avoiding recomputation of already completed tasks.

0.2.6 (2017-11-07)

  • Fixed an issue from the previous hotfix that could cause taskgraph to exceed the number of available threads if enough tasks were added with long running dependencies.
  • Additional error checking and flow control ensures that a TaskGraph will catastrophically fail and report useful exception logging a task fails during runtime.
  • Fixed a deadlock issue where a failure on a subtask would occasionally cause a TaskGraph to hang.
  • Task.is_complete raises a RuntimeError if the task is complete but failed.
  • More efficient handling of topological progression of task execution to attempt to maximize total possible CPU load.
  • Fixing an issue from the last release that caused the test cases to fail. (Don’t use 0.2.5 at all).

0.2.5 (2017-10-11)

  • Fixed a bug where tasks with satisfied dependencies or no dependencies were blocked on dependent tasks added to the task graph earlier in the main thread execution.
  • Indicating that psutil is an optional dependency through the setup function.

0.2.4 (2017-09-19)

  • Empty release. Possible bug with PyPI release, so re-releasing with a bumped up version.

0.2.3 (2017-09-18)

  • More robust testing on a chain of tasks that might fail because an ancestor failed.

0.2.2 (2017-08-15)

  • Changed how TaskGraph determines of work is complete. Now records target paths in file token with modified time and file size. When checking if work is complete, the token is loaded and the target file stats are compared for each file.

0.2.1 (2017-08-11)

  • Handling cases where a function might be an object or something else that can’t import source code.
  • Using natcap.versioner for versioning.

0.2.0 (2017-07-31)

  • Fixing an issue where types.StringType is not the same as types.StringTypes.
  • Redefined target in add_task to func to avoid naming collision with target_path_list in the same function.

0.1.1 (2017-07-31)

  • Fixing a TYPO on __version__ number scheme.
  • Importing psutil if it exists.

0.1.0 (2017-07-29)

  • Initial release.

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