Signal module

• The signal module in Python provides a way to handle system signals, allowing you to execute custom code when specific signals are received by your program.
• Signals are asynchronous notifications sent to a process, often triggered by external events or user actions (e.g., pressing Ctrl+C).

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Basic Usage

Signals

• Signals are identified by constants like signal.SIGINT, signal.SIGTERM, etc.
• SIGINT is sent when you press Ctrl+C.
• SIGTERM is sent to terminate a program (e.g., via kill command).

Handlers

• You can register a custom function (signal handler) to be executed when a specific signal is received.
• Use signal.signal(signal_name, handler_function) to register a handler.

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Handling SIGINT (Ctrl+C)

import signal
import time

def handle_sigint(signum, frame):
    print(f"Received signal {signum}. Exiting gracefully!")
    exit(0)

# Register the handler for SIGINT
signal.signal(signal.SIGINT, handle_sigint)

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

Output when you press Ctrl+C:

Running program. Press Ctrl+C to interrupt.
Received signal 2. Exiting gracefully!

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Handling SIGTERM

import signal
import time

def handle_sigterm(signum, frame):
    print("Received SIGTERM. Cleaning up resources...")
    exit(0)

# Register the handler for SIGTERM
signal.signal(signal.SIGTERM, handle_sigterm)

print("Running program. Use 'kill -15 <PID>' to terminate.")
while True:
    time.sleep(1)

Output when sending SIGTERM:

Received SIGTERM. Cleaning up resources...

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Ignoring Signals

You can ignore specific signals by setting their handler to signal.SIG_IGN:

import signal
import time

# Ignore SIGINT (Ctrl+C)
signal.signal(signal.SIGINT, signal.SIG_IGN)

print("Try pressing Ctrl+C. It will be ignored.")
time.sleep(10)
print("Program finished.")

Output:

Try pressing Ctrl+C. It will be ignored.
Program finished.

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Restoring Default Behavior

You can restore the default behavior for a signal using signal.SIG_DFL:

import signal
import time

def custom_handler(signum, frame):
    print("Custom handler for SIGINT!")

# Set custom handler
signal.signal(signal.SIGINT, custom_handler)

print("Press Ctrl+C to trigger the custom handler.")
time.sleep(5)

# Restore default behavior
signal.signal(signal.SIGINT, signal.SIG_DFL)
print("Default behavior restored. Press Ctrl+C again.")
time.sleep(5)

Output:

Press Ctrl+C to trigger the custom handler.
Custom handler for SIGINT!
Default behavior restored. Press Ctrl+C again.
# Exits program on second Ctrl+C

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Using signal.alarm

You can set an alarm to send a SIGALRM signal after a specific time:

import signal
import time

def handle_alarm(signum, frame):
    print("Alarm triggered!")

# Register handler and set alarm
signal.signal(signal.SIGALRM, handle_alarm)
signal.alarm(5)  # Trigger SIGALRM after 5 seconds

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

Output after 5 seconds:

Waiting for alarm...
Alarm triggered!

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Key Points to Remember

1. Signals

   Signals are used for asynchronous events like interrupts or termination requests.

2. Handlers

   Register handlers using signal.signal(signal_name, handler_function).

   Handlers must accept two arguments: signum (signal number) and frame (current stack frame).

3. Special Constants

   signal.SIG_IGN: Ignore the signal.

   signal.SIG_DFL: Restore the default behavior.

4. Common Signals

   SIGINT: Interrupt (Ctrl+C).

   SIGTERM: Termination request.

   SIGALRM: Alarm timer expiration.

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Summary

• The signal module provides fine-grained control over how your Python program responds to external signals.
• You can customize behavior for interrupts (Ctrl+C), termination requests, or other signals, making it ideal for building robust, interactive, and cleanup-aware applications.