2. Rules and dice rolling

In EvAdventure we have decided to use the Knave RPG ruleset. This is commercial, but released under Creative Commons 4.0, meaning it’s okay to share and adapt Knave for any purpose, even commercially. If you don’t want to buy it but still follow along, you can find a free fan-version here.

2.1. Summary of Knave rules

Knave, being inspired by early Dungeons & Dragons, is very simple.

  • It uses six Ability bonuses Strength (STR), Dexterity (DEX), Constitution (CON), Intelligence (INT), Wisdom (WIS) and Charisma (CHA). These are rated from +1 to +10.

  • Rolls are made with a twenty-sided die (1d20), usually adding a suitable Ability bonus to the roll.

  • If you roll with advantage, you roll 2d20 and pick the highest value, If you roll with disadvantage, you roll 2d20 and pick the lowest.

  • Rolling a natural 1 is a critical failure. A natural 20 is a critical success. Rolling such in combat means your weapon or armor loses quality, which will eventually destroy it.

  • A saving throw (trying to succeed against the environment) means making a roll to beat 15 (always). So if you are lifting a heavy stone and have STR +2, you’d roll 1d20 + 2 and hope the result is higher than 15.

  • An opposed saving throw means beating the enemy’s suitable Ability ‘defense’, which is always their Ability bonus + 10. So if you have STR +1 and are arm wrestling someone with STR +2, you roll 1d20 + 1 and hope to roll higher than 2 + 10 = 12.

  • A special bonus is Armor, +1 is unarmored, additional armor is given by equipment. Melee attacks test STR versus the Armor defense value while ranged attacks uses WIS vs Armor.

  • Knave has no skills or classes. Everyone can use all items and using magic means having a special ‘rune stone’ in your hands; one spell per stone and day.

  • A character has CON + 10 carry ‘slots’. Most normal items uses one slot, armor and large weapons uses two or three.

  • Healing is random, 1d8 + CON health healed after food and sleep.

  • Monster difficulty is listed by hy many 1d8 HP they have; this is called their “hit die” or HD. If needing to test Abilities, monsters have HD bonus in every Ability.

  • Monsters have a morale rating. When things go bad, they have a chance to panic and flee if rolling 2d6 over their morale rating.

  • All Characters in Knave are mostly randomly generated. HP is <level>d8 but we give every new character max HP to start.

  • Knave also have random tables, such as for starting equipment and to see if dying when hitting 0. Death, if it happens, is permanent.

2.2. Making a rule module

Create a new module mygame/evadventure/rules.py

There are three broad sets of rules for most RPGS:

  • Character generation rules, often only used during character creation

  • Regular gameplay rules - rolling dice and resolving game situations

  • Character improvement - getting and spending experience to improve the character

We want our rules module to cover as many aspeects of what we’d otherwise would have to look up in a rulebook.

2.3. Rolling dice

We will start by making a dice roller. Let’s group all of our dice rolling into a structure like this (not functional code yet):

class EvAdventureRollEngine:

   def roll(...):
       # get result of one generic roll, for any type and number of dice
   def roll_with_advantage_or_disadvantage(...)
       # get result of normal d20 roll, with advantage/disadvantage (or not)
   def saving_throw(...):
       # do a saving throw against a specific target number
   def opposed_saving_throw(...):
       # do an opposed saving throw against a target's defense

   def roll_random_table(...):
       # make a roll against a random table (loaded elsewere)
   def morale_check(...):
       # roll a 2d6 morale check for a target
   def heal_from_rest(...):
       # heal 1d8 when resting+eating, but not more than max value.
   def roll_death(...):
       # roll to determine penalty when hitting 0 HP. 
dice = EvAdventureRollEngine() 

This structure (called a singleton) means we group all dice rolls into one class that we then initiate into a variable dice at the end of the module. This means that we can do the following from other modules:

    from .rules import dice 


2.3.1. Generic dice roller

We want to be able to do roll("1d20") and get a random result back from the roll.

# in mygame/evadventure/rules.py 

from random import randint

class EvAdventureRollEngine:
    def roll(self, roll_string):
        Roll XdY dice, where X is the number of dice 
        and Y the number of sides per die. 
            roll_string (str): A dice string on the form XdY.
            int: The result of the roll. 
        # split the XdY input on the 'd' one time
        number, diesize = roll_string.split("d", 1)     
        # convert from string to integers
        number = int(number) 
        diesize = int(diesize)
        # make the roll
        return sum(randint(1, diesize) for _ in range(number))

The randint standard Python library module produces a random integer
in a specific range. The line

sum(randint(1, diesize) for _ in range(number))

works like this:

  • For a certain number of times …

  • … create a random integer between 1 and diesize

  • … and sum all those integers together.

You could write the same thing less compactly like this:

rolls = []
for _ in range(number): 
   random_result = randint(1, diesize)
return sum(rolls)

We don’t ever expect end users to call this method; if we did, we would have to validate the inputs much more - We would have to make sure that number or diesize are valid inputs and not crazy big so the loop takes forever!

2.3.2. Rolling with advantage

Now that we have the generic roller, we can start using it to do a more complex roll.

# in mygame/evadventure/rules.py 

# ... 

class EvAdventureRollEngine:

    def roll(roll_string):
        # ... 
    def roll_with_advantage_or_disadvantage(self, advantage=False, disadvantage=False):
        if not (advantage or disadvantage) or (advantage and disadvantage):
            # normal roll - advantage/disadvantage not set or they cancel 
            # each other out 
            return self.roll("1d20")
        elif advantage:
             # highest of two d20 rolls
             return max(self.roll("1d20"), self.roll("1d20"))
             # disadvantage - lowest of two d20 rolls 
             return min(self.roll("1d20"), self.roll("1d20"))

The min() and max() functions are standard Python fare for getting the biggest/smallest of two arguments.

2.3.3. Saving throws

We want the saving throw to itself figure out if it succeeded or not. This means it needs to know the Ability bonus (like STR +1). It would be convenient if we could just pass the entity doing the saving throw to this method, tell it what type of save was needed, and then have it figure things out:

result, quality = dice.saving_throw(character, Ability.STR)

The return will be a boolean True/False if they pass, as well as a quality that tells us if a perfect fail/success was rolled or not.

To make the saving throw method this clever, we need to think some more about how we want to store our data on the character.

For our purposes it sounds reasonable that we will be using Attributes for storing the Ability scores. To make it easy, we will name them the same as the Enum values we set up in the previous lesson. So if we have an enum STR = "strength", we want to store the Ability on the character as an Attribute strength.

From the Attribute documentation, we can see that we can use AttributeProperty to make it so the Attribute is available as character.strength, and this is what we will do.

So, in short, we’ll create the saving throws method with the assumption that we will be able to do character.strength, character.constitution, character.charisma etc to get the relevant Abilities.

# in mygame/evadventure/rules.py 
# ...
from .enums import Ability

class EvAdventureRollEngine: 

    def roll(...)
        # ...
    def roll_with_advantage_or_disadvantage(...)
        # ...
    def saving_throw(self, character, bonus_type=Ability.STR, target=15, 
                     advantage=False, disadvantage=False):
        Do a saving throw, trying to beat a target.
           character (Character): A character (assumed to have Ability bonuses
               stored on itself as Attributes).
           bonus_type (Ability): A valid Ability bonus enum.
           target (int): The target number to beat. Always 15 in Knave.
           advantage (bool): If character has advantage on this roll.
           disadvantage (bool): If character has disadvantage on this roll.
            tuple: A tuple (bool, Ability), showing if the throw succeeded and 
                the quality is one of None or Ability.CRITICAL_FAILURE/SUCCESS
        # make a roll 
        dice_roll = self.roll_with_advantage_or_disadvantage(advantage, disadvantage)
        # figure out if we had critical failure/success
        quality = None
        if dice_roll == 1:
            quality = Ability.CRITICAL_FAILURE
        elif dice_roll == 20:
            quality = Ability.CRITICAL_SUCCESS 

        # figure out bonus
        bonus = getattr(character, bonus_type.value, 1) 

        # return a tuple (bool, quality)
        return (dice_roll + bonus) > target, quality

The getattr(obj, attrname, default) function is a very useful Python tool for getting an attribute off an object and getting a default value if the attribute is not defined.

