Esper is a lightweight Entity System module for Python, with a focus on performance

Esper is an MIT licensed Entity System, or, Entity Component System (ECS). The design is based on the Entity System concepts outlined by Adam Martin in his blog at http://t-machine.org/, and others. The primary focus is on keeping it as lightweight and performant as possible, while handling common use cases.

The following Wikipedia article provides a summary of the ECS pattern: https://en.wikipedia.org/wiki/Entity_component_system

API documentation is hosted at ReadTheDocs: https://esper.readthedocs.io Due to the small size of the project, this README currently serves as general usage documentation.

:warning: Esper 3.0 introduces breaking changes. Version 3.0 removes the World object, and migrates its methods to module level functions. Multiple contexts can be created and switched between. The v2.x README can be found here: https://github.com/benmoran56/esper/blob/v2_maintenance/README.md

• Compatibility
• Installation
• Design
• Quick Start
• General Usage
  • Adding and Removing Processors
  • Adding and Removing Components
  • Querying Specific Components
  • Boolean and Conditional Checks
  • More Examples
• Event Dispatching
• Contributing

Compatibility

Esper attempts to target all currently supported Python releases (not EOL). Esper is written in 100% pure Python, so any compliant interpreter should work. Automated testing is currently done for both CPython and PyPy3.

Installation

Esper is a pure Python package with no dependencies, so installation is not strictly necessary. You can simply copy the esper folder into your project, and import esper. If you do want to install it into your site-packages, you can do so by using setup.py::

python setup.py install --user

Or from PyPi via pip::

pip install --user --upgrade esper

Design

• World Context

Esper uses the concept of "World" contexts. When you import esper, a default context is active. You create Entities, assign Components, register Processesors, etc., by calling functions on the esper module. Entities, Components and Processors can be created, assigned, or deleted while your game is running. A simple call to esper.process() is all that's needed for each iteration of your game loop. Advanced users can switch contexts, which can be useful for isolating different game scenes that have different Processor requirements.

• Entities

Entities are simple integer IDs (1, 2, 3, 4, etc.). Entities are "created", but they are generally not used directly. Instead, they are simply used as IDs in the internal Component database to track collections of Components. Creating an Entity is done with the esper.create_entity() function.

• Components

Components are defined as simple Python classes. In keeping with a pure Entity System design philosophy, they should not contain any logic. They might have initialization code, but no processing logic whatsoever. A simple Component can be defined as::

class Position:
    def __init__(self, x=0.0, y=0.0):
        self.x = x
        self.y = y

In addition, the excellent dataclass decorator is available in Python 3.7+. https://docs.python.org/3/library/dataclasses.html#module-dataclasses This decorator simplifies defining your Component classes. The attribute names don't need to be repeated, and you can still instantiate the Component with positional or keyword arguments::

from dataclasses import dataclass as component

@component
class Position:
    x: float = 0.0
    y: float = 0.0

• Processors

Processors, also commonly known as "Systems", are where all processing logic is defined and executed. All Processors must inherit from the esper.Processor class, and have a method called process. Other than that, there are no restrictions. You can define any additional methods you might need. A simple Processor might look like::

class MovementProcessor(esper.Processor):

    def process(self):
        for ent, (vel, pos) in esper.get_components(Velocity, Position):
            pos.x += vel.x
            pos.y += vel.y

In the above code, you can see the standard usage of the esper.get_components() function. This function allows efficient iteration over all Entities that contain the specified Component types. This function can be used for querying two or more components at once. Note that tuple unpacking is necessary for the return component pairs: (vel, pos). In addition the Components, you also get a reference to the Entity ID (the ent object) for the current pair of Velocity/Position Components. This entity ID can be useful in a variety of cases. For example, if your Processor will need to delete certain Entites, you can call the esper.delete_entity() function on this Entity ID. Another common use is if you wish to add or remove a Component on this Entity as a result of some condition being met.

