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A 5e D&D encounter simulator written for my own amusement to test some hypotheses.


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DnD Encounter simulator

Simulate who would win in a Dungeons and Dragons encounter

This is a python 3 script and is not intended to work with 2. Some folk may have made forks that do —I don't know. This code was my first project switching from Perl to Python, so was rather messy. Due to the interest I have refactored it to make it cleaner (see changelog 0.2). For work-in-progress see dev branch.

Welcome to the D&D 5e Encounter simulator. It was written to determine victory probabilities and to test some hypotheses. An online version of the simulator.

NB. The server goes down occassionally —primarily due to the fact that if a request times out the encounter simulation continues.
So if it is down, please feel free to email matteo dot ferla at and I'll reboot it. NB. A repository of the server is available here.


This module is not pip released because it was never finished —the main branch will be stable though. Two big requests are to make the combat on a grid, and the other make the tactics trained —the logic is conditionals based atm. Also the code became quickly sprawling so I started doing a refactor two years ago and I have only given it 2-3 hours since because work projects, work-related projects, other projects (like a Raspberry Pi Furby) and real life take more of my free time.

Monster manual

The simulator relies on creature information present in the beastiary.csv file. This file was kindly compiled by Jeff Fox. It contains all creature present in the D&D 5e SDR and is distributed under the following licence: Open Game License v 1.0a Copyright 2000, Wizards of the Coast, Inc. Copyright 2016, Wizards of the Coast, Inc.

#Documentation This module allows the simulation of a D&D encounter. It has three main classes: Dice (and its derivatives), Character, Encounter. It also has a csv file (beastiary.csv) containing all 5e SDR monsters.

Teams. Multiple creatures of the same alignment will team up to fight creatures of different alignments in a simulation (Encounter().battle() for a single iteration or Encounter().go_to_war() for multiple). Gridless. The game assumes everyone is in contact with everyone and not on a grid. The reason being is tactics. Tactics. Tactics are highly problematic both in targetting and actions to take. Players do not play as strategically as they should due to heroism and kill tallies, while the DM might play monsters really dumbly to avoid a TPK. Targetting. The simulator is set up as a munchkin combat where everyone targets the weakest opponent (The global variable TARGET="enemy alive weakest" makes the find_weakest_target method of the Encounter be used, but could be changed (unwisely) to a permutation of enemy/ally alive/dead weakest/random/fiercest. The muchkinishness has a deleterious side-effect when the method deathmatch of the Encounter class is invoked —this allocates each Creature object in the Encounter object to a different team. Actions. Action choice is dictated by turn economy. A character of a team with the greater turn economy will dodge (if it knows itself a target) or throw a net (if it has one), and so forth while a creature on the opposed side will opt for a slugfest.

>>> from DnD_battler import Creature, Encounter
>>> Creature.load('aboleth') # get from beastiary
>>> level1 = Creature(name="buff peasant", abilities = {'str': 15,'dex': 14,'con':13,'int':12,'wis':10,'cha': 8}, alignment ="good", attack_parameters=['longsword'])
>>> billybob = Creature(name = "lich")
>>> billybob.alignment = "good"  #the name of the alignment means only what team name they are in.  
>>> arena = DnD.Encounter(level1, 'badger')  #Encounter accepts both Creature and strings.
>>> print(arena.go_to_war(10000)) #simulate 10,000 times
>>> print(arena.battle()) # simulate one encounter and tell what happens.
>>> print(Creature.load('tarrasque').generate_character_sheet())  #md character sheet.
>>> print(Encounter.load("ancient blue dragon").addmob(85).go_to_war(10))  #An ancient blue dragon is nearly a match for 85 commoners (who crit evenutally)...

