Python wait until function finishes24.10.2020
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Python-wait for one process to finish before starting next one.
Hello, i'm new to python I have script which go through JSON file,look for values in email field, for each value found it should create new EC2 instance with field in email adrress part after removed as user name, for every user it should prompt for number of instances. It all works fine when one user is in JSON file, but how to make python process one user at a time if we have more than one email address in JSON file, first create machine for one user,wait until finishes, then prompt for number of instances for other user, wait until machine is created, then prompt for number of instances for 3-rd user, and so on Code:.
If I understand this correctly. Take each "customer" found in data['customers'] and run the below code? Last edited by linux starter; at PM.It is done for back-to-back execution of functions acting like a queue or chain of functions. To do that there is two popular way described below. Use of setTimeout function: In order to wait for a promise to finish before returning the variable, the function can be set with setTimeoutso that the function waits for a few milliseconds.
Use of async or await function: This method can be used if the exact time required in setTimeout cannot be specified. The async keyword is used to create an asynchronous function that returns a promise that is either rejected or resolved. The promise is rejected when there is an uncaught exception thrown from that function or it is resolved otherwise.
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Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. Inside my for there is another for loop then that 2nd for loop calls a function that does another for loop.
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Now I want the 2nd for loop to wait until the 3rd loops finish its job. How to do that? By default python runs synchronously this is true of nearly all programming languages.
This means that any statement will completely finish before the next statement begins. You would have to go out of your way to change this behavior. As an aside: you have a problem where you refer to x inside your function but it is not passed in. This might work at the moment, because x is being treated as a global variable, but it would be more appropriate to let your function accept it, or to change your glob.
Learn more. Wait until function finish executing in Python Ask Question. Asked 3 years, 5 months ago. Active 3 years, 5 months ago. Viewed 19k times. I am new to Python I have the codes below: Inside my for there is another for loop then that 2nd for loop calls a function that does another for loop. Thanks def GetArea img : os. It'll do that whether you want it to or not. You have to work extra hard to get programs to do two things simultaneously.
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Post as a guest Name. Email Required, but never shown. The Overflow Blog. Featured on Meta. Feedback on Q2 Community Roadmap. Technical site integration observational experiment live on Stack Overflow. Question Close Updates: Phase 1. Related Hot Network Questions.For many types of applications, at times it is necessary to pause the running of the program until some external condition occurs.
You may need to wait until another thread finishes, or maybe until a new file appears in a directory on disk that is being watched. In these and many other situations you will need to figure out a way to make your script wait, and this isn't as easy as it sounds if you want to do it properly!
In this article I'm going to show you a few different ways to wait. I'm going to use Python for all the examples, but the concepts I'm going to present apply to all programming languages. To keep this example simple, I have coded this function with a time. When the function reaches the end, a result global variable is set with the result of this made-up calculation, which is going to be, obviously, the number forty-two. The main application function starts the background calculation in a separate thread, then waits for the thread to complete its work and finally prints the result global variable.
The version of this function that you see above does not have the waiting part implemented, you can see a TODO comment in the place where the wait needs to take place. In the following sections I'm going to show you a few different ways to implement this wait, starting from the worst and working my way up to the best. If you want to experience it, you can try the script on your system. As soon as the script runs, open your Task Manager on Windows, Activity Monitor on Mac or maybe top if you prefer the command line.
Look at CPU usage and note how it goes through the roof. This while-loop appears to be an empty loop, and in fact it mostly is, with the only exception that the loop exit condition needs to be checked over and over again to determine when the loop should exit.
So while the loop body is completely empty, Python is forced to continuously evaluate result is Noneand actually, the fact that the loop is empty makes Python concentrate fully on repeating this evaluation as fast as it possibly can, burning a lot of CPU cycles, and making everything else running on that CPU much slower! This type of wait loop is often called a busy wait. And a CPU that is stuck doing a lot of work over nothing as in this case is said to be spinning.
Never do this. It is interesting that in the busy waiting example from the previous section, you would think that having an empty loop should give less work to the CPU, but in fact the contrary happens. So the obvious improvement to the previous solution is to add something inside the while-loop that puts a brake to the CPU frantically evaluating the while-loop exit condition.