Quick Start

To get started, simply import esper::

import esper

From there, define some Components, and create Entities that use them::

player = esper.create_entity()
esper.add_component(player, Velocity(x=0.9, y=1.2))
esper.add_component(player, Position(x=5, y=5))

Optionally, Component instances can be assigned directly to the Entity on creation::

player = esper.create_entity(Velocity(x=0.9, y=1.2), Position(x=5, y=5))

Design some Processors that operate on these Component types, and then register them with Esper for processing. You can specify an optional priority (higher numbers are processed first). All Processors are priority "0" by default::

movement_processor = MovementProcessor()
collision_processor = CollisionProcessor()
rendering_processor = RenderingProcessor()
esper.add_processor(collision_processor, priority=2)
esper.add_processor(movement_processor, priority=3)
esper.add_processor(rendering_processor)
# or just add them in one line: 
esper.add_processor(SomeProcessor())

Executing all Processors is done with a single call to esper.process(). This will call the process method on all assigned Processors, in order of their priority. This is usually called once per frame update of your game (every tick of the clock).::

esper.process()

Note: You can pass any arguments (or keyword arguments) you need to esper.process(), but you must also make sure to receive them properly in the process() methods of your Processors. For example, if you pass a delta time argument as esper.process(dt), your Processor's process() methods should all receive it as: def process(self, dt): This is appropriate for libraries such as pyglet, which automatically pass a delta time value into scheduled functions.

General Usage

World Contexts

Esper has the capability of supporting multiple "World" contexts. On import, a "default" World is active. All creation of Entities, assignment of Processors, and all operations exist within the confines of a World. For advanced use cases Esper allows you to switch between multiple Worlds, which are completely isolated from each other. This can be useful when different scenes in your game have different Entities and Processor requirements. World context operations are done with the following functions::

• esper.list_worlds()
• esper.switch_world(name)
• esper.delete_world(name)

When switching Worlds, be careful of the name. If a World doesn't exist, it will be created. You can delete old Worlds which are no longer needed, but you cannot delete the currently active World.

Adding and Removing Processors

You have already seen examples of adding Processors in an earlier section. There is also a remove_processor function available:

• esper.add_processor(processor_instance)
• esper.remove_processor(ProcessorClass)

Depending on the structure of your game, you may want to add or remove certain Processors when changing scenes, etc.

Adding and Removing Components

In addition to adding Components to Entities when you're creating them, it's a common pattern to add or remove Components inside your Processors. The following functions are available for this purpose:

• esper.add_component(entity_id, component_instance)
• esper.remove_component(entity_id, ComponentClass)

As an example of this, you could have a "Blink" component with a duration attribute. This can be used to make certain things blink for s specific period of time, then disappear. For example, the code below shows a simplified case of adding this Component to an Entity when it takes damage in one processor. A dedicated BlinkProcessor handles the effect, and then removes the Component after the duration expires::

class BlinkComponent:
    def __init__(self, duration):
        self.duration = duration


.....


class CollisionProcessor(esper.Processor):

    def process(self, dt):
        for ent, enemy in esper.get_component(Enemy):
            ...
            is_damaged = self._some_method()
            if is_damaged:
                esper.add_component(ent, BlinkComponent(duration=1))
            ...


class BlinkProcessor(esper.Processor):

    def process(self, dt):
        for ent, (rend, blink) in esper.get_components(Renderable, BlinkComponent):
            if blink.duration < 0:
                # Times up. Remove the Component:
                rend.sprite.visible = True
                esper.remove_component(ent, BlinkComponent)
            else:
                blink.duration -= dt
                # Toggle between visible and not visible each frame:
                rend.sprite.visible = not rend.sprite.visible

Querying Specific Components

If you have an Entity ID and wish to query one specific, or ALL Components that are assigned to it, the following functions are available:

• esper.component_for_entity
• esper.components_for_entity

The component_for_entity function is useful in a limited number of cases where you know a specific Entity ID, and wish to get a specific Component for it. An error is raised if the Component does not exist for the Entity ID, so it may be more useful when combined with the has_component function that is explained in the next section. For example::

if esper.has_component(ent, SFX):
    sfx = esper.component_for_entity(ent, SFX)
    sfx.play()