Creature: parameters and attributes

The creature class can be started from scratch or from a monster from the manual:

from DnD_battler import Creature

Both accept several arguments.

from DnD_battler import Creature
Creature(name="Achilles", alignment='Achaeans')
Creature.load(creature_name='commoner', name="Achilles", alignment='Achaeans')

Technically, these are set via apply_parameters. These are:

  • name (str) name of creature for logs, stored in
  • base (str) No longer accepted as parameter. Please use Creature.load(creature_name). The attribute .base is just a keepsake, altering does nothing.
  • xp (int) Experience points, does nothing. Stored in creature.xp.
  • size (str) sets the size. Note the attribute creature.size is a Size instance. alter for effects.
  • alignment (str) not quite the alignment but the side in the encounter. In future I may split these and use alignment to determine side.
  • arena (Encounter) the encounter object itself. Stored in creature.arena.
  • level (int) the level. Stored in creature.level, but use set_level to alter and it affects the .proficiency attribute.
  • proficiency (int) the proficiency bonus, however, the .proficiency attribute is a Proficiency instance. the .proficiency.bonus attribute is the bonus. It scales automatically with level.
  • hd (int) hit dice number of faces. Alters .hit_die.num_faces. Trigger hp recalculation if no hp specified.
  • hp (int) hit points. Note this is calculated automatically otherwise. .hp is the current .hp, .starting_hp is the pre-battle one.
  • abilities (dict), ability_bonuses (dict), str (int), dex (int) etc. ab_str (int) etc.: abilities and ability_bonuses are potentially incomplete dict of 3-letter ability and score/bonus. 3-letter ability take precedence. Bonus takes precedence over score (note that if a mismatching score/bonus is given the score will be kept and not corrected —it has no effect. The abilities are stored as 3-letter attributes with a unique Ability die. Proficiency is already added. so creature.str.bonus is the bonus, creature.str.temp_modifier is a temp modifier and creature.str.score is the score. Note that derived abilities, such as attack rolls and skill checks (AttackRoll and SkillRoll) are dependent on this die, so change the properties as opposed to setting a new one. This allows AttackRolls to have a weapon-specific attack bonus (in addition to a damage bonus) which gets added to a .proficiency.bonus, ability_die.temp_modifier and ability_die.bonus.
  • initiative_bonus (int): this alters the .initiative.modifier as initiative is a SkillRoll.


The Dice object is easy. It has .num_faces, a .bonus and a .avg boolean flag which controls whether rolls are always an average (NPC style). The method roll rolls the dice and adds the bonus (base_roll does not).

Then Ability extends this by taking into account Proficiency stored in the .proficiency attribute. temp_modifier and taking account of advantage. score does nothing really.

Then SkillRoll wraps around an ability die adding a modifier. Note that bonuses (plural) gives the sum of the bonuses. The attribute bonus is not used. Altering an ability die will automatically affect the dependent skill rolls.

Then AttackRoll extends SkillRoll further and has a bound damage dice. attack against an AC value rolls and returns damage.


The module uses a shared logging.Logger with a sys.stdout stream set to logging.INFO.

from DnD_battler import Creature, log

log is Creature.log

Therefore, alter the logging to a different handler if needed as per usual.

from DnD_battler import log
import logging, io

# change the default...
log.handlers[0].setLevel(logging.DEBUG)  # logging.DEBUG = 20
# add a new one.
stream = io.StringIO('started')
handler = logging.StreamHandler(stream)
handler.setFormatter(logging.Formatter('[%(asctime)s] %(levelname)s - %(message)s'))
log.addHandler(handler)'Next battle!')

Note on altering methods

The behaviour of a Creature is dictated by act() class method. Specifically, the simulations runs n encounters via Encounter(<...>).go_to_war(n)_, which runs .battle() n times. The latter method iterates across the creatures running their Creature().act() method, which makes them decide whether to heal, dodge, attack, free themselves, buff, throw net etc. The attack is called .multiattack() If you want to override the behaviour of say a creature to attack regardlessly and at random you can change the class's method act()

import random, DnD, types

donald=Creature("Donald", alignment='Murica')
rex=DnD.Creature.load(creature_name="owlbear", name="Rex", alignment='Murica', int=1, wis=1)

# new method
def trumpconomy(self, assess=0):
    if not self.arena.find('alive enemy') and len(self.arena.find('alive ally')) == 1: #TrumpMod: Win when all bar one.
        raise Encounter.Victory()
    for i in range(len(self.attacks)):
            opponent = random.choice([other for other in self.arena.combattants if other is not self]) #TrumpMod kill all bar self.
        except IndexError:
            raise self.arena.Victory()
        self.log.debug( + ' attacks ' + + ' with ' + str(self.attacks[i]['name']))
        # This was the hit method. put here for now.
        self.attacks[i]['attack'].advantage = self.check_advantage(opponent)
        if self.attacks[i]['attack'].roll() >=
            # self.attacks[i]['damage'].crit = self.attacks[i]['attack'].crit  #Pass the crit if present.
            h = self.attacks[i]['damage'].roll()
            self.tally['damage'] += h
            self.tally['hits'] += 1
            self.tally['misses'] += 1

# adding the unbound method as a bound method...
donald.act=types.MethodType(trumpconomy, donald)


In round one Donald attacks his ally Rex, thus proving the behavior is altered.

