python theading.Timer: how to pass argument to the callback?

Question

I tried this code:

import threading

def hello(arg, kargs):
    print(arg)

t = threading.Timer(2, hello, "bb")
t.start()

while 1:
    pass

The output is just b.

How can I pass arguments to the callback properly?

If I remove the kargs parameter from hello, I get an exception that says TypeError: hello() takes 1 positional argument but 2 were given. Why? Where did the kargs value come from in the first code?

Asked By: Bin Chen

||

Source

Answer 1

Timer expects a sequence (normally, a list or tuple) of arguments and a mapping (normally, a dict) of keyword arguments, so pass a list instead:

import threading

def hello(arg):
    print(arg)

t = threading.Timer(2, hello, ["bb"])
t.start()

while 1:
    pass

Since "bb" is an iterable, the Timer will iterate over it and use each element as a separate argument; threading.Timer(2, hello, ["bb"]) is equivalent to threading.Timer(2, hello, ["b", "b"]).

Use a dictionary to pass any keyword arguments to the callback, for example:

def hello(arg, kwarg):
    print('arg is', arg, 'and kwarg is', kwarg)

t = threading.Timer(2, hello, ["bb"], {'kwarg': 1})

Answered By: Glenn Maynard

Answer 2

The third argument to Timer is a sequence. Passing "bb" as that sequence means that hello gets the elements of that sequence ("b" and "b") as separate arguments (arg and kargs). Put "bb" in a list, and hello will get the string as the first argument:

t = threading.Timer(2, hello, ["bb"])

Presumably, hello was intended to have parameters like:

def hello(*args, **kwargs):

See What does ** (double star/asterisk) and * (star/asterisk) do for parameters? for a detailed explanation of this syntax.

Answered By: outis

python theading.Timer: how to pass argument to the callback?

Question:

Answers: