# This happens most often in testing. We would like to make sure that
# a block of code raises an exception of the certain type.
# If it does not raise an exception, *we* raise an error.
# If it raises an unexpected exception, we raise an error *too*.

# Write a context manager which does checks that an exception is
# raised properly and convert the example below. Make things
# less ugly too.

import math
import contextlib

def log_of_sqrt(n):
    return math.log(math.sqrt(n))

@contextlib.contextmanager
def assert_raises(exception_type):
    try:
        yield
    except exception_type:
        pass
    except Exception as e:
        raise ValueError('unexpected exception ' + e.__class__.__name__)
    else:
        raise ValueError('no expected exception')

def test_sqrt():
    with assert_raises(ValueError):
        log_of_sqrt(1/0)

# Note: this is implemented by
#  pytest.raises,
#  unittest.TestCase.assertRaises.