Python sleep(): How to Add Time Delays to Your Code

Adding a Python sleep() Call With time.sleep()

Python has built-in support for putting your program to sleep. The time module has a function sleep() that you can use to suspend execution of the calling thread for however many seconds you specify.

Here’s an example of how to use time.sleep():

>>> import time
>>> time.sleep(3) # Sleep for 3 seconds

If you run this code in your console, then you should experience a delay before you can enter a new statement in the REPL.

You can test how long the sleep lasts by using Python’s timeit module:

$ python3 -m timeit -n 3 "import time; time.sleep(3)"
3 loops, best of 5: 3 sec per loop

Here, you run the timeit module with the -n parameter, which tells timeit how many times to run the statement that follows. You can see that timeit ran the statement 3 times and that the best run time was 3 seconds, which is what was expected.

The default number of times that timeit will run your code is one million. If you were to run the above code with the default -n, then at 3 seconds per iteration, your terminal would hang for approximately 34 days! The timeit module has several other command line options that you can check out in its documentation.

Let’s create something a bit more realistic. A system administrator needs to know when one of their websites goes down. You want to be able to check the website’s status code regularly, but you can’t query the web server constantly or it will affect performance. One way to do this check is to use a Python sleep() system call:

import time
import urllib.request
import urllib.error

def uptime_bot(url):
    while True:
        try:
            conn = urllib.request.urlopen(url)
        except urllib.error.HTTPError as e:
            # Email admin / log
            print(f'HTTPError: {e.code} for {url}')
        except urllib.error.URLError as e:
            # Email admin / log
            print(f'URLError: {e.code} for {url}')
        else:
            # Website is up
            print(f'{url} is up')
        time.sleep(60)

if __name__ == '__main__':
    url = 'http://www.google.com/py'
    uptime_bot(url)

Here you create uptime_bot(), which takes a URL as its argument. The function then attempts to open that URL with urllib. If there’s an HTTPError or URLError, then the program catches it and prints out the error. (In a live environment, you would log the error and probably send out an email to the webmaster or system administrator.)

If no errors occur, then your code prints out that all is well. Regardless of what happens, your program will sleep for 60 seconds. This means that you only access the website once every minute. The URL used in this example is bad, so it will output the following to your console once every minute:

HTTPError: 404 for http://www.google.com/py

Go ahead and update the code to use a known good URL, like http://www.google.com. Then you can re-run it to see it work successfully. You can also try to update the code to send an email or log the errors. For more information on how to do this, check out Sending Emails With Python and Logging in Python.

Adding a Python sleep() Call With Decorators

There are times when you need to retry a function that has failed. One popular use case for this is when you need to retry a file download because the server was busy. You usually won’t want to make a request to the server too often, so adding a Python sleep() call between each request is desirable.

Another use case that I’ve personally experienced is where I need to check the state of a user interface during an automated test. The user interface might load faster or slower than usual, depending on the computer I’m running the test on. This can change what’s on the screen at the moment my program is verifying something.

In this case, I can tell the program to sleep for a moment and then recheck things a second or two later. This can mean the difference between a passing and failing test.

You can use a decorator to add a Python sleep() system call in either of these cases. If you’re not familiar with decorators, or if you’d like to brush up on them, then check out Primer on Python Decorators. Let’s look at an example:

import time
import urllib.request
import urllib.error

def sleep(timeout, retry=3):
    def the_real_decorator(function):
        def wrapper(*args, **kwargs):
            retries = 0
            while retries < retry:
                try:
                    value = function(*args, **kwargs)
                    if value is None:
                        return
                except:
                    print(f'Sleeping for {timeout} seconds')
                    time.sleep(timeout)
                    retries += 1
        return wrapper
    return the_real_decorator

sleep() is your decorator. It accepts a timeout value and the number of times it should retry, which defaults to 3. Inside sleep() is another function, the_real_decorator(), which accepts the decorated function.

Finally, the innermost function wrapper() accepts the arguments and keyword arguments that you pass to the decorated function. This is where the magic happens! You use a while loop to retry calling the function. If there’s an exception, then you call time.sleep(), increment the retries counter, and try running the function again.

