NOTE: Flax is being actively improved and has a growing community of researchers and engineers at Google who happily use Flax for their daily research. Flax is "early release" but now is a good time to start using it. We want to smooth out any rough edges so please report any issues, questions or concerns as GitHub issues. Expect changes to the API, but we'll use deprecation warnings when we can, and keep track of them in our Changelog.

In case you need to reach us directly, we're at [email protected].

⟶ Full documentation and API reference

⟶ Annotated full end-to-end MNIST example

⟶ The Flax Guide -- a guided walkthrough of the parts of Flax

JAX is NumPy + autodiff + GPU/TPU

It allows for fast scientific computing and machine learning with the normal NumPy API (+ additional APIs for special accelerator ops when needed)

JAX comes with powerful primitives, which you can compose arbitrarily:

• Autodiff (jax.grad): Efficient any-order gradients w.r.t any variables
• JIT compilation (jax.jit): Trace any function ⟶ fused accelerator ops
• Vectorization (jax.vmap): Automatically batch code written for individual samples
• Parallelization (jax.pmap): Automatically parallelize code across multiple accelerators (including across hosts, e.g. for TPU pods)

Flax is a high-performance neural network library for JAX that is designed for flexibility: Try new forms of training by forking an example and by modifying the training loop, not by adding features to a framework.

Flax is being developed in close collaboration with the JAX team and comes with everything you need to start your research, including:

• Common layers (flax.nn): Dense, Conv, {Batch|Layer|Group} Norm, Attention, Pooling, {LSTM|GRU} Cell, Dropout

• Optimizers (flax.optim): SGD, Momentum, Adam, LARS

• Utilities and patterns: replicated training, serialization and checkpointing, metrics, prefetching on device

• Educational examples that work out of the box: MNIST, LSTM seq2seq, Graph Neural Networks, Sequence Tagging

• HOWTO guides -- diffs that add functionality to educational base exampless

• Fast, tuned large-scale end-to-end examples: CIFAR10, ResNet ImageNet, Transformer LM1b

See docs/annotated_mnist.md for an MNIST example with detailed annotations for each code block.

The core of Flax is the Module abstraction. Modules allow you to write parameterized functions just as if you were writing a normal numpy function with JAX. The Module api allows you to declare parameters and use them directly with the JAX api’s.

Modules are the one part of Flax with "magic" -- the magic is constrained, and enables a very ergonomic style, where modules are defined in a single function with minimal boilerplate.

A few things to know about Modules:

1. Create a new module by subclassing flax.nn.Module and implementing the apply method.

2. Within apply, call self.param(name, shape, init_func) to register a new parameter and returns its initial value.

3. Apply submodules by calling MySubModule(...args...) within MyModule.apply. Parameters of MySubModule are stored as a dictionary under the parameters MyModule. NOTE: this returns the output of MySubModule, not an instance. To get an access to an instance of MySubModule for re-use, use Module.partial or Module.shared

4. MyModule.init(rng, ...) is a pure function that calls apply in "init mode" and returnes a nested Python dict of initialized parameter values

5. MyModule.call(params, ...) is a pure function that calls apply in "call mode" and returnes the output of the module.

For example you can define a learned linear transformation as follows:

from flax import nn
import jax.numpy as jnp

class Linear(nn.Module):
  def apply(self, x, num_features, kernel_init_fn):
    input_features = x.shape[-1]
    W = self.param('W', (input_features, num_features), kernel_init_fn)
    return jnp.dot(x, W)

You can also use nn.module as a function decorator to create a new module, as long as you don't need access to self for creating parameters directly:

@nn.module
def DenseLayer(x, features):
  x = flax.nn.Dense(x, features)
  x = flax.nn.relu(x)
  return x

⟶ Read more about Modules in the Flax Guide

You will need Python 3.5 or later.

Now install flax from PyPi:

> pip install flax

First install jaxlib; please follow the instructions in the JAX readme. If they are not already installed, you will need to install CUDA and CuDNN runtimes.

Now install flax from PyPi:

> pip install flax

NOTE: We are still testing these examples across all supported hardware configurations.

• ResNet on ImageNet

• Language Modeling on LM1b with a Transformer architecture

• WIP: WMT translation with a Transformer architecture and on-device beam decoding

This is not an official Google product.