Class summary


Dice accepts bonus plus an int —8 is a d8— or a list of dice —[6,6] is a 2d6— or nothing —d20. roll() distinguishes between a d20 and not. d20 crits have to be passed manually.


Character has a boatload of attributes. It can be initialised with a dictionary or an unpacked one... or a single name matching a preset.


Encounter includes the following method: battle(reset=1) does a single battle (after a reset of values if asked). it calls a few other fuctions such as roll_for_initiative() go_to_war(rounds=1000) performs many battles and gives the team results

There is some code messiness resulting from the unclear distinction between Encounter and Creature object, namely a Creature interacting with another is generally a Creature method, while a Creature searching across the list of Creatures in the Encounter is an Encounter method.

There are one or two approximations that are marked #NOT-RAW. In the Encounter.battle method there are some thought on the action choices.

class Creature(builtins.object)

Creature class handles the creatures and their actions and some interactions with the encounter.

Methods defined here:

TBA_act(self, verbose=0) # TODO

init(self, wildcard, **kwargs) Creature object creation. A lot of parameters make a creature so a lot of assumptions are made (see init`). :param wildcard: the name of the creature. If nothing else is passed it will take it from the beastiary. If a dictionary is passed, it will process it like **kwargs, If a Creature object is passed it will make a copy :param kwargs: a lot of arguments... :return: a creature.

The arguments are many.
>>> print(Creature(Creature('aboleth'), ac=20).__dict__)
`{'abilities': None, 'dex': 10, 'con_bonus': 10, 'cr': 17, 'xp': 5900, 'ac': 20, 'starting_healing_spells': 0, 'starting_hp': 135, 'condition': 'normal', 'initiative': <__main__.Dice object at 0x1022542e8>, 'str': 10, 'wis': 10, 'ability_bonuses': {'int': 0, 'cha': 0, 'dex': 0, 'con': 0, 'str': 0, 'wis': 0}, 'custom': [], 'hd': <__main__.Dice object at 0x102242c88>, 'hurtful': 36.0, 'tally': {'rounds': 0, 'hp': 0, 'battles': 0, 'hits': 0, 'damage': 0, 'healing_spells': 0, 'dead': 0, 'misses': 0}, 'hp': 135, 'proficiency': 5, 'cha_bonus': 10, 'able': 1, 'healing_spells': 0, 'copy_index': 1, 'int': 10, 'concentrating': 0, 'wis_bonus': 10, 'con': 10, 'int_bonus': 10, 'sc_ab': 'con', 'str_bonus': 10, 'level': 18, 'settings': {}, 'arena': None, 'dex_bonus': 10, 'log': '', 'cha': 10, 'dodge': 0, 'alt_attack': {'attack': None, 'name': None}, 'alignment': 'lawful evil ', 'attacks': [{'attack': <__main__.Dice object at 0x1022545f8>, 'damage': <__main__.Dice object at 0x1022545c0>, 'name': 'tentacle'}, {'attack': <__main__.Dice object at 0x102254668>, 'damage': <__main__.Dice object at 0x102254630>, 'name': 'tentacle'}, {'attack': <__main__.Dice object at 0x1022546d8>, 'damage': <__main__.Dice object at 0x1022546a0>, 'name': 'tentacle'}], 'attack_parameters': [['tentacle', 9, 5, 6, 6], ['tentacle', 9, 5, 6, 6], ['tentacle', 9, 5, 6, 6]], 'buff_spells': 0, 'temp': 0, 'name': 'aboleth'}`

str(self) Return str(self).