Now rewrite uptime_bot() to use your new decorator:

@sleep(3)
def uptime_bot(url):
    try:
        conn = urllib.request.urlopen(url)
    except urllib.error.HTTPError as e:
        # Email admin / log
        print(f'HTTPError: {e.code} for {url}')
        # Re-raise the exception for the decorator
        raise urllib.error.HTTPError
    except urllib.error.URLError as e:
        # Email admin / log
        print(f'URLError: {e.code} for {url}')
        # Re-raise the exception for the decorator
        raise urllib.error.URLError
    else:
        # Website is up
        print(f'{url} is up')

if __name__ == '__main__':
    url = 'http://www.google.com/py'
    uptime_bot(url)

Here, you decorate uptime_bot() with a sleep() of 3 seconds. You’ve also removed the original while loop, as well as the old call to sleep(60). The decorator now takes care of this.

One other change you’ve made is to add a raise inside of the exception handling blocks. This is so that the decorator will work properly. You could write the decorator to handle these errors, but since these exceptions only apply to urllib, you might be better off keeping the decorator the way it is. That way, it will work with a wider variety of functions.

There are a few improvements that you could make to your decorator. If it runs out of retries and still fails, then you could have it re-raise the last error. The decorator will also wait 3 seconds after the last failure, which might be something you don’t want to happen. Feel free to try these out as an exercise!

Adding a Python sleep() Call With Threads

There are also times when you might want to add a Python sleep() call to a thread. Perhaps you’re running a migration script against a database with millions of records in production. You don’t want to cause any downtime, but you also don’t want to wait longer than necessary to finish the migration, so you decide to use threads.

To prevent customers from noticing any kind of slowdown, each thread needs to run for a short period and then sleep. There are two ways to do this:

1. Use time.sleep() as before.
2. Use Event.wait() from the threading module.

Let’s start by looking at time.sleep().

import logging
import threading
import time

def worker(arg):
    while not arg["stop"]:
        logging.debug("worker thread checking in")
        time.sleep(1)

def main():
    logging.basicConfig(
        level=logging.DEBUG,
        format="%(relativeCreated)6d %(threadName)s %(message)s"
    )
    info = {"stop": False}
    thread = threading.Thread(target=worker, args=(info,))
    thread_two = threading.Thread(target=worker, args=(info,))
    thread.start()
    thread_two.start()

    while True:
        try:
            logging.debug("Checking in from main thread")
            time.sleep(0.75)
        except KeyboardInterrupt:
            info["stop"] = True
            logging.debug('Stopping')
            break
    thread.join()
    thread_two.join()

if __name__ == "__main__":
    main()

 0 Thread-1 worker thread checking in
 1 Thread-2 worker thread checking in
 1 MainThread Checking in from main thread
752 MainThread Checking in from main thread
1001 Thread-1 worker thread checking in
1001 Thread-2 worker thread checking in
1502 MainThread Checking in from main thread
2003 Thread-1 worker thread checking in
2003 Thread-2 worker thread checking in
2253 MainThread Checking in from main thread
3005 Thread-1 worker thread checking in
3005 MainThread Checking in from main thread
3005 Thread-2 worker thread checking in

import logging
import threading

def worker(event):
    while not event.isSet():
        logging.debug("worker thread checking in")
        event.wait(1)

def main():
    logging.basicConfig(
        level=logging.DEBUG,
        format="%(relativeCreated)6d %(threadName)s %(message)s"
    )
    event = threading.Event()

    thread = threading.Thread(target=worker, args=(event,))
    thread_two = threading.Thread(target=worker, args=(event,))
    thread.start()
    thread_two.start()

    while not event.isSet():
        try:
            logging.debug("Checking in from main thread")
            event.wait(0.75)
        except KeyboardInterrupt:
            event.set()
            break

if __name__ == "__main__":
    main()

Adding a Python sleep() Call With Async IO

Asynchronous capabilities were added to Python in the 3.4 release, and this feature set has been aggressively expanding ever since. Asynchronous programming is a type of parallel programming that allows you to run multiple tasks at once. When a task finishes, it will notify the main thread.

asyncio is a module that lets you add a Python sleep() call asynchronously. If you’re unfamiliar with Python’s implementation of asynchronous programming, then check out Async IO in Python: A Complete Walkthrough and Python Concurrency & Parallel Programming.

Here’s an example from Python’s own documentation:

import asyncio

async def main():
    print('Hello ...')
    await asyncio.sleep(1)
    print('... World!')

# Python 3.7+
asyncio.run(main())

In this example, you run main() and have it sleep for one second between two print() calls.

