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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11In Python, use ThreadPoolExecutor.map() when you want results in the same order as the input items. If you need to submit tasks individually, keep the returned futures in a list and call result() in that list’s order. Use as_completed() only when you want to handle results as tasks finish; add each result to its original index if the final collection must still be ordered.
Use Executor.map() for ordered results
map() is the most direct option when every item goes through the same function. It runs calls asynchronously, potentially concurrently, but yields results in the order of the input iterables—not the order in which tasks finish. See the Python 3.13 concurrent.futures documentation.
from concurrent.futures import ThreadPoolExecutor
def work(item):
return process(item)
with ThreadPoolExecutor() as executor:
results = list(executor.map(work, items))Here, results[i] corresponds to items[i]. Converting the iterator to a list collects all results before execution continues past the block. You can also iterate over the returned iterator directly if you want to consume results one at a time while preserving input order.
Keep futures in submission order when using submit()
Use submit() when calls need different arguments or otherwise need to be created individually. Each call returns a Future. Store those futures in the order you submit them, then retrieve their results in that same order:
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from concurrent.futures import ThreadPoolExecutor
with ThreadPoolExecutor() as executor:
futures = [executor.submit(work, item) for item in items]
results = [future.result() for future in futures]result() waits for its future if necessary, returns its value when ready, and raises the task’s exception when that result is retrieved. Because this code waits on futures in list order, a slow earlier task can hold up access to later results that have already finished. This affects when the caller receives results, not the order in which the tasks run.
Handle results as they finish, but assemble them in order
as_completed() yields futures in completion order. To process results immediately as they become available and still build an ordered final list, associate each future with its input index, then place its value in the matching slot:
from concurrent.futures import ThreadPoolExecutor, as_completed
with ThreadPoolExecutor() as executor:
futures = {
executor.submit(work, item): index
for index, item in enumerate(items)
}
results = [None] * len(items)
for future in as_completed(futures):
index = futures[future]
results[index] = future.result()Each completed value is handled as soon as its future finishes; after the loop, results is in input order. If None is a valid task result, the initialized slots are still unambiguous once all futures have been processed. An exception from future.result() is raised at that point, so decide whether to let it stop the loop or catch and record it for your application.
Exceptions, timeouts, and Python-version details
With map(), an exception from a task is raised when the iterator reaches that task’s result. In Python 3.13, the documented timeout is measured from the original map() call: requesting a result that has not become available within that interval raises TimeoutError. See the Python 3.13 API documentation for the precise behavior.
Python 3.14 documentation adds buffersize to Executor.map(), limiting the number of submitted tasks whose results have not yet been yielded. The same documentation specifies that chunksize has no effect for ThreadPoolExecutor; it is not a thread-pool batching control. Check the Python 3.14 concurrent.futures documentation before using version-specific arguments.
Quick Recap
Which pattern should you choose?
| Need | Pattern | Ordering behavior |
|---|---|---|
| Same function applied to input items; ordered output | executor.map(work, items) |
Yields results in input order |
| Individually configured calls; ordered output | Store submit() futures in a list and call result() in list order |
Retrieves results in submission order; may wait behind an earlier slow task |
| Handle each result as soon as it finishes; ordered final collection | as_completed() plus an index-to-future mapping |
Processes in completion order and places values into input-order slots |
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