Windows typically uses spawn for multiprocessing; modern POSIX defaults differ by version, and macOS also favors spawn.
Understanding the Concept
Windows typically uses spawn for multiprocessing; modern POSIX defaults differ by version, and macOS also favors spawn. Code should work when the child imports the main module.
Never select a start method only to make a faulty imported module appear to work.
How to reason about concurrent execution
Workers may start and finish in a different order on every run. Focus on ownership of data, how work and errors move between workers, and what happens when one worker stops early. Locks protect invariants, queues exchange messages, and explicit shutdown logic prevents blocked or orphaned workers.
Concurrency adds overhead and failure modes. Begin with a working sequential solution, choose bounded worker counts and explicit timeouts, then measure whether overlap or parallel execution improves the target workload.
Worked Example
Read the code from the first line to the last. Identify which values are inputs, which operation is being demonstrated and what should happen before running it.
import multiprocessing as mp
import platform
print(platform.system())
print(mp.get_all_start_methods())
# Always protect worker startup with if __name__ == "__main__":Walkthrough: What Each Part Does
- The imports make multiprocessing, platform available. These are standard-library building blocks unless the example explicitly names an external tool.
- The last lines coordinate completion and inspect results. Actual worker scheduling may differ between executions.
This demonstration focuses on one practical behavior. Before changing several lines, vary one input or one configuration choice and check whether the result changes for the reason you expected.
Example Output
Linux
['fork', 'spawn', 'forkserver']The output above was captured by running the example in the available Python environment. The order of concurrent events, durations and platform-dependent values can legitimately differ on your computer.
Important Behavior and Edge Cases
Writing Linux-fork-dependent code that unexpectedly recurses or fails on Windows.
Never select a start method only to make a faulty imported module appear to work.
Practice Exercise
Test one guarded process worker on Windows and Linux and compare the start methods.
Write down the result you expect before running your changed code. Check at least one normal case and one boundary or failure case. If your example starts workers or opens resources, verify how they are cleaned up.
Further reading: official documentation and API reference.
Common Beginner Mistakes
- Writing Linux-fork-dependent code that unexpectedly recurses or fails on Windows.
- Forgetting to handle worker errors and shutdown, even when the normal completion path appears correct.
Key Takeaways
- Windows typically uses spawn for multiprocessing; modern POSIX defaults differ by version, and macOS also favors spawn.
- Never select a start method only to make a faulty imported module appear to work.
- Practice: Test one guarded process worker on Windows and Linux and compare the start methods.