# Getting Started with Cherry

This document provides an overview of the philosophy behind cherry, the tools it provides, and a small illustrative example. By the end of this tutorial, you should be well-equiped to incorporate cherry in your research workflow.

We assume that you are already familiar with reinforcement learning. If, instead, you're looking for an introduction to the field we recommend looking at Josh Achiam's Spinning Up in Deep RL.

## Installation

The first step in getting started with cherry is to install it. You can easily do that by typing the following command in your favorite shell.

pip install cherry-rl


By default cherry only has two dependencies: torch and gym. However, more dependencies might be required if you plan to use some specific functionalities. For example, the OpenAIAtari wrapper requires OpenCV (pip install opencv-python) and the VisdomLogger requires visdom (pip install visdom).

Note While cherry depends on Gym for its environment wrappers, it doesn't restrict you to Gym environments. For instance, check the examples using simple_rl and pycolab environments for Gym-free usage of cherry.

## Overview

Why do we need cherry ? There are many reinforcement learning libraries, many of which feature high-quality implementations. However, few of them provide the kind of low-level utilities useful to researchers. Cherry aims to alleviate this issue. Instead of an interface akin to PPO(env_name).train(1000), it provides researchers with a set of tools they can use to write readable, replicable, and flexible implementations. Cherry prioritizes time-to-correct-implementation over time-to-run, by explicitely helping you check, debug, and reliably report your results.

What's the future of cherry ? Reinforcement learning is a fast moving field, and it is difficult to predict which advances are safe bets for the future. Our long-term development strategy can be summarized as follows.

1. Have as many recent and high-quality examples as possible.
2. Merge advances that turn up to be fundamental in theory or practice into the core library.

We hope to combat the reproducibility crisis by extensively testing and benchmarking our implementations.

Note Cherry is in its early days and is still missing some of the well-established methods from the past 60 years. Those ones are being implemented as fast as we can :)

## Core Features

The following features are fundamental components of cherry.

#### Transitions and Experience Replay

A majority of algorithms needs to store, retrieve, and sample past experience. To that end, you can use cherry's ExperienceReplay. An experience replay is implemented as a wrapper around a standard Python list. The major difference is that the append() method expects arguments used to create a Transition. In addition to behaving like a list, it exposes methods that act on this list, such as to(device) (moves the replay to a device), sample() (randomly samples some experience), or load()/save() (for convenient serialization). An ExperienceReplay contains Transitions, which are akin to (state, action, reward, next_state, done) named tuples with possibly additional custom fields. Those fields are easily accessible directly from the replay by accessing the method named after them. For example, calling replay.action() will fetch the action field from every transition stored in replay, stack them along the first dimension, and return that large tensor. The same is true for custom fields; if all transitions have a logprob field, replay.logprob() will return the result of stacking them.

#### Temporal Difference and Policy Gradients

Many low-level utilities used to implement temporal difference and policy gradient algorithms are available in the cherry.td and cherry.pg modules, respectively. Those modules include classical methods such as discounting rewards or computing the temporal difference, as well as more recent advances such as the generalized advantage estimator. We tried our best to avoid philosophical dissonance when a method belonged to both families of algorithms.

#### Models and PyTorch

Similar to PyTorch, we provide differentiable modules in cherry.nn, domain-specific initialization schemes in cherry.nn.init, and optimization utilities in cherry.optim. In addition, popular higher-level models are available in cherry.models; for instance, those include tabular modules, the Atari CNN features extractor, and a Multi-Layer Perceptron for continuous control.

#### Gym Wrappers

Given the popularity of OpenAI Gym environment in modern reinforcement learning benchmarks, cherry includes convenient wrappers in the cherry.envs package. Examples include normalization of states and actions, Atari frames pre-processing, customizable state / action processing, and automatic collection of experience in a replay.

#### Plots

Reporting comparable results has become a central problem in modern reinforcement learning. In order to alleviate this issue, cherry provides utilities to smooth and compute confidence intervals over lists of rewards. Those are available in the cherry.plot submodule.

As an introductory example let us dissect the following snippet, which demonstrates how to implement the policy gradient theorem using cherry.

import cherry as ch

env = gym.make('CartPole-v0')
env = ch.envs.Logger(env, interval=1000)
env = ch.envs.Torch(env)
env = ch.envs.Runner(env)
env.seed(42)

policy = PolicyNet()
action_dist = ch.distributions.ActionDistribution(env)

def get_action(state):
mass = action_dist(policy(state))
action = mass.sample()
log_prob = mass.log_prob(action)
return action, {'log_prob': log_prob}

for step in range(1000):
replay = env.run(get_action, episodes=1)

rewards = ch.td.discount(0.99, replay.reward(), replay.done())
rewards = ch.normalize(rewards)

loss = -th.sum(replay.log_prob() * rewards)
loss.backward()
optimizer.step()


After importing cherry, the first step is to instanciate, wrap, and seed the desired gym environment.

env = gym.make('CartPole-v0')
env = ch.envs.Logger(env, interval=1000)
env = ch.envs.Torch(env)
env = ch.envs.Runner(env)
env.seed(42)


The Logger, Torch, and Runner classes are Gym environment wrappers that systematically modify the behaviour of an environment:

• Logger keeps track of metrics and prints them at a given interval.
• Torch converts Gym states into PyTorch tensors, and action tensors into numpy arrays.
• Runner implements a run() method which allows to easily gather transitions for a number of steps or episodes.

One particularity of wrappers is that they automatically expose methods of the wrapped environment: env.seed(42) calls the seed() method from the CartPole-v0 environment.

Second, we instanciate the policy, optimizer, as well as the action distribution. The action distribution is created with

action_dist = ch.distributions.ActionDistribution(env)


which will automatically choose a diagonal Gaussian for continuous action-spaces and a categorical distribution for discrete ones.

Next, we define get_action() which specifies how to get an action from our agent and will be used in conjuction to env.run() to quickly collect experience data:

replay = env.run(get_action, episodes=1)


env.run() assumes that the first returned value by get_action is the action to be passed to the environment and the second, optional, returned value is a dictionary to be saved into the experience replay. Under the hood, env.run() creates a new ExperienceReplay and fills it with the desired number of transitions; instead of episodes=1 we could have passed steps=100.

Finally, we discount and normalize the rewards and take an optimization step on the policy gradient loss.

rewards = ch.td.discount(0.99, replay.reward(), replay.done())
rewards = ch.normalize(rewards)

loss = -th.sum(replay.log_prob() * rewards)

When calling replay.reward(), replay.done(), or replay.log_prob(), the experience replay will concatenate the corresponding attribute across all of its transitions and return a new tensor. This means that this operation is rather expensive (you should cache it when possible) and that modifying this tensor does not modify the corresponding transitions in replay. Note that in this case log_prob is a custom attribute which is not declared in the original implementation of ExperienceReplay, and we could have given it any name by changing the dictionary key in get_action().