Esper is a lightweight Entity System for Python, with a focus on performance.
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 T-Machines.org, and others. Efforts were made to keep it as lightweight and performant as possible.
There is a fairly accurate writeup of what Entity Systems are in this Wikipedia article: https://en.wikipedia.org/wiki/Entity_component_system
- 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).
- 0.9.6 - A new method has been added: World.get_processor which returns a Processor
- instance by type. This could be useful if you wish to call a specific method from another Processor from within a Processor. For example: self.world.get_processor(OtherProcessorType).some_method()
- 0.9.5 - A new method was added: World.components_for_entity which returns a tuple of all
- Components that are assigned to an Entity. This is probably not useful within standard Processor logic, but could be useful for serializing a specific Entity’s state. For example: saving player status to disk, or passing to another scene in your game that might have a separate World. You can recreate the Entity using these exported Component instances. In addition to the new method, the behavior of the World.component_for_entity method has changed slightly. It will not raise a KeyError if the Entity ID doesn’t exist, instead of passing silently.
- 0.9.4 - A new method was added: World.has_component which returns a Boolean (True/False).
- This is mostly a simple convenience method to make writing some types of logic in Processors. This release also has a bit better test coverage, including coverage for the undocumented esper.CachedWorld. As the name implies, this is a alternate World that uses the functools.lru_cache module from the Python standard library. Feel free to try this out in your program, or run the benchmark in the examples folder. The API is the same, and it should be faster than the standard World in most cases. After more testing/feedback, it should be documented in a future release. Thanks to Christopher Arndt for his work on this.
Esper is developed for Python 3. It will also work on Pypy3. Being written in pure Python, it should work on any compliant interpreter. Current automated testing is done for both CPython and PyPy. Python 2 is not supported in the main branch, but Christopher Arndt is currently maintaining a branch here: https://github.com/SpotlightKid/esper/tree/python2
No installation is necessary. Esper is a tiny library with no dependencies. Simply copy the esper directory 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
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 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 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, 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, to allow easy querying of Components. A simple Processor might look like:
class MovementProcessor(esper.Processor): def __init__(self): super().__init__() 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. You also get a reference to the Entity ID for the current pair of Velocity/Position Components, in case you should need it. For example, you may have a Processor that will delete certain Entites. You could add these Entity IDs to a list, and call the self.world.delete_entity() method on them after you’re done iterating over the Components.
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)
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) )
Running 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:
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):
- Additional Methods
Have a look through esper/world.py for an idea of what additional functionality is available. All methods have docstrings with details on usage, which will be put into a real API document at some point. Here is a quick list of the methods, whose names should be semi-explanitory:
World.create_entity() World.delete_entity(entity) World.add_processor(processor_instance) World.remove_processor(ProcessorType) World.add_component(entity, component_instance) World.remove_component(entity, ComponentType) World.get_component(ComponentType) World.get_components(ComponentTypeA, ComponentTypeB, Etc) World.component_for_entity(entity, ComponentType) World.components_for_entity(entity) World.has_component(entity, ComponentType) World.process()
See the /examples folder to get an idea of how the basic structure of a game looks.
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