Question

I am reading this

http://docs.python.org/dev/library/multiprocessing.html

In particular this

p = multiprocessing.Process(target=time.sleep, args=(1000,))

I tried the same thing, but if I remove the “,” after 1000 it doesn’t work. So my question is what is the semantic behind the args=(1000,) in this case? What is the difference if I put a comma and not ?

p/s: I believe it’s a fundamental issue, if it is can someone point me to some further reading if possible? thanks.

Thanks.

Regards,
Andy.

Answer 1

If you just put (1000), Python assumes you’re just evaluating the expression as math, hence it gets simplified to just 1000. Think of the result of 5 + (1000) + 4.

Just as the expression above would get simplified to 1009, here is what your line looks like once things have been simplified:

p = multiprocessing.Process(target=time.sleep, args=1000)

You can see that this is not the same thing at all. args is supposed to be a tuple of arguments, not a single integer.

If you put (1000,), Python can tell you are looking for a tuple which only contains one element, since that expression is differentiable from a simple arithmetic expression, so you end up passing in the correct thing.