In python 3.4.1, how can I assert an error in unittest like this?

For example, I define a function like

def f(n): return 1+'1'

and in the unittest, I define test_error:

assertRaises(TypeError,f(1))

But it just showed that this is a error. But when I define f with no argument like

def f(): return 1+'1'

and in unittest

assertRaises(TypeError,f)

it worked just fine. Why is that? How can I deal with function with arguments?