Here’s a more compelling example from the Coroutines and Tasks portion of the asyncio documentation:

import asyncio
import time

async def output(sleep, text):
    await asyncio.sleep(sleep)
    print(text)

async def main():
    print(f"Started: {time.strftime('%X')}")
    await output(1, 'First')
    await output(2, 'Second')
    await output(3, 'Third')
    print(f"Ended: {time.strftime('%X')}")

# Python 3.7+
asyncio.run(main())

In this code, you create a worker called output() that takes in the number of seconds to sleep and the text to print out. Then, you use Python’s await keyword to wait for the output() code to run. await is required here because output() has been marked as an async function, so you can’t call it like you would a normal function.

When you run this code, your program will execute await 3 times. The code will wait for 1, 2, and 3 seconds, for a total wait time of 6 seconds. You can also rewrite the code so that the tasks run in parallel:

import asyncio
import time

async def output(text, sleep):
    while sleep > 0:
        await asyncio.sleep(1)
        print(f'{text} counter: {sleep} seconds')
        sleep -= 1

async def main():
    task_1 = asyncio.create_task(output('First', 1))
    task_2 = asyncio.create_task(output('Second', 2))
    task_3 = asyncio.create_task(output('Third', 3))
    print(f"Started: {time.strftime('%X')}")
    await task_1
    await task_2
    await task_3                                 
    print(f"Ended: {time.strftime('%X')}")

if __name__ == '__main__':
    asyncio.run(main())

Now you’re using the concept of tasks, which you can make with create_task(). When you use tasks in asyncio, Python will run the tasks asynchronously. So, when you run the code above, it should finish in 3 seconds total instead of 6.

Adding a Python sleep() Call With GUIs

Command-line applications aren’t the only place where you might need to add Python sleep() calls. When you create a Graphical User Interface (GUI), you’ll occasionally need to add delays. For example, you might create an FTP application to download millions of files, but you need to add a sleep() call between batches so you don’t bog down the server.

GUI code will run all its processing and drawing in a main thread called the event loop. If you use time.sleep() inside of GUI code, then you’ll block its event loop. From the user’s perspective, the application could appear to freeze. The user won’t be able to interact with your application while it’s sleeping with this method. (On Windows, you might even get an alert about how your application is now unresponsive.)

Fortunately, there are other methods you can use besides time.sleep(). In the next few sections, you’ll learn how to add Python sleep() calls in both Tkinter and wxPython.

import tkinter
import time

class MyApp:
    def __init__(self, parent):
        self.root = parent
        self.root.geometry("400x400")
        self.frame = tkinter.Frame(parent)
        self.frame.pack()
        b = tkinter.Button(text="click me", command=self.delayed)
        b.pack()

    def delayed(self):
        time.sleep(3)

if __name__ == "__main__":
    root = tkinter.Tk()
    app = MyApp(root)
    root.mainloop()

import tkinter

class MyApp:
    def __init__(self, parent):
        self.root = parent
        self.root.geometry("400x400")
        self.frame = tkinter.Frame(parent)
        self.frame.pack()
        self.root.after(3000, self.delayed)

    def delayed(self):
        print('I was delayed')

if __name__ == "__main__":
    root = tkinter.Tk()
    app = MyApp(root)
    root.mainloop()

import wx

class MyFrame(wx.Frame):
    def __init__(self):
        super().__init__(parent=None, title='Hello World')
        wx.CallLater(4000, self.delayed)
        self.Show()

    def delayed(self):
        print('I was delayed')

if __name__ == '__main__':
    app = wx.App()
    frame = MyFrame()
    app.MainLoop()

Conclusion

With this tutorial, you’ve gained a valuable new technique to add to your Python toolbox! You know how to add delays to pace your applications and prevent them from using up system resources. You can even use Python sleep() calls to help your GUI code redraw more effectively. This will make the user experience much better for your customers!

To recap, you’ve learned how to add Python sleep() calls with the following tools:

• time.sleep()
• Decorators
• Threads
• asyncio
• Tkinter
• wxPython

Now you can take what you’ve learned and start putting your code to sleep!

Take the Quiz: Test your knowledge with our interactive “Python time.sleep()” quiz. You’ll receive a score upon completion to help you track your learning progress:

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Interactive Quiz

Python time.sleep()

In this quiz, you'll revisit how to add time delays to your Python programs.