45. Handling Signals in Python

Here are 10 Python code snippets demonstrating how to handle signals using the signal module. The signal module allows you to capture and respond to various system signals, such as termination signals, alarms, etc.

1. Basic Signal Handling

Handling the SIGINT (Ctrl+C) signal.

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import signal
import time

def signal_handler(sig, frame):
    print('SIGINT received, exiting...')
    exit(0)

# Register signal handler for SIGINT
signal.signal(signal.SIGINT, signal_handler)

print("Press Ctrl+C to exit")
while True:
    time.sleep(1)

This example captures SIGINT and gracefully exits the program when Ctrl+C is pressed.

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2. Handling Multiple Signals

Handling both SIGINT and SIGTERM signals.

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import signal
import time

def signal_handler(sig, frame):
    if sig == signal.SIGINT:
        print('Received SIGINT')
    elif sig == signal.SIGTERM:
        print('Received SIGTERM')
    exit(0)

# Register signal handlers
signal.signal(signal.SIGINT, signal_handler)
signal.signal(signal.SIGTERM, signal_handler)

print("Waiting for signals (Ctrl+C or kill -TERM pid)")
while True:
    time.sleep(1)

In this example, both SIGINT and SIGTERM are handled by the same handler.

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3. Setting a Timeout with SIGALRM

Using SIGALRM to set a timeout for a task.

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import signal
import time

def timeout_handler(sig, frame):
    print('Timeout reached!')
    exit(1)

# Register signal handler for SIGALRM
signal.signal(signal.SIGALRM, timeout_handler)

# Set an alarm for 5 seconds
signal.alarm(5)

print("Waiting for timeout")
time.sleep(10)

This example sets an alarm that triggers SIGALRM after 5 seconds, calling timeout_handler.

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4. Ignoring a Signal

Using signal.SIG_IGN to ignore a signal.

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import signal
import time

def signal_handler(sig, frame):
    print("This won't be printed because SIGINT is ignored.")

# Ignore SIGINT signal
signal.signal(signal.SIGINT, signal.SIG_IGN)

print("Press Ctrl+C. Nothing will happen.")
while True:
    time.sleep(1)

This code ignores the SIGINT signal (Ctrl+C) and does nothing when it is received.

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5. Handling Signals with a Custom Function

Define a custom function for handling signals.

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import signal
import time

def custom_handler(signal, frame):
    print(f"Signal {signal} received!")
    exit(0)

# Handle SIGQUIT with a custom function
signal.signal(signal.SIGQUIT, custom_handler)

print("Press Ctrl+\\ to send SIGQUIT")
time.sleep(10)

This example defines a custom function custom_handler to respond to the SIGQUIT signal.

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6. Catching Signals in a Child Process

Capture signals in a child process spawned by os.fork().

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import os
import signal
import time

def child_handler(sig, frame):
    print("Child process received signal")
    exit(0)

if os.fork() == 0:
    # Child process
    signal.signal(signal.SIGUSR1, child_handler)
    print("Child waiting for signal")
    time.sleep(10)
else:
    # Parent process
    time.sleep(1)
    os.kill(os.getpid(), signal.SIGUSR1)

This snippet demonstrates how a child process can catch signals using the signal module.

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7. Using signal.sigtimedwait for Handling Signals

Blocking until a signal is received.

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import signal
import time

def signal_handler(sig, frame):
    print(f"Signal {sig} received!")
    exit(0)

# Register signal handler
signal.signal(signal.SIGALRM, signal_handler)

# Set an alarm
signal.alarm(5)

# Wait for the signal
signal.sigtimedwait([signal.SIGALRM], timeout=10)
print("Waiting completed.")

In this example, the process waits for SIGALRM with a timeout using sigtimedwait.

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8. Handling Signals in Threads

Handling signals inside a thread.

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import signal
import time
import threading

def signal_handler(sig, frame):
    print("Signal received in main thread!")
    exit(0)

def thread_function():
    time.sleep(5)
    print("Thread finished")

# Register signal handler
signal.signal(signal.SIGINT, signal_handler)

# Start a background thread
thread = threading.Thread(target=thread_function)
thread.start()

# Main thread waits for signal
print("Waiting for signal (Ctrl+C)...")
time.sleep(10)

This demonstrates how to handle signals in the main thread while running a background thread.

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9. Using signal.pause

Pause the program until a signal is received.

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import signal

def signal_handler(sig, frame):
    print("Signal received, exiting.")
    exit(0)

# Register signal handler
signal.signal(signal.SIGINT, signal_handler)

# Wait for signals indefinitely
print("Waiting for signal...")
signal.pause()

The signal.pause() function causes the program to wait indefinitely for any signal to be received.

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10. Handling Signals in a Daemon

Handling signals in a daemon process.

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import signal
import os
import time

def signal_handler(sig, frame):
    print(f"Daemon received signal {sig}")
    exit(0)

# Register signal handler
signal.signal(signal.SIGTERM, signal_handler)

# Create a daemon process
pid = os.fork()
if pid == 0:
    # Child (daemon) process
    print("Daemon running...")
    time.sleep(60)  # Simulate daemon work
else:
    # Parent process
    print("Parent sending SIGTERM")
    os.kill(pid, signal.SIGTERM)
    time.sleep(2)

In this example, the parent process sends a SIGTERM signal to the daemon (child process), which handles it with a signal handler.

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These examples demonstrate how you can use Python's signal module to handle various types of signals in different scenarios, making your program more flexible and responsive to external events.