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Flax: A neural network library for JAX designed for flexibility

Project description

Flax: A neural network library for JAX designed for flexibility


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 in "early release stage" -- if that's your style, now could be 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


Full documentation and API reference

Annotated full end-to-end MNIST example

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

Background: JAX

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)

What is Flax?

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 examples

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

Try Flax now by forking one of our starter examples

We keep here a limited list of canonical examples maintained by the Flax team. If you are looking for more examples, or others built by the community, please check the examples folder for further guidance.

Image Classification

MNIST (also see annotated version)

CIFAR-10 (Wide ResNet w/ and w/o Shake-Shake, PyramidNet w/ShakeDrop)

ResNet50 on ImageNet

Transformer Models

Sequence tagging on Universal Dependencies

LM1b language modeling (try on a TPU in Colab)

WMT translation


LSTM text classifier on SST-2

LSTM seq2seq on number addition

Generative Models

Basic VAE

Graph Neural Networks

Semi-supervised node classification on Zachary's karate club

The Flax Module abstraction in a nutshell

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 APIs.

Modules are the one part of Flax with "magic" -- the magic is constrained, and enables a very ergonomic model construction 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 with MySubModule(name=..., ...) within MyModule.apply. Parameters of MySubModule are stored as a dictionary under the parameters MyModule and accessible via self.get_param(name=...). This applies MySubmodule once -- to re-use parameters, use Module.shared

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

  5., ...) is a pure function that calls apply in "call mode" and returns 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, 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:

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

A full ResNet implementation

(from examples/imagenet/

class ResidualBlock(nn.Module):
  def apply(self, x, filters, strides=(1, 1), train=True, dtype=jnp.float32):
    needs_projection = x.shape[-1] != filters * 4 or strides != (1, 1)
    batch_norm = nn.BatchNorm.partial(
        use_running_average=not train, momentum=0.9, epsilon=1e-5, dtype=dtype)
    conv = nn.Conv.partial(bias=False, dtype=dtype)

    residual = x
    if needs_projection:
      residual = conv(residual, filters * 4, (1, 1), strides, name='proj_conv')
      residual = batch_norm(residual, name='proj_bn')

    y = conv(x, filters, (1, 1), name='conv1')
    y = batch_norm(y, name='bn1')
    y = nn.relu(y)
    y = conv(y, filters, (3, 3), strides, name='conv2')
    y = batch_norm(y, name='bn2')
    y = nn.relu(y)
    y = conv(y, filters * 4, (1, 1), name='conv3')

    y = batch_norm(y, name='bn3', scale_init=nn.initializers.zeros)
    y = nn.relu(residual + y)
    return y

class ResNet(nn.Module):
  def apply(self, x, num_classes, num_filters=64, num_layers=50,
            train=True, dtype=jnp.float32):
    if num_layers not in _block_size_options:
      raise ValueError('Please provide a valid number of layers')
    block_sizes = _block_size_options[num_layers]
    x = nn.Conv(
        x, num_filters, (7, 7), (2, 2), padding=[(3, 3), (3, 3)],
        bias=False, dtype=dtype, name='init_conv')
    x = nn.BatchNorm(
        x, use_running_average=not train, momentum=0.9,
        epsilon=1e-5, dtype=dtype, name='init_bn')
    x = nn.max_pool(x, (3, 3), strides=(2, 2), padding='SAME')
    for i, block_size in enumerate(block_sizes):
      for j in range(block_size):
        strides = (2, 2) if i > 0 and j == 0 else (1, 1)
        x = ResidualBlock(
            x, num_filters * 2 ** i, strides=strides,
            train=train, dtype=dtype)
    x = jnp.mean(x, axis=(1, 2))
    x = nn.Dense(x, num_classes)
    x = nn.log_softmax(x)
    return x


You will need Python 3.6 or later.

For GPU support, 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.

Then install flax from PyPi:

> pip install flax

TPU support

We currently have a LM1b/Wikitext-2 language model with a Transformer architecture that's been tuned. You can run it directly via Colab.

At present, Cloud TPUs are network-attached, and Flax users typically feed in data from one or more additional VMs

When working with large-scale input data, it is important to create large enough VMs with sufficient network bandwidth to avoid having the TPUs bottlenecked waiting for input

TODO: Add an example for running on Google Cloud.

Getting involved

We welcome pull requests, in particular for those issues marked as PR-ready. For other proposals, we ask that you first open an Issue to discuss your planned contribution.


This is not an official Google product.

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