python assertRaises don't pass test if function with parameters

Question

assertRaises give an assertion error with the following code. Is there something I'm doing wrong?

class File_too_small(Exception):
    "Check file size"

def foo(a,b):
    if a<b:
        raise File_too_small
class some_Test(unittest.TestCase):

    def test_foo(self):
        self.assertRaises(File_too_small,foo(1,2))

The test seems to pass with the following modification though

def foo:
    raise File_too_small

def test_foo(self):
    self.assertRaises(File_too_small,foo)

Answer 1

Try like this:

def test_foo(self):
    with self.assertRaises(File_too_small):
        foo(1, 2)

or:

def test_foo(self):
    self.assertRaises(File_too_small, foo, 1, 2):

Answer 2

You need to pass the callable, not the result, to assertRaises:

self.assertRaises(File_too_small, foo, 1, 2)

Alternatively use it as a context manager:

with self.assertRaises(File_too_small):
    foo(1, 2)