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An ECS (Entity Component System) for Python
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Benjamin Moran
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README.rst

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Esper

Esper is a lightweight Entity System 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. Efforts were made to keep it as lightweight and performant as possible.

There is a fairly accurate writeup describing Entity Systems in this Wikipedia article: https://en.wikipedia.org/wiki/Entity_component_system

Inspired by Sean Fisk's ecs https://github.com/seanfisk/ecs, and Marcus von Appen's ebs https://bitbucket.org/marcusva/python-utils.

What's New

1.2 - Calls to super() are no longer necessary in your Processor subclasses.
This should eliminate a fair amount of boilerplate. The README has also been updated with more usage examples. All methods should now have at least one example. And finally, wheels are now uploaded to PyPi. This should help with packaging systems that only support wheels. Addresses issue #38.
1.0.0 - Esper is now using simple lru_caching internally by default. The cache is currently
flushed when adding or deleting Entities or Components from the World, Component queries are much faster otherwise. This will likely be improved in a Future version. In addition to caching, Esper now supports passing kwargs to Processors. Continuous Integration testing is now being done for Python 3.7.
0.9.9 - The big change in this release is that esper has been condensed into a single
file: esper.py. This will make it simple to just drop into your project folder, without cluttering your project with additional folders that didn't really need to exist. You can still install it from PyPi via pip if you wish, but it's easy enough to just ship with your project (and of course the license allows for this).
0.9.8 - This release contains a new timer that can be enabled to profile Processor execution
time. Simply pass the "timed=True" parameter to the World on instantiation, and a new World.process_times dictionary will be available. This contains the total execution time of each Processor in milliseconds, and can be logged, printed, or displayed on screen as is useful. It's useful to see a quick profile of which processors are using the most cpu time, without fully profiling your game. This release also contains some consolidations and cleanups for the benchmarks.
0.9.7 - By default, entities are now lazily deleted. When calling World.delete_entity(entity_id),
Entities are now placed into a queue to be deleted at the beginning of the next call to World.process(). This means it is now safe to delete entities even while iterating over components in your processors. This should allow for cleaner Processor classes, by removing the need to manually track and delete "dead" Entities after iteration. If you do wish to delete an Entity immediately, simply pass the new optional immediate=True argument. Ie: self.world.delete_entity(entity, immediate=True).

1) Compatibility

Esper is a Python 3 library only. Specifically, all currently supported versions of Python 3. It also supports Pypy3. Being written in pure Python, it should work on any compliant interpreter. Continuous Integration (automated testing) is done for both CPython and PyPy3.

2) Installation

No installation is necessary. Esper is a tiny library with no dependencies. Simply copy esper.py into the top level of your project folder, and import esper.

If you prefer, Esper is also available on PyPI for easy installation via pip.

3) Project Structure

  • World

A World is the main point of interaction in Esper. After creating a World object, you will use that object to create Entities and assigning Components to them. A World is also assigned all of your Processor instances, and handles smoothly running everything with a single call per frame. Of course, Entities, Components and Processors can be created and assigned, or deleted while your application is running.

  • 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 World.create_entity() method.

  • 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 might look like:

class Position:
    def __init__(self, x=0.0, y=0.0):
        self.x = x
        self.y = y
  • 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. All Processors will have access to the World instance, which is how you query Components to operate on. A simple Processor might look like:

class MovementProcessor(esper.Processor):

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

In the above code, you can see the standard usage of the World.get_components() method. This method allows efficient iteration over all Entities that contain the specified Component types. This method 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 self.world.delete_entity() method 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.

4) Basic Usage

The first step after importing Esper is to create a World instance. You can have a single World instance for your entire game, or you can have a separate instance for each of your game scenes. Whatever makes sense for your design. Create a World instance like this:

world = esper.World()

Create some Processor instances, and assign them to the World. You can specify an optional processing priority (higher numbers are processed first). All Processors are priority "0" by default:

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

Create an Entity, and assign some Component instances to it:

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

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

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

Executing all Processors is done with a single call to world.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.:

world.process()

Note: You can pass any args you need to world.process(), but you must also make sure to recieve them properly in the process() methods of your Processors. For example, if you pass a delta time argument as world.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 methods.

5) Additional methods

Adding and Removing Processors

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

  • World.add_processor(processor_instance)
  • World.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 of your Processors. The following methods are availble for this purpose:

  • World.add_component(entity_id, component_instance)
  • World.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 dissapear. 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 self.world.get_component(Enemy):
            ...
            is_damaged = self._some_method()
            if is_damaged:
                self.world.add_component(ent, BlinkComponent(duration=1))
            ...


class BlinkProcessor(esper.Processor):

    def process(self, dt):
        for ent, (rend, blink) in self.world.get_components(Renderable, BlinkComponent):
            if blink.duration < 0:
                # Times up. Remove the Component:
                rend.sprite.visible = True
                self.world.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 Engity ID and wish to query one specific, or ALL Components that are assigned to it, the following methods are available:

  • World.component_for_entity
  • World.components_for_entity

The component_for_entity method 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 method that is explained in the next section. For example:

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

The components_for_entity method is a special method that returns ALL of 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 of your Processor.process method. It can be useful, however, if you wanted to transfer all of a specific Entity's Components between two separate World instances (such as when changing Scenes, or Levels). For example:

player_components = old_world.components_for_entity(player_entity_id) ... player_entity_id = new_world.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 two methods are available for this task:

  • World.has_component(entity, ComponentType)
  • World.try_component(entity, ComponentType)

For example, you may want projectiles (and only projectiles) to dissapear when hitting a wall in your game. The simplified code below shows how that might look:

class CollisionProcessor(esper.Processor):

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

The above example is easy enough, as you don't want to actually do anything to the Component - just check if it's there. In cases where you want to both check if a Component exists, and then operate on it if so, the try_component method is useful. Consider the following example, where you want to first check if an Entity has a Component, get it if so, then operate on it. You could write it this way:

if self.world.has_component(ent, Stun):
    stun = self.world.get_component(ent, Stun)
    stun.duration -= dt

The above code works fine, but the try_component method is more concise and slightly faster. It allows you to get specific Components only if they exist, but passes silently if they do not:

for stun in self.world.try_component(ent, Stun):
    stun.duration -= dt

5) More Examples

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

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