Why isn't assertRaises catching my Attribute Error using python unittest?

Question

I\'m trying to run this test: self.assertRaises(AttributeError, branch[0].childrennodes), and branch[0] does not have an attribute childrenno

Answer 1

When the test is running, before calling self.assertRaises, Python needs to find the value of all the method's arguments. In doing so, it evaluates branch[0].children_nodes, which raises an AttributeError. Since we haven't invoked assertRaises yet, this exception is not caught, causing the test to fail.

The solution is to wrap branch[0].children_nodes in a function or a lambda:

self.assertRaises(AttributeError, lambda: branch[0].children_nodes)

assertRaises can also be used as a context manager (Since Python 2.7, or in PyPI package 'unittest2'):

with self.assertRaises(AttributeError):
    branch[0].children_nodes
    # etc

This is nice because it can be used on arbitrary blocks of code in the middle of a test, rather than having to create a new function just to define the block of code to which it applies.

It can give you access to the raised exception for further processing, if needed:

with self.assertRaises(AttributeError) as cm:
    branch[0].children_nodes

self.assertEquals(cm.exception.special_attribute, 123)

Answer 2

pytest also has a similar decorator:

from pytest import raises

def test_raising():
    with raises(AttributeError):
        branch[0].childrennodes

Answer 3

I think its because assert raises only accepts a callable. It evalutes to see if the callable raises an exception, not if the statement itself does.

self.assertRaises(AttributeError, getattr, branch[0], "childrennodes")

should work.

EDIT:

As THC4k correctly says it gathers the statements at collection time and will error then, not at testing time.

Also this is a reason why I like nose, it has a decorator (raises) that is useful and clearer for these kind of tests.

@raises(AttributeError)
def test_1(self)
    branch[0].childrennodes