# 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

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

def assert_raises(exception_type):
    ...

def test_sqrt():
    try:
        log_of_sqrt(-5)
    except Exception as e:
        if isinstance(e, ValueError):  # what is Pythonic way
                                       # to check exception type?
            pass
        else:
            raise AssertionError('bad exception')
    finally:
        raise AssertionError('no expected exception')

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