Effects of spells

The next step with spell casting takes me to effects. There is already a effect subsystem in place that is being used for healing potions and spider poisonings for example. Effects can be one off or they can have a duration (finite or even infinite). So it seemed like a good idea to reuse those effects for spells. It gives the possibility to have immediate spell effects (like magic missiles and fire balls) or longer lasting ones (blessings and curses).

First step is to create some effects, so I will start with a test:

def test_creating_effect(self):
    Casting a spell should create effects it has
    effects_factory = mock(EffectsFactory)
    effect = mock(Effect)
    dying_rules = mock(Dying)
    when(effects_factory).create_effect(key = 'healing wind',
                                        target = self.character).thenReturn(effect)

    effect_handle = EffectHandle(trigger = 'on spell hit',
                                 effect = 'healing wind',
                                 parameters = {},
                                 charges = 1)

    spell = (SpellBuilder()


    verify(effects_factory).create_effect(key = 'healing wind',
                                          target = self.character)

Essential parts are where we tell the effect_factory to return a mocked effect when it is being called with correct parameters. EffectHandles are sort of specifications that tell when to create what kind of effect. In this case, when ‘on spell hit’ event happens, we want to create a healing wind. EffectFactory on the other hand can be used to create effect by name, which in this case is ‘healing wind’.

I’m using a SpellBuilder to create instance of Spell object. The reason for this is to decouple tests and the code they are testing to a degree. Obviously there has to be some sort of connectivity, but I try to limit it a bit. If internal representation of spells change, it is enough to change the builder and all the tests are hopefully up and running again (at least if the verification part of the test works).

Executing tests at this point gives an error:

Traceback (most recent call last):
  File "C:\Python32\lib\site-packages\nose\case.py", line 198, in runTest
  File "C:\programming\pyHack\src\pyherc\test\unit\test_spells.py", line 85, in test_triggering_effect
    target = self.character)
  File "C:\Python32\lib\site-packages\mockito\invocation.py", line 98, in __call__
    verification.verify(self, len(matched_invocations))
  File "C:\Python32\lib\site-packages\mockito\verification.py", line 51, in verify
    raise VerificationError("\nWanted but not invoked: %s" % (invocation))
Wanted but not invoked: create_effect()

One noteworthy thing about how mockito-python verifies interactions between objects. If I have a verification like:

verify(effects_factory).create_effect(key = 'healing wind',
                                      target = character)

It does not match to a call like this:

effects_factory.create_effect('healing wind',

If the verification is specified with keyword arguments, then the call must be made with keyword arguments:

effects_factory.create_effect(key = 'healing wind',
                              target = character)

This is a limitation in mockito-python and so far I have not found a workaround for it. It is something that needs to be remembered when working with keyword arguments.

Another advantage of reusing effects subsystem is that I already have an EffectsCollection class ready that is used to store and query Effects and EffectHandles. Instead of writing all that logic again, I can just reuse it in Spell class:

def get_effect_handles(self, trigger = None):
    Get effect handles

    :param trigger: optional trigger type
    :type trigger: string

    :returns: effect handles
    :rtype: [EffectHandle]
    return self.effects.get_effect_handles(trigger)

In order to make the test pass, I made following modifications to Spell class:

def cast(self, effects_factory):
    Cast the spell

    :param effects_factory: factory for creating effects
    :type effects_factory: EffectsFactory
    handles = self.effects.get_effect_handles('on spell hit')
    effects = []

    for target in self.target:
        for handle in handles:
                                            key = handle.effect,
                                            target = target))

At this point the test passes. The test verifies only that the EffectsFactory was called with parameters that it should be called with. It does not verify what is being done with the results of the call. That is left for the next step, when I want to write the code to actually trigger the created effects.

Code for this change (including the SpellBuilder) can be found in commit 62512fb694a93e81e792ec2cf41c1d10901f4c32.

Does it make sense to take this tiny steps? Couldn’t I have just written a test that checks that the effects being created are being triggered and hit two birds with a single stone? I could have done that and I think that would have been as valid approach as dividing it to two separate tests. I prefer to work with very small steps and small tests. While they help me to pinpoint the exact spot where the system is misbehaving, the drawback is the amount of code I have to write.


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