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@hgrecco hgrecco/tasks.py
Last active Aug 15, 2017

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Simple concurrent tasks with dependencies in Python.
from collections import defaultdict
import concurrent.futures
class TasksGroup(object):
"""A group of tasks with dependencies.
>>> tasks = TasksGroup()
>>> @tasks.add()
... def first():
... print(1)
>>> @tasks.add(first)
... def second():
... print(2)
>>> import concurrent.futures
>>> with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
... tasks.run(executor)
1
2
"""
def __init__(self):
#: Maps functions to their upstream connections (dependencies)
self._upstream = {}
#: Maps functions to their downstream connections
self._downstream = defaultdict(set)
def add(self, *depends_on):
"""Add a task to the group.
"""
def decorator(func):
self._upstream[func] = depends_on
for f in depends_on:
self._downstream[f].add(func)
return func
return decorator
def run(self, executor):
"""Run tasks respecting the dependencies using an executor.
"""
count_deps = {task: len(deps) for task, deps in self._upstream.items()}
base_tasks = {task for task, count in count_deps.items() if count == 0}
future_to_task = {executor.submit(task): task for task in base_tasks}
not_done = set(future_to_task.keys())
wait = concurrent.futures.wait
FIRST_COMPLETED = concurrent.futures.FIRST_COMPLETED
while not_done:
done, not_done = wait(not_done, return_when=FIRST_COMPLETED)
for future in done:
task = future_to_task[future]
for f in self._downstream[task]:
count_deps[f] -= 1
if count_deps[f] == 0:
fut = executor.submit(f)
future_to_task[fut] = f
not_done.add(fut)
# Example
import time
tasks = TasksGroup()
@tasks.add()
def f1():
time.sleep(.4)
print(time.time(), 1)
@tasks.add(f1)
def f2():
time.sleep(4)
print(time.time(), 2)
@tasks.add(f1)
def f3():
time.sleep(.4)
print(time.time(), 3)
@tasks.add(f3)
def f4():
time.sleep(.4)
print(time.time(), 4)
with concurrent.futures.ThreadPoolExecutor(max_workers=2) as executor:
tasks.run(executor)
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