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 instead of bb.
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?
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})
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.