2.3.4. Opposed saving throw

With the building pieces we already created, this method is simple. Remember that the defense you have to beat is always the relevant bonus + 10 in Knave. So if the enemy defends with STR +3, you must roll higher than 13.

# in mygame/evadventure/rules.py 

from .enums import Ability

class EvAdventureRollEngine:
    def roll(...):
        # ... 

    def roll_with_advantage_or_disadvantage(...):
        # ... 

    def saving_throw(...):
        # ... 

    def opposed_saving_throw(self, attacker, defender, 
                             attack_type=Ability.STR, defense_type=Ability.ARMOR,
                             advantage=False, disadvantage=False):
        defender_defense = getattr(defender, defense_type.value, 1) + 10 
        result, quality = self.saving_throw(attacker, bonus_type=attack_type,
                                            advantage=advantage, disadvantage=disadvantage)
        return result, quality 

2.3.5. Morale check

We will make the assumption that the morale value is available from the creature simply as monster.morale - we need to remember to make this so later!

In Knave, a creature have roll with 2d6 equal or under its morale to not flee or surrender when things go south. The standard morale value is 9.

# in mygame/evadventure/rules.py 

class EvAdventureRollEngine:

    # ...
    def morale_check(self, defender): 
        return self.roll("2d6") <= getattr(defender, "morale", 9)

2.3.6. Roll for Healing

To be able to handle healing, we need to make some more assumptions about how we store health on game entities. We will need hp_max (the total amount of available HP) and hp (the current health value). We again assume these will be available as obj.hp and obj.hp_max.

According to the rules, after consuming a ration and having a full night’s sleep, a character regains 1d8 + CON HP.

# in mygame/evadventure/rules.py 

from .enums import Ability

class EvAdventureRollEngine: 

    # ... 
    def heal_from_rest(self, character): 
        A night's rest retains 1d8 + CON HP  
        con_bonus = getattr(character, Ability.CON.value, 1)
        character.heal(self.roll("1d8") + con_bonus)

We make another assumption here - that character.heal() is a thing. We tell this function how much the character should heal, and it will do so, making sure to not heal more than its max number of HPs

Knowing what is available on the character and what rule rolls we need is a bit of a chicken-and-egg problem. We will make sure to implement the matching Character class next lesson.

2.3.7. Rolling on a table

We occasionally need to roll on a ‘table’ - a selection of choices. There are two main table-types we need to support:

Simply one element per row of the table (same odds to get each result).






This we will simply represent as a plain list

["item1", "item2", "item3", "item4"]

Ranges per item (varying odds per result):











This we will represent as a list of tuples:

[("1-5", "item1"), ("6-15", "item2"), ("16-19", "item4"), ("20", "item5")]

We also need to know what die to roll to get a result on the table (it may not always be obvious, and in some games you could be asked to roll a lower dice to only get early table results, for example).

# in mygame/evadventure/rules.py 

from random import randint, choice

class EvAdventureRollEngine:
    # ... 

    def roll_random_table(self, dieroll, table_choices): 
             dieroll (str): A die roll string, like "1d20".
             table_choices (iterable): A list of either single elements or 
                of tuples.
            Any: A random result from the given list of choices.
            RuntimeError: If rolling dice giving results outside the table.
        roll_result = self.roll(dieroll) 
        if isinstance(table_choices[0], (tuple, list)):
            # the first element is a tuple/list; treat as on the form [("1-5", "item"),...]
            for (valrange, choice) in table_choices:
                minval, *maxval = valrange.split("-", 1)
                minval = abs(int(minval))
                maxval = abs(int(maxval[0]) if maxval else minval)
                if minval <= roll_result <= maxval:
                    return choice 
            # if we get here we must have set a dieroll producing a value 
            # outside of the table boundaries - raise error
            raise RuntimeError("roll_random_table: Invalid die roll")
            # a simple regular list
            roll_result = max(1, min(len(table_choices), roll_result))
            return table_choices[roll_result - 1]

Check that you understand what this does.

This may be confusing:

minval, *maxval = valrange.split("-", 1)
minval = abs(int(minval))
maxval = abs(int(maxval[0]) if maxval else minval)

If valrange is the string 1-5, then valrange.split("-", 1) would result in a tuple ("1", "5"). But if the string was in fact just "20" (possible for a single entry in an RPG table), this would lead to an error since it would only split out a single element - and we expected two.

