Welcome to utilitai!

This package is a minimal(ish) implementation of Utility AI in Python. It is designed to complement btreeny, my Behaviour Trees implementation!

There actually aren't many options for utility AI in Python, which surprises me! In general for serious systems you should be looking at C++ (or, maybe, Rust 🦀), but hopefully this library will help on the prototyping journey!

The idea

List the things your agent could do, score each of them against the current state of the world, and do whichever scores highest.

from dataclasses import dataclass

from utilitai import ToConsider, curves


# In order to make decisions, we need context - this can be anything but
# a dataclass is a logical choice
@dataclass
class Context:
    hunger: int
    money: int
    food: int


MAX_HUNGER = 10

# We create a registry object which is generic over the context
things: ToConsider[Context] = ToConsider()


# And then add options to consider
@things.option("go to the shops")
def go_to_the_shops(ctx: Context):
    hunger_level = curves.logistic(ctx.hunger / MAX_HUNGER, midpoint=0.5)
    return hunger_level * curves.is_gt_zero(ctx.money)


# Names can be inferred from function name
@things.option
def eat_food(ctx: Context):
    hunger_level = curves.logistic(ctx.hunger / MAX_HUNGER, midpoint=0.5)
    return hunger_level * curves.is_gt_zero(ctx.food)


# We can also add anoption that has a constant utility
things.constant_option("do nothing", 0.1)

current_context = Context(hunger=5, money=1, food=0)
action = things.consider(current_context)
assert action == "go to the shops"

Scoring functions

A scoring function takes your context and returns a float. That's the whole interface - no subclassing, no DSL, and the decorated function is returned unchanged so you can still call, test, or reuse it elsewhere.

Scores are only ever compared against each other, but keeping them in [0, 1] makes them much easier to reason about. The utilitai.curves module has the usual suspects for shaping a raw value into that range:

Because scoring functions are plain Python, curves compose by calling them and combining the results - multiply to say "and", max to say "or", and multiply by a constant to weight an option against its rivals:

@things.option("order a takeaway")
def order_a_takeaway(ctx: Context):
    # Weighted below the other options - it's a treat, not a necessity
    return 0.5 * curves.smoothstep(ctx.hunger) * curves.is_gt_zero(ctx.money)

Choosing between options

• things.consider(context) returns the name of the highest scoring option. If two options tie, they fall back to priority and then alphabetic on name. Take this into consideration!
• things.constant_option(name, value) adds an option with a fixed score. This is the usual way to express "if nothing else appeals, do this".
• things.score(context) returns every option's score as a dict, which is useful in tests and when tuning curves.
• consider raises ValueError if nothing has been added, and option raises ValueError on a duplicate name.

Each call to consider logs the full set of scores at DEBUG level under the utilitai logger, so you can see why a decision was made:

import logging

logging.basicConfig(level=logging.DEBUG)

Considerations and shared dependencies

In the above, we've repeated ourselves in a couple of places - for instance computing hunger_level. We can see leverage utilitai's "considerations" to define dependencies for our options:

# We create a registry object which is generic over the context
things: ToConsider[Context] = ToConsider()


@things.consideration
def has_money(ctx: Context):
    # Note that by returning `None` we signal to `utilitai` that
    # any dependencies must also be None, and bypass any downstream
    # maths.
    # Feel free to return 0.0 if you don't want this.
    return None if ctx.money == 0 else 1.0


@things.consideration
def has_food(ctx: Context):
    # Note that by returning `None` we signal to `utilitai` that
    # any dependencies must also be None, and bypass any downstream
    # maths.
    # Feel free to return 0.0 if you don't want this.
    return None if ctx.food == 0 else 1.0


@things.consideration
def hunger_level(ctx: Context):
    return curves.logistic(hunger / MAX_HUNGER, midpoint=0.5)


# Note that now the utility of eat_food and go_to_the_shops will be
# identical - making it easy to spot undesirable ties.
# Ties are broken using a priority flag (which defaults to 0)
@things.option("eat_food", priority=1)
def eat_food(ctx: Context, has_food: float, hunger_level: float):
    return hunger_level


@things.option("go to the shops")
def go_to_the_shops(ctx: Context, has_money: float, hunger_level: float):
    return hunger_level


things.constant_option("do nothing", 0.1)

current_context = Context(hunger=5, money=1, food=0)
action = things.consider(current_context)
assert action == "go to the shops"

A slightly bigger example

actions: ToConsider[RobotState] = ToConsider()

# If we have no good choices, do nothing
actions.constant_option("do nothing", 1e-6)


@actions.option("recharge")
def recharge(state: RobotState) -> float:
    # As our battery gets lower, our desire to stop and recharge gets higher
    battery_used = 1 - state.remaining_battery_percentage / 100
    return curves.inverse_quadratic(battery_used)


@actions.option("move to goal")
def move_to_goal(state: RobotState) -> float:
    if is_at_goal(state) or not has_valid_path(state):
        return 0.0
    return 0.9 if can_reach_goal_with_battery(state) else 0.1


@actions.option("do a dance")
def do_a_dance(state: RobotState) -> float:
    # Dancing before we make it to the goal would be silly!
    return 1.0 if is_at_goal(state) else 0.0


while True:
    # Perception
    state = update_state_from_sensors(state)
    # Planning (ish)
    best_action = actions.consider(state)
    # Control - maybe using btreeny!
    control_robot_from_action(best_action, state)

There are also a growing number of examples available for you to peruse.