gentimer -- generator based timer

Install

pip install gentimer

Description

Consume generator based timer within event-loop.

This utility function provide a way to use time.sleep like behavior inside the event loop.

• tkinter
• wxPython
• PyQt5
• sched (standard library)

time.sleep such function will block thread in Python interpreter.

This utility improve the coding, callback based timer code usually recursive call. generator based timer code is much easier to read.

This utility does not promise to avoid the blocking. users must take case of keep event-loop running. If you want to do something heavy tasks then you should use multi-thread or multi-process instead.

Step by step guide

# basic timer by time.sleep

def count_down(num):
    for num in range(num, 0, -1):
        print(num)
        time.sleep(1)

count_down(10)

This code is simple good. at least, it works well in threaded code. However, if we are now in an event-loop context, time.sleep will stop the current thread,

# Bad code: time.sleep inside eventloop

import tkinter

root = tkinter.Tk()
# This button does not work while blocking by time.sleep
tkinter.Button(root, text="quit", command=root.quit).pack()

def count_down(num):
    # This function is called inside tkinter event-loop.
    for num in range(num, 0, -1):
        print(num)
        time.sleep(1) # XXX: will block event-loop

root.after_idle(count_down, 10)
root.mainloop()

The most of GUI libraries provides timer feature on the event-loop. Here is tkinter example.

# nested callback based timer code

import tkinter

root = tkinter.Tk()
tkinter.Button(root, text="quit", command=root.quit).pack()

def count_down(num):
    if num > 0:
        print(num)
        # give back the control-flow to event-loop.
        root.after(1000, count_down, num-1)

root.after_idle(count_down, 10)
root.mainloop()

• every iterations check if num > 0:
• every iterations make temporary stack-frame.
• when stop the timer? when code was grown, it hard to detect the loop by nested callbacks.

# generator based timer code

import tkinter

root = tkinter.Tk()
root.withdraw()

def count_down(num):
    for num in range(num, 0, -1):
        print(num)
        yield 1

gen_timer(root, count_down(10))
root.mainloop()

It's almost same as the first time.sleep code. This code can avoid the event-loop block. Because, the generator will be consumed inside event-loop timer.

• keep the function context, that avoid unnecessary function calls.

When it's useful? let's think more complex situation

import tkinter

root = tkinter.Tk()
root.withdraw()

def count_down(num):
    print("count down start")
    for num in range(num, 0, -1):
        print(num)
        yield 1  # sleep 1 sec without blocking event loop
    print("count down end")

gen_timer(root, count_down(10), done=root.quit)
root.mainloop()

How to call the pre-post procedure in timer callback?

import tkinter

root = tkinter.Tk()
root.withdraw()

def count_down(num):
    if num == 10: # XXX: how to detect it's starting?
        print("count down start")
    
    print(num)

    if num > 0:
        root.after(1000, count_down, num-1)
    else:
        print("count down end")
        root.quit()

root.after_idle(count_down, 10)
root.mainloop()

Cons:

• redundant double checks every iterations
• can't reuse local variables it's recursive callbacks, each iterations has different stack-frame.
• can't follow control-flow without understand how root.after works.

Coding Design history

This design concept are used to discussed, like "async/await" syntax improved promise based callbacks. "yield" can replace recursive timer callback to generator based code.