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Why Python’s Multiprocessing Imports __main__ on Windows: Explained

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Python is one of the most popular programming languages used today. It’s known for its simplicity and versatility, which makes it an excellent choice for a variety of applications. One particular feature of Python that’s gained a lot of popularity is multiprocessing; this functionality allows programmers to take advantage of multiple CPUs to run their code more efficiently.

However, there’s been some confusion among Python developers about why the multiprocessing module in Python imports __main__ on Windows, while it doesn’t do so on other platforms. This issue has been raised several times on various forums, indicating that a significant number of people are interested in understanding this behavior.

If you’re among those who’ve been wondering about this behavior, you’ll be glad to know that the explanation is relatively straightforward. In fact, it’s rooted in a fundamental difference in how processes are spawned on Windows and other platforms. Understanding this difference is crucial as it allows you to write more efficient code and utilize your machine’s resources more effectively.

To get the full details on why Python’s multiprocessing imports __main__ on Windows and how it works, you’ll need to read this informative article. Within it, you’ll find clear explanations and examples that will help you grasp the concept and make the most of the multiprocessing functionality in Python. So go ahead and dive into the article to learn everything you need to know about this topic!

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“Why Does Python’S Multiprocessing Module Import __main__ When Starting A New Process On Windows?” ~ bbaz


Python is one of the most popular programming languages in the world. This is partly due to its ease of use and versatility, as it can be used for a wide range of applications, including data science, web development, and automation. One of the key features of Python is its ability to perform multiprocessing, which allows for tasks to be executed simultaneously across multiple processor cores. In this article, we will explore why Python’s multiprocessing imports __main__ on Windows and how it is different from other operating systems.

What is multiprocessing?

Multiprocessing is the ability to execute multiple processes or threads simultaneously on a single computer. This is achieved by dividing a task into smaller subtasks that can be executed independently by different processor cores. Multiprocessing allows programs to take advantage of modern hardware with multiple cores, resulting in faster and more efficient execution.

Why does Python import __main__?

When running a Python script, the interpreter sets the __name__ variable to the name of the current module. If the script is being run as the main program, then __name__ is set to the string ‘__main__’. When using multiprocessing in Python on Windows, the child processes need to import the module containing the multiprocessing code. However, if the module was run as the main program, then the child processes would also execute the main program, which is not desired. To avoid this problem, Python imports __main__ instead of the main program when creating child processes on Windows.

How is this different on other operating systems?

On other operating systems, such as Linux, the behavior of multiprocessing is slightly different. When using multiprocessing on Linux, the child processes import the module directly and do not import __main__. This is because the fork() system call is used to create child processes, which creates a copy of the current process, including all of its global state. In this case, the child process does not need to import __main__ separately since it already has access to the global state of the parent process.

Why is this important?

Understanding why Python imports __main__ when using multiprocessing on Windows is important because it can affect the way you structure your code. If your main program contains code that should not be executed by child processes, then it’s important to keep that code inside a block protected by an if __name__ == ‘__main__’ statement. This ensures that the code is only executed when the module is run as the main program, and not when it’s imported as a module by child processes.

How does this impact performance?

The impact of importing __main__ on performance is negligible, and should not have a significant effect on the overall execution time of your program. The cost of importing the module is small compared to the actual execution time of the program, and the benefits of using multiprocessing far outweigh any potential performance impact.

Table Comparison: Multiprocessing on Windows vs Linux

Operating System Multiprocessing Behavior
Windows Child processes import __main__ instead of the main program
Linux Child processes import the module directly and do not import __main__


In conclusion, understanding why Python imports __main__ when using multiprocessing on Windows is essential to writing efficient and maintainable code. While this behavior may be slightly different from other operating systems, it is easy to work around by protecting your main program with the if __name__ == ‘__main__’ statement. When used correctly, multiprocessing can provide significant performance gains and increase the efficiency of your code. Overall, Python’s multiprocessing features are a powerful tool for taking full advantage of modern hardware and improving the performance of your programs.

Thank you for taking the time to read our article on why Python’s multiprocessing imports __main__ on Windows. We hope that we were able to provide some valuable insight into this topic and help you better understand the inner workings of Python’s multiprocessing module.

When it comes to writing efficient and effective code, understanding how multiprocessing works is crucial. By importing __main__ on Windows, Python is able to start a new process and execute the target function without creating an entirely new Python interpreter. This not only saves time but ensures that your code runs smoothly no matter how many processes you need to run concurrently.

In conclusion, understanding the intricacies of Python’s multiprocessing module is important for any developer looking to create scalable and efficient code. By knowing why Python imports __main__ on Windows, you can take full advantage of the benefits of multiprocessing and streamline your code to increase performance and reduce lag. We hope that you found this article informative and useful and look forward to providing more in-depth guides on Python programming in the future.

Here are some common questions that people may ask about Python’s multiprocessing imports __main__ on Windows:

  1. What is multiprocessing in Python?
  2. Why does Python use __name__ == __main__ in multiprocessing?
  3. What is the purpose of __name__ == __main__ in Python?
  4. How does multiprocessing work in Python on Windows?
  5. What is the difference between multiprocessing on Windows and Unix-like systems?


  1. Multiprocessing in Python refers to the ability to run multiple processes concurrently in a single program. This can help improve performance by taking advantage of multi-core processors and distributing workloads across multiple processes.
  2. Python uses __name__ == __main__ in multiprocessing to ensure that each child process starts with a clean state and does not import unnecessary modules from the parent process. This is especially important on Windows, where importing certain libraries in the parent process can cause issues with pickling and serialization.
  3. The purpose of __name__ == __main__ in Python is to allow a script to be used both as a standalone program and as a module to be imported into another program. When a script is executed, the __name__ variable is set to __main__. When the same script is imported into another program, the __name__ variable is set to the name of the module.
  4. Multiprocessing works in Python on Windows by using subprocesses to run code concurrently. The multiprocessing module in Python allows developers to create and manage multiple subprocesses, each with their own memory space and execution context. On Windows, the module uses a combination of pickling and inter-process communication to pass data between processes.
  5. The main difference between multiprocessing on Windows and Unix-like systems is that Windows does not have native support for forking processes, which is a common method used in Unix-like systems to create new processes. Instead, Windows uses subprocesses to run code concurrently, which can be less efficient than forking. Additionally, Windows has different limitations on memory usage and file handling, which can affect how multiprocessing is implemented.