I'm sure a lot of you can guess what we can do inside the loop to slow things down a bit.I am using a particular python code base which, in a Linux system, solves the problem I am working on I am using Windows.
A major part of the python code's task is to generate and run certain matlab scripts. The code starts matlab subprocesses by an os. I am wondering how I could rewrite the code to ensure that the python script waits for matlab subprocess to terminate.
Module subprocess, specifically subprocess. Popenhas a wait method. Here is a typical example:. This code doesn't crash but it doesn't wait either, I'm starting to wonder if the problem lies with matlabb I am currently using time. I just wish something less ugly could be done. The problem apparently has to do with the way the command line matlab launcher operates in Windows. The launcher spawns a separate process and then immediately exits unless you use the -wait flag.
Use communicate to avoid that. I had a similar problem when I was trying to run Excel. If would work if Excel wasn't running, but would fail when other instances of Excel were already in memory. The code shown below worked for me. I think it might work with programs like Matlab that spawn subprocesses, because child processes inherit stdin and stdout from their parent.
Running a subprocess and waiting for it to return. Hello, First of all, I'm sorry if my question seems silly, but I am very unfamiliar with Python. Any help would be greatly appreciated, Thank you very much in advance, Ilya. I tried to substitute os. Ilya P. S: I posted this before Ene Uran's post, I'll try your suggestion now.
Popen [self. Any suggestions? Thank you, Ilya Module subprocess, specifically subprocess. May be because your code block indentations are screwed up. Hello, This code doesn't crash but it doesn't wait either, I'm starting to wonder if the problem lies with matlabb PIPE p.
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Waiting until a Thread is ready in Python
Promote Question.Concurrency in Python can be tricky. Users of the threading module know how easy it is to get wrong. One thing that bit me quite a bit was how easy it was to get races when using threading. The following example is a fairly common race condition developers run into when using threading. Thread - we start a program in a thread, then operate on it before it is fully ready, causing our program to explode:. Turns out thread. Instead, it continued on, causing us to call program.
That's because program. So how can we be sure our ChildProgram is fully ready before operating on it from our main thread?
The correct way to solve this problem is by utilizing a threading. Eventwhich allows:. Now that we've setup our ready Eventwe were able to ensure that our ChildProgram is fully initialized, and program.
One might be tempted to call Thread. This, however, would be a mistake because:. The Race Condition The following example is a fairly common race condition developers run into when using threading. We define a ChildProgram to run in a separate Thread. Then we call thread. Finally, we call program. But why? Using the Event Synchronization Primitive The correct way to solve this problem is by utilizing a threading.
Eventwhich allows In our case, we want the main thread to wait until the child thread to signal that it is ready. This, however, would be a mistake becauseIn this tutorial example we propose to discover threads using a bank account use case : we will provide a BankAccount class in charge of managing your money — deposit only!
Then we will create customers that will make deposit on your account using threads, all at the same time. So quickly we should be facing data concurrent access issues that we will clearly show and then solve using locks. To do this we will use threads by importing the threading module. To create a thread we just have to instantiate an object from threading.
The Thread class accepts many arguments. The Thread class also accepts args or kwargs arguments containing the attributes to pass to the target object when running the thread. Finally the Thread.
Running a subprocess and waiting for it to return.
Below is the code to run 10 threads at once making a deposit :. By the way, you could also create the thread by using instance method in place of the class method, this way :. You can also provide a timeout parameter in seconds real numbers accepted to the join method. This will prevent from waiting too long or block your program if a thread is in an undefined state. So to wait until all the threads have finished their deposit, we could update our code like this :.
When managing many processes accessing and updating the same data at the same time we may see concurrent access issues. That is a concurrent access issue. Before seing how to solve and prevent from these, we are going to try to create one. It could have been in any order. This means that our system is too fast or the task so short that each thread execution is finished before the next thread creation and execution starts or before Python switch to another thread.
So we could try to overhead the system by running or deposits at the same time :. But no side effect appears on my computer. Task is certainly too fast. But, once again, on my computer : no side effect. So lets update the deposit method like this :. And run it again! This time it is clearly slower and in history logs we see that threads executions are mixed all together:. But, once again final result is good with a value of after the last thread execution.