The components_for_entity function is a special function that returns ALL the Components that are assigned to a specific Entity, as a tuple. This is a heavy operation, and not something you would want to do each frame or inside your Processor.process method. It can be useful, however, if you wanted to transfer all of a specific Entity's Components between two separate contexts (such as when changing Scenes, or levels). For example::

player_components = esper.components_for_entity(player_entity_id)
esper.switch_world('context_name')
player_entity_id = esper.create_entity(player_components)

Boolean and Conditional Checks

In some cases you may wish to check if an Entity has a specific Component before performing some action. The following functions are available for this task:

• esper.has_component(entity, ComponentType)
• esper.has_components(entity, ComponentTypeA, ComponentTypeB)
• esper.try_component(entity, ComponentType)
• esper.try_components(entity, ComponentTypeA, ComponentTypeB)

For example, you may want projectiles (and only projectiles) to disappear when hitting a wall in your game. We can do this by checking if the Entity has a Projectile Component. We don't want to do anything to this Component, simply check if it's there. Consider this example::

class CollisionProcessor(esper.Processor):

    def process(self, dt):
        for ent, body in esper.get_component(PhysicsBody):
            ...
            colliding_with_wall = self._some_method(body):
            if colliding_with_wall and esper.has_component(ent, Projectile):
                esper.delete_entity(ent)
            ...

In a different scenario, we may want to perform some action on an Entity's Component, if it has one. For example, a MovementProcessor that skips over Entities that have a Stun Component::

class MovementProcessor(esper.Processor):

    def process(self, dt):
        for ent, (body, vel) in esper.get_components(PhysicsBody, Velocity):

            if esper.has_component(ent, Stun):
                stun = esper.component_for_entity(ent, Stun)
                stun.duration -= dt
                if stun.duration <= 0:
                    esper.remove_component(ent, Stun)
                continue    # Continue to the next Entity

            movement_code_here()
            ...

Let's look at the core part of the code::

if esper.has_component(ent, Stun):
    stun = esper.component_for_entity(ent, Stun)
    stun.duration -= dt

This code works fine, but the try_component function can accomplish the same thing with one less function call. The following example will get a specific Component if it exists, or return None if it does not::

stun = esper.try_component(ent, Stun)
if stun:
    stun.duration -= dt

With Python 3.8+, the new "walrus" operator (:=) can also be used, making the try_component functions even more concise ::

if stun :=  esper.try_component(ent, Stun):
    stun.duration -= dt

More Examples

See the /examples folder to get an idea of how the basic structure of a game might look.

Event Dispatching

Esper includes basic support for event dispatching and handling. This functionality is provided by three functions to set (register), remove, and dispatch events. Minimal error checking is done, so it's left up to the user to ensure correct naming and number of arguments are used when dispatching and receiving events.

Events are dispatched by name::

esper.dispatch_event('event_name', arg1, arg2)

In order to receive the above event, you must register handlers. An event handler can be a function or class method. Registering a handler is also done by name::

esper.set_handler('event_name', my_func)
# or
esper.set_handler('event_name', self.my_method)

Note: Only weak-references are kept to the registered handlers. If a handler is garbage collected, it will be automatically un-registered by an internal callback.

Handlers can also be removed at any time, if you no longer want them to receive events::

esper.remove_handler('event_name', my_func)
# or
esper.remove_handler('event_name', self.my_method)

Registered events and handlers are part of the current World context.

Contributing

Contributions to Esper are always welcome, but there are some specific project goals to keep in mind:

• Pure Python code only: no binary extensions, Cython, etc.
• Try to target all non-EOL Python versions. Exceptions can be made if there is a compelling reason.
• Avoid bloat as much as possible. New features will be considered if they are commonly useful. Generally speaking, we don't want to add functionality that is better handled in another module or library.
• Performance is preferrable to readability.

If you have any questions before contributing, feel free to open an issue.