{"id":1551,"date":"2019-07-05T20:34:23","date_gmt":"2019-07-05T20:34:23","guid":{"rendered":"http:\/\/www.afternerd.com\/blog\/?p=1551"},"modified":"2019-07-05T20:34:23","modified_gmt":"2019-07-05T20:34:23","slug":"python-sleep-function-explained","status":"publish","type":"post","link":"https:\/\/www.afternerd.com\/blog\/python-sleep-function-explained\/","title":{"rendered":"Python: Sleep Function Explained"},"content":{"rendered":"

In Python, or any other programming language, sometimes you want to add a time delay in your code before you proceed to the next section of the code.<\/p>\n

If this is what you want to do, then you should use the sleep<\/strong> function from the time<\/a> module.<\/p>\n

First, I will start by discussing how to use Python’s sleep<\/strong> function. After that I will talk more about some frequently asked questions and how the sleep<\/strong> function is actually implemented under the hood.<\/p>\n

Sleep function<\/h2>\n

Like I mentioned, Sleep is a Python built-in function in the time<\/strong> module.<\/p>\n

So in order to use sleep<\/strong>, you will need to import the time<\/strong> module first.<\/p>\n

sleep takes only one argument which is the period of time in seconds that you want to sleep for.<\/p>\n

Using the sleep<\/strong> function in Python 2<\/strong> and Python 3<\/strong> is exactly the same, so you won’t need to worry about which version of Python your code is running on.<\/p>\n

It is really very simple and straight-forward. Let’s walk through some examples.<\/p>\n

Python 2<\/h3>\n

Let’s say you want to use sleep<\/strong> to add a 10-second delay.<\/p>\n

In the following example, I print out the elapsed time between invoking<\/em> the sleep function and after it returns<\/em>.<\/p>\n

import time\r\n\r\n# record start time\r\nstart_time = time.time()\r\n# sleep for 10 seconds\r\ntime.sleep(10)\r\n# print elapsed time\r\nelapsed_time = time.time() - start_time\r\nprint(elapsed_time)\r\n\r\n# output: 10.000149965286255<\/code><\/pre>\n
As you can see, the elapsed time is very close to 10 seconds which is what we expect.<\/p>\n
Python 3<\/h3>\n
No changes in Python 3.<\/p>\n
You can use the sleep<\/span> function in Python 2 exactly as you do in Python 2.<\/p>\n
Frequently Asked Questions<\/h2>\n
Q: How to use the sleep function to sleep for a time period that is less than a second, for example milliseconds or microseconds?<\/h3>\n
Great question.<\/p>\n
The sleep function actually takes a floating point number as an argument, not an integer.<\/p>\n
And that means, you can pass any fraction of a second as an argument.<\/p>\n
For example:<\/p>\n
import time\r\n\r\n# sleep for half a second\r\ntime.sleep(0.5)\r\n\r\n# sleep for 1 millisecond\r\ntime.sleep(1 * 10**(-3))\r\n\r\n# sleep for 1 microsecond\r\ntime.sleep(1 * 10**(-6))<\/code><\/pre>\n
Q: How to use the sleep function to sleep until a specific time?<\/h3>\n
This is a very common question.<\/p>\n
Depending on what you are trying to do, there are two different ways to achieve this.<\/p>\n
The straight-forward way is to precompute the delay (in seconds) until that specific time before you call the sleep function.<\/p>\n
Another way is to use the event.wait<\/a> function from the event<\/strong> module.<\/p>\n
If what you are trying to do is to block a thread until a condition<\/strong> happens, it is much better to use wait()<\/span><\/strong>.<\/p>\n
Q: [Advanced] How is sleep actually implemented?<\/h3>\n
This is a fantastic question.<\/p>\n
sleep<\/strong> is not actually implemented by Python.<\/p>\n
Well, it is, but it is simply a wrapper around something else that is implemented by the Operating System.<\/p>\n
All Operating Systems have a Sleep()<\/span> system call.<\/p>\n
If you don’t know what a System Call<\/a> is, then you should!<\/p>\n
here is the book that I recommend for learning about Operating Systems concepts.<\/a><\/p>\n
But either way, you can think of a System Call as an API or an interface that the OS provides for user-space programs to interact with the OS.<\/p>\n
So what does an OS do when it receives a Sleep System Call?<\/em><\/p>\n
Basically what the OS will do is that it will suspend the process (your program) from being scheduled on the CPU for the time period that you specified.<\/p>\n
Notice that this is completely different from adding delay using a dummy for loop that does nothing (which is something you should NEVER do).<\/p>\n
In the case of a dummy for loop, your process is actually still running<\/em> on the CPU.<\/p>\n
But in the case of Sleep()<\/strong>, your process will be effectively dormant<\/em> for a while until the OS starts scheduling it again on the CPU.<\/p>\n
Learning Python?<\/h3>\n
Check out the Courses section!<\/a><\/p>\n\n
Featured Posts<\/h2>\n