Handling Exceptions in Python Behave Testing framework

Question

I've been thinking about switching from nose to behave for testing (mocha/chai etc have spoiled me). So far so good, but I can't seem to figure out any way of testing for exceptions besides:

@then("It throws a KeyError exception")
def step_impl(context):
try:
    konfigure.load_env_mapping("baz", context.configs)
except KeyError, e:
    assert (e.message == "No baz configuration found") 

With nose I can annotate a test with

@raises(KeyError)

I can't find anything like this in behave (not in the source, not in the examples, not here). It sure would be grand to be able to specify exceptions that might be thrown in the scenario outlines.

Anyone been down this path?

Answer 1

I'm pretty new to BDD myself, but generally, the idea would be that the tests document what behaves the client can expect - not the step implementations. So I'd expect the canonical way to test this would be something like:

When I try to load config baz
Then it throws a KeyError with message "No baz configuration found"

With steps defined like:

@when('...')
def step(context):
    try:
        # do some loading here
        context.exc = None
    except Exception, e:
        context.exc = e

@then('it throws a {type} with message "{msg}"')
def step(context, type, msg):
    assert isinstance(context.exc, eval(type)), "Invalid exception - expected " + type
    assert context.exc.message == msg, "Invalid message - expected " + msg

If that's a common pattern, you could just write your own decorator:

def catch_all(func):
    def wrapper(context, *args, **kwargs):
        try:
            func(context, *args, **kwargs)
            context.exc = None
        except Exception, e:
            context.exc = e

    return wrapper

@when('... ...')
@catch_all
def step(context):
    # do some loading here - same as before