Python Multithreading Tutorial: Concurrency and Parallelism

Python is a prevalent language for writing concurrent and parallel applications. In this article, we will look at the differences between Python threading vs. multiprocessing. We will focus on how both of these methods can be used to improve concurrency in your applications. We will also look at some of the differences between them and how they can be used together to create better solutions. There is a difference in the nature of concurrency in multithreading vs asyncio. OS controls when one thread is kicked out and the other is given a chance (allocated CPU).

Multithreading is useful for IO-bound processes, such as reading files from a network or database since each thread can run the IO-bound process concurrently. Threading is a mechanism for creating independent threads within a process. Each thread can have its own memory space and access to disk storage. If a program needs to access shared resources like files or database connections, it must do so using locks that restrict access to those resources for all of the threads inside the program at once.

This is the central difference between the two modules and supports all other differences. The focus of the threading module is a naive thread managed by the operating system. The rest of the “threading” module provides tools to work with threading.Thread instances. The Python examples are skeleton code snippets that you can replace with your functions and you’re good to go. You’ll often want your threads to be able to use or modify variables common between threads. Connect and share knowledge within a single location that is structured and easy to search.

  • Threading is one of the most well-known approaches to attaining parallelism and concurrency in Python.
  • You can and are free to do so, but your code will not benefit from concurrency because of the GIL.
  • This means that only one thread can run at a time in a Python process, unless the GIL is released, such as during I/O or explicitly in third-party libraries.
  • Therefore, prepare_main() supports setting multiple
    values at once.
  • When run, you will see that multiple Python processes are created, one per core.

Since each of the CPUs runs in parallel, you’re effectively able to run multiple tasks simultaneously. An example would be trying to calculate a sum of all elements of a huge list. If your machine has 8 cores, you can “cut” the list into 8 smaller lists and calculate the sum of each of those lists separately on separate core and then just add up those numbers. To summarize you would typically want to use threading when your operations are IO bound. As we know the GIL would prevent 20 parallel threads from running.

Use Cases for Threading

This works by using the current thread state as
the runtime context. You can see that the order is predictable and it is always same and synchronous. Whereas with multithreading you cannot predict the order (always different). There is a nice discussion on this link regarding the advantages of asyncio over threads. Just and addition here, would not working in say jupyter notebook very well, as the notebook already has a asyncio loop running. In some cases, it may not help (though it likely does not hurt), while in other cases it may help a lot.

  • The processor is hardly breaking a sweat while downloading these images, and the majority of the time is spent waiting for the network.
  • The new class can be created and the inherited Process.start() method called to execute the contents of the run() method in a new thread of execution.
  • In this Python multithreading example, we will write a new module to replace
  • On my laptop, this script took 19.4 seconds to download 91 images.

As you all know, data science is the science of dealing with large amounts of data and extracting useful insights from them. Using processes and process pools via the multiprocessing module in Python is probably the best path toward achieving this end. The multiprocessing module provides powerful and flexible concurrency, although is not suited for all situations where you need to run a background task. You can and are free to do so, but your code will not benefit from concurrency because of the GIL. It will likely perform worse because of the additional overhead of context switching (the CPU jumping from one thread of execution to another) introduced by using threads. CPUs are very fast, and we often have more than one CPU core on a single chip in modern computer systems.

The new class can be created and the inherited Thread.start() method called to execute the contents of the run() method in a new thread of execution. In threaded programming the typical synchronization primitives are
types like mutexes. However, interpreters cannot share objects which means they cannot
share threading.Lock objects. Most objects will be shared through queues (Queue), as interpreters
communicate information between each other.

What is Multiprocessing in Python

This can trip up new users of asyncio a lot and cause a lot of rework to be done if you didn’t start programming with asyncio in mind from the beginning. After knowing the relationship between processes and threads, now we could forward to dig into the details of them as well as practically using python code to see how they speed up the program. In this article we will look at threading vs multiprocessing within Python, and when you should use one over the other. Now we’ll look into how we can reduce the running time of this algorithm.

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Now, when you need to deal with a process, there might be a case when you are stuck with a mutual exclusion lock. These locks are brought into effect at times of critical section problems. When a thread has to stop during its waiting time (for a thread primitive), the thread lock is accessed. With these factors in mind, together with the takeaways above, you should be able to make the decision.

Threading vs Multiprocessing in Python

After the download is finished, the worker signals the queue that that task is done. This is very important, because the Queue keeps track of how many tasks were enqueued. The call to queue.join() would block the main thread forever if the workers did not signal that they completed a task. A function can be run in a new process by creating an instance of the multiprocessing.Process class and specifying the function to run via the “target” argument, just like we would with a threading.Thread. The Process.start() method can then be called which will execute the target function in a new thread of execution.

A Minimal API

You can find this client ID from the dashboard of the application that you have registered on Imgur, and the response will be JSON encoded. Downloading the image is an even simpler task, as all you have to do is fetch the image by its URL and write it to a file. Therefore, the tasks we execute with a threading.Thread should be tasks that involve IO operations. An IO-bound task is a type of task that involves reading from or writing to a device, file, or socket connection. Thread-based concurrency supports parallelism, whereas process-based concurrency supports full parallelism.

This includes Interpreter objects that represent
the underlying interpreters. The module will also provide a basic
Queue class for communication between interpreters. Finally, we
will add a new concurrent.futures.InterpreterPoolExecutor based
on the interpreters module. With Python with GIL, the only way to unleash the power of multicores is to use multiprocessing (there are exceptions to this as mentioned below). If the nature of your problem requires intense communication between concurrent routines, multithreading is the natural way to go. Unfortunately with CPython, true, effectively parallel multithreading is not possible due to the GIL.

The interpreters module does not provide any such dedicated
synchronization primitives. Supporting sharing of all objects is possible (via pickle)
but not part of this proposal. For one thing, it’s helpful to know
that only an efficient implementation is being used. Furthermore,
for mutable objects pickling python multiprocessing vs threading would violate the guarantee that “shared”
objects be equivalent (and stay that way). Without a stdlib module, users are limited to the
C API, which restricts how much
they can try out and take advantage of multiple interpreters. This shared state is called the interpreter state

Now that we have all these images downloaded with our Python ThreadPoolExecutor, we can use them to test a CPU-bound task. We can create thumbnail versions of all the images in both a single-threaded, single-process script and then test a multiprocessing-based solution. This is almost the same as the previous one, with the exception that we now have a new class, DownloadWorker, which is a descendent of the Python Thread class.

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