How to Kill a Subprocess in Python

While working in Python, there might be situations where you need to execute external programs or scripts during runtime. In such cases, you can use the subprocess module, which allows you to trigger new processes.

In such situations, you may need to terminate or kill a subprocess due to various reasons, such as a subprocess being unresponsive or taking much time to get completed.

In this article, we will discuss various methods to kill a subprocess in Python with code examples.

What is a Subprocess in Python?

Before we delve into the methods of killing subprocesses, let’s briefly understand what a subprocess is in Python. A subprocess is an additional process that is created by a main process (your Python script) to execute a separate program or script.

The subprocess module provides a simple and consistent interface to create and interact with additional processes.

 

1) Terminating a subprocess using terminate() method

The terminate() method is a part of the Popen class in the subprocess module. It sends a SIGTERM signal to the process, allowing the process to perform cleanup operations before terminating.

However, it does not guarantee that the process will be terminated, as it depends on how the process handles the SIGTERM signal.

Example

Let’s see an example of how to use the terminate() method:

import subprocess
import time

process = subprocess.Popen(['sleep', '10'])
time.sleep(2)

process.terminate()
process.wait()

In this example, we started a subprocess using the sleep command, which causes the subprocess to wait for 10 seconds. After a 2-second sleep, we use the terminate() method to send the SIGTERM signal to the subprocess. Thereafter, we used process.wait() to wait for the process to finish if its not finished.

 

2) Killing a subprocess using kill() method

The kill() method is also a part of the Popen class in the subprocess module. It sends a SIGKILL signal to the process, forcefully terminating it without giving it a chance to perform any cleanup operations. This method guarantees that the process will be terminated.

Example

Here’s an example using the kill() method:

import subprocess
import time

process = subprocess.Popen(['sleep', '10'])
time.sleep(2)

process.kill()
process.wait()

In this example, we again started a subprocess using the sleep command, which causes the subprocess to wait for 10 seconds and after a 2-seconds sleep, we called the kill() method to send the SIGKILL signal to the subprocess, forcefully terminating it. Lastly, we use process.wait() to wait for the process to finish, if it hasn’t already.

 

3) Using os.kill() method

The os.kill() method is part of the os module and allows you to send signals to a process using its process ID (PID). This method can be used to send both SIGTERM and SIGKILL signals.

Example

Here’s an example using the os.kill() method:

import subprocess
import os
import signal
import time

process = subprocess.Popen(['sleep', '10'])
time.sleep(2)

os.kill(process.pid, signal.SIGTERM)
process.wait()

 

4) Using signal module

The signal module provides mechanisms to use signals in Python. It can be used in conjunction with the os module to send signals such as SIGTERM and SIGKILL to a process.

Example

Here’s an example using the signal module:

import subprocess
import os
import signal
import time

process = subprocess.Popen(['sleep', '10'])
time.sleep(2)

os.kill(process.pid, signal.SIGKILL)
process.wait()

5) Handling subprocess timeouts

The subprocess module provides a timeout parameter for some of its functions, such as run(), call(), and check_output(). If the timeout is reached, a TimeoutExpired exception is raised.

Example

Here’s an example of handling a subprocess timeout:

import subprocess

try:
    result = subprocess.run(['sleep', '10'], timeout=5)
except subprocess.TimeoutExpired:
    print("Subprocess timed out.")

In this example, we start a subprocess using the sleep command, which causes the subprocess to wait for 10 seconds. We set the timeout parameter to 5 seconds. If the subprocess takes longer than 5 seconds to complete, a TimeoutExpired exception is raised, and we print “Subprocess timed out.”

 

6) Gracefully stopping a subprocess

Gracefully stopping a subprocess involves giving the process an opportunity to perform cleanup operations before terminating. This can be achieved by sending a SIGTERM signal, waiting for a specified period, and then sending a SIGKILL signal if the process has not terminated.

Example

import subprocess
import os
import signal
import time

process= subprocess.Popen(['sleep', '10'])
time.sleep(2)

os.kill(process.pid, signal.SIGTERM)
try:
process.wait(timeout=5)
except subprocess.TimeoutExpired:
os.kill(process.pid, signal.SIGKILL)
process.wait()

print("Subprocess terminated.")

Above we followed the same patter after that we wait for the process to finish with a timeout of 5 seconds. If the process does not terminate within the timeout, we send the SIGKILL signal to forcefully terminate it. Finally, we print “Subprocess terminated.”

 

Conclusion

In this article, we have discussed different methods to kill a subprocess in Python, including the terminate() and kill() methods provided by the subprocess module, the os.kill() method, the signal module, and handling subprocess timeouts. Each method has its use cases and trade-offs, so choose the one that best fits your needs.