RecursionError: maximum recursion depth exceeded while using thread

Question

So I am getting the error

[Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded

The code I am running is

import threading

def hello_world(a):
    threading.Timer(2.0, hello_world(a)).start() # called every minute
    print(a)
    print("Hello, World!")

hello_world('a')

I noticed that when there are no arguments in the hello_world function the error doesn't occur. But as soon as I need to pass in a parameter to the function I get the error. Can someone explain why that is the case and how to fix it?

Green Cloak Guy · Accepted Answer · 2019-10-26 05:41:37Z

6

The threading.Timer() constructor expects the function and the arguments to pass to that function as separate parameters. The proper way to call it is this:

threading.Timer(2.0, hello_world, (a,)).start()

You can see that we refer to hello_world without calling it, and we list the parameters we want to pass separately in a 1-tuple (a,).

The way you're currently doing it, it's evaluating hello_world(a) immediately, before it gets to the end of the expression, trying to figure out what the return value of hello_world(a) would be - as opposed to starting the timer and then evaluating the expression every time the timer goes off.

answered Oct 26, 2019 at 5:41

Green Cloak Guy

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1 Comment

DTK Over a year ago

I have seen the same error with a simple print function, can anyone reproduce?

def print_path(path):     print("In Python")     print("Path:"+ path)     print("Out Python")     return True if __name__ == '__main__':     message = "Hey D"     print_path(message)

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