A debounce function decorator in Python similar to the one in underscore.js, tested with 2.7

debounce.py

from threading import Timer


def debounce(wait):
    """ Decorator that will postpone a functions
        execution until after wait seconds
        have elapsed since the last time it was invoked. """
    def decorator(fn):
        def debounced(*args, **kwargs):
            def call_it():
                fn(*args, **kwargs)
            try:
                debounced.t.cancel()
            except(AttributeError):
                pass
            debounced.t = Timer(wait, call_it)
            debounced.t.start()
        return debounced
    return decorator

test_debounce.py

import unittest
import time
from debounce import debounce


class TestDebounce(unittest.TestCase):

    @debounce(10)
    def increment(self):
        """ Simple function that
            increments a counter when
            called, used to test the
            debounce function decorator """
        self.count += 1

    def setUp(self):
        self.count = 0

    def test_debounce(self):
        """ Test that the increment
            function is being debounced.
            The counter should only be incremented
            once 10 seconds after the last call
            to the function """
        self.assertTrue(self.count == 0)
        self.increment()
        self.increment()
        time.sleep(9)
        self.assertTrue(self.count == 0)
        self.increment()
        self.increment()
        self.increment()
        self.increment()
        self.assertTrue(self.count == 0)
        time.sleep(10)
        self.assertTrue(self.count == 1)

if __name__ == '__main__':
    unittest.main()

Didn't know about threading.Timer, I like this solution!

An optimization would be to protect the Timer creation using a threading.Lock. This would prevent problems with debounce being called between Timer's initialization and calling .start().

Another would be to use functools.wraps to preserve docstrings.

Hi everyone ! A few months ago I had the same need to have a working debounce annotation, after stumbling upon this discussion I created this open source project: https://github.com/salesforce/decorator-operations

The idea of the project is to regroup useful annotations such as debounce, throttle, filter... There are only 4 annotations available for now, but you're more than welcome to suggest new features or suggestions on how to improve the existing ones :D

👍 thanks for sharing

@KarlPatach that sounds good 👍

// feature 1
In addition to debounce and throttle
also has a situation, which can has both feature.

if calls too frequently, the executor may delayed indefinitely.
some states cannot be updated in time.
then wish debounce has a max delay time allow execute one time.
// I don't known how to call it, has a debounce time, and a max delay time

// feature 2
if asyncio version has supported, that will be nice.

// finally
I wonder is them thread safe

@KarlPatach That's awesome 🎉

My implementation (fully-typed and thread-safe):

import threading
from typing import Any, Callable, Optional, TypeVar, cast

class Debouncer:
    def __init__(self, f: Callable[..., Any], interval: float):
        self.f = f
        self.interval = interval
        self._timer: Optional[threading.Timer] = None
        self._lock = threading.Lock()

    def __call__(self, *args, **kwargs) -> None:
        with self._lock:
            if self._timer is not None:
                self._timer.cancel()
            self._timer = threading.Timer(self.interval, self.f, args, kwargs)
            self._timer.start()


VoidFunction = TypeVar("VoidFunction", bound=Callable[..., None])


def debounce(interval: float):
    """
    Wait `interval` seconds before calling `f`, and cancel if called again.
    The decorated function will return None immediately,
    ignoring the delayed return value of `f`.
    """

    def decorator(f: VoidFunction) -> VoidFunction:
        if interval <= 0:
            return f
        return cast(VoidFunction, Debouncer(f, interval))

    return decorator