EvoStrat

A library that makes Evolutionary Strategies (ES) simple to use.

Installation

pip install evostrat

Usage

pop = PopulationImpl(...) # See complete examples for implementations. 
optim = torch.optim.Adam(pop.parameters()) # Use any torch.optim optimizer
for i in range(N):
    optim.zero_grads()
    pop.fitness_grads(n_samples=200) # Computes approximate gradients
    optim.step()

For complete examples that solves 'LunarLander-v2' see the examples/.

Description

Evolutionary Strategies is a powerful approach to solve reinforcement learning problems and other optimization problems where the gradients cannot be computed with backprop. See "Evolution strategies as a scalable alternative to reinforcement learning" for an excellent introduction.

In ES the objective is to maximize the expected fitness of a distribution over individuals, referred to as the population. With a few math tricks this objective can be maximized with gradient ascent, even if the fitness function itself is not differentiable.

This library offers

1. A flexible and natural interface for ES that cleanly separates the environment, the reinforcement learning agent, the population distribution and the optimization.
   
2. A plug-and-play approach for reinforcement learning agents with

   torch.nn.Module

   policy networks. See examples/lunar_lander.py and examples/normallunarlander.py.
3. Several population distributions and variants
   1. Independent Normal. equivalent to OpenAI ES or PEPG depending on whether the standard deviation is fixed or learned. See examples/normallunarlander.py
   2. Multivariate Normal with a full covariance matrix. Similar to CMA-ES. See examples/multivariatenormallunar_lander.py
   3. Categorical. For agents with categorical parameters, demonstrating the ability to handle non-normal distributions. See the examples/binarylunarlander.py.
4. A simple interface for creating your own populations, without having to derive any gradients! Just subclass Population and implement the sampling process. See the built in populations for inspiration.

Attribution

If you use this software in your academic work please cite

@misc{palm2020,
  author = {Palm, Rasmus Berg},
  title = {EvoStrat},
  year = {2020},
  publisher = {GitHub},
  journal = {GitHub repository},
  howpublished = {\url{https://github.com/rasmusbergpalm/evostrat}}
}