act(self, verbose=0)

assess_wounded(self, verbose=0)


cast_healing(self, weakling, verbose=0)

cast_nothing(self, state='activate')

check_action(self, action, verbose) # TODO

check_advantage(self, opponent)

copy(self) :return: a copy of the creature.

do_action(self, action, verbose) # TODO

generate_character_sheet(self) An markdown character sheet. :return: a string

heal(self, points, verbose=0)


multiattack(self, verbose=0, assess=0)

net(self, opponent, verbose=0)



set_level(self, level=None) Alter the level of the creature. :param level: opt. int, the level. if absent it will set it to the stored level. :return: nothing. changes self.

take_damage(self, points, verbose=0)

Static methods defined here:

clean_settings(dirtydex) Sanify the settings :return: a cleaned dictionary

Data descriptors defined here:

dict dictionary for instance variables (if defined)

weakref list of weak references to the object (if defined)

Data and other attributes defined here:

ability_names = ['str', 'dex', 'con', 'wis', 'int', 'cha']

beastiary = {'aboleth': {'AB_Cha': '4', 'AB_Con': '0', 'AB_Dex': '0', ...

class Dice(builtins.object)

Methods defined here:

init(self, bonus=0, dice=20, avg=False, twinned=None, role='ability') Class to handle dice and dice rolls :param bonus: int, the bonus added to the attack roll :param dice: list of int, the dice size. :param avg: boolean flag marking whether the dice always rolls average, like NPCs and PCs on Mechano do for attack rolls. :param twinned: a dice. ja. ehrm. this is the other dice. The crits are passed to it. It should be a weak ref or the crits passed more pythonically. :param role: string, but actually on a restricted vocabulary: ability, damage, hd or healing. Extras can be added, but they won't trigger some things :return: a rollable dice!

The parameters are set to attributes. Other attributes are:
* critable: determined from `role` attribute
* cirt: 0 or 1 ... or more if you want to go 3.5 and crit train.
* advantage: trinary int. -1 is disadvantage, 0 normal, 1 is advantage.

str(self) This is rather inelegant piece of code and is not overly flexible. If the dice fail to show, they will still work. :return: string in dice notation.

icosaroll(self, verbose=0) A roll that is a d20. It rolls advantage and disadvatage and calls _critcheck. :param verbose: :return:

multiroll(self, verbose=0) A roll that is not a d20. It adds the bonus and rolls (x2 if a crit). :param verbose: :return:

roll(self, verbose=0) The roll method, which calls either icosaroll or multiroll. :param verbose: debug :return: the value rolled (and alters the dice too if need be)

class Encounter(builtins.object)

In a dimensionless model, move action and the main actions dash, disengage, hide, shove back/aside, tumble and overrun are meaningless. weapon attack —default two-weapon attack — Good when the opponent has low AC (<12) if 2nd attack is without proficiency. Stacks with bonuses such as sneak attack or poisoned weapons -neither are in the model. Due to the 1 action for donning/doffing a shield, switch to two handed is valid for unshielded folk only. Best keep two weapon fighting as a prebuild not a combat switch. AoE spell attack — Layout… targetted spell attack —produce flame is a cantrip so could be written up as a weapon. The bigger ones. Spell slots need to be re-written. spell buff —Barkskin is a druidic imperative. Haste? Too much complication. spell debuff —Bane… dodge —targeted and turn economy help —high AC target (>18), turn economy, beefcake ally ready —teamwork preplanning. No way. grapple/climb —very situational. grapple/shove combo or barring somatic. disarm —disarm… grey rules about whether picking it up or kicking it away is an interact/move/bonus/main action. netting is a better option albeit a build. called shot —not an official rule. Turn economy.

Methods defined here:

add(self, other)

getitem(self, item)

init(self, *lineup) Initialize self. See help(type(self)) for accurate signature.



str(self) Return str(self).

addmob(self, n)

append(self, newbie)

battle(self, reset=1, verbose=1)


extend(self, iterable)

find(self, what, searcher=None, team=None)

go_to_war(self, rounds=1000)




roll_for_initiative(self, verbose=0)


Data and other attributes defined here:

Victory = <class 'DnD_Battler.Encounter.Victory'> The way the encounter ends is a victory error is raised to stop the creatures from acting further.


A 5e D&D encounter simulator written for my own amusement to test some hypotheses.







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