By using *maxval (with the *), maxval is told to expect 0 or more elements in a tuple. So the result for 1-5 will be ("1", ("5",)) and for 20 it will become ("20", ()). In the line

maxval = abs(int(maxval[0]) if maxval else minval)

we check if maxval actually has a value ("5",) or if its empty (). The result is either "5" or the value of minval.

2.3.8. Roll for death

While original Knave suggests hitting 0 HP means insta-death, we will grab the optional “death table” from the “prettified” Knave’s optional rules to make it a little less punishing. We also changed the result of 2 to ‘dead’ since we don’t simulate ‘dismemberment’ in this tutorial:



-1d4 Loss of Ability






















All the non-dead values map to a loss of 1d4 in one of the six Abilities (but you get HP back). We need to map back to this from the above table. One also cannot have less than -10 Ability bonus, if you do, you die too.

# in mygame/evadventure/rules.py 

death_table = (
    ("1-2", "dead"),
    ("3", "strength"),
    ("4", "dexterity"),
    ("5", "constitution"),
    ("6", "intelligence"),
    ("7", "wisdom"),
    ("8", "charisma"),
class EvAdventureRollEngine:
    # ... 

    def roll_random_table(...)
        # ... 
    def roll_death(self, character): 
        ability_name = self.roll_random_table("1d8", death_table)

        if ability_name == "dead":
            # TODO - kill the character! 
            loss = self.roll("1d4")
            current_ability = getattr(character, ability_name)
            current_ability -= loss
            if current_ability < -10: 
                # TODO - kill the character!
                # refresh 1d4 health, but suffer 1d4 ability loss
                self.heal(character, self.roll("1d4"))
                setattr(character, ability_name, current_ability)
                    "You survive your brush with death, and while you recover "
                    f"some health, you permanently lose {loss} {ability_name} instead."
dice = EvAdventureRollEngine()

Here we roll on the ‘death table’ from the rules to see what happens. We give the character a message if they survive, to let them know what happened.

We don’t yet know what ‘killing the character’ technically means, so we mark this as TODO and return to it in a later lesson. We just know that we need to do something here to kill off the character!

2.4. Testing

Make a new module mygame/evadventure/tests/test_rules.py

Testing the rules module will also showcase some very useful tools when testing.

# mygame/evadventure/tests/test_rules.py 

from unittest.mock import patch 
from evennia.utils.test_resources import BaseEvenniaTest
from .. import rules 

class TestEvAdventureRuleEngine(BaseEvenniaTest):
    def setUp(self):
        """Called before every test method"""
        self.roll_engine = rules.EvAdventureRollEngine()
    def test_roll(self, mock_randint):
        mock_randint.return_value = 4 
        self.assertEqual(self.roll_engine.roll("1d6"), 4)     
        self.assertEqual(self.roll_engine.roll("2d6"), 2 * 4)     
    # test of the other rule methods below ...

As before, run the specific test with

evennia test --settings settings.py .evadventure.tests.test_rules

2.4.1. Mocking and patching

The setUp method is a special method of the testing class. It will be run before every test method. We use super().setUp() to make sure the parent class’ version of this method always fire. Then we create a fresh EvAdventureRollEngine we can test with.

In our test, we import patch from the unittest.mock library. This is a very useful tool for testing. Normally the randint function we imported in rules will return a random value. That’s very hard to test for, since the value will be different every test.

With @patch (this is called a decorator), we temporarily replace rules.randint with a ‘mock’ - a dummy entity. This mock is passed into the testing method. We then take this mock_randint and set .return_value = 4 on it.

Adding return_value to the mock means that every time this mock is called, it will return 4. For the duration of the test we can now check with self.assertEqual that our roll method always returns a result as-if the random result was 4.

There are many resources for understanding mock, refer to them for further help.

The EvAdventureRollEngine have many methods to test. We leave this as an extra exercise!

2.5. Summary

This concludes all the core rule mechanics of Knave - the rules used during play. We noticed here that we are going to soon need to establish how our Character actually stores data. So we will address that next.