How Can I Implement Timeouts When Using Python's `subprocess` Module?

Python's subprocess Module and Timeouts

The subprocess module offers a convenient way to execute external commands, capturing their outputs and managing their lifecycles. However, by default, its communicate() method does not support timeouts. This poses a challenge when executing long-running commands that could potentially deadlock the calling process.

Implementing Timeouts with check_output

Python 3.3 and later provides check_output() as a more efficient alternative to Popen() and communicate(). This function evaluates a command, merges its stdout and stderr outputs into a byte string, and raises a CalledProcessError if the command exits with a non-zero status. Crucially, it also supports timeouts, allowing you to specify a maximum execution time for the command.

from subprocess import STDOUT, check_output

seconds = 10  # Timeout in seconds
output = check_output(cmd, stderr=STDOUT, timeout=seconds)

In this example, the check_output() function will execute the command specified in cmd and wait for it to complete within 10 seconds. If the command takes longer than 10 seconds, a TimeoutExpired error will be raised.

Using subprocess32 for Timeouts in Python 2.x

For Python 2.x, the subprocess32 backport provides the same timeout functionality as check_output() in Python 3.3 . To install subprocess32, use pip:

pip install subprocess32

Once installed, you can use subprocess32's call() function to execute commands with timeouts:

import subprocess32

seconds = 10  # Timeout in seconds
subprocess32.call(cmd, timeout=seconds)

Additional Considerations

• Remove shell=True from your subprocess calls when possible. This will improve security and performance.
• Timeout behavior may vary based on platform and process complexity. See the Subprocess timeout failure documentation for more details.

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