🔪 Flax - The Sharp Bits 🔪#

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Flax exposes the full power of JAX. And just like when using JAX, there are certain “sharp bits” you may experience when working with Flax. This evolving document is designed to assist you with them.

First, install and/or update Flax:

!pip install -qq flax

🔪 flax.linen.Dropout layer and randomness#


When working on a model with dropout (subclassed from Flax Module), add the 'dropout' PRNGkey only during the forward pass.

  1. Start with jax.random.split() to explicitly create PRNG keys for 'params' and 'dropout'.

  2. Add the flax.linen.Dropout layer(s) to your model (subclassed from Flax Module).

  3. When initializing the model (flax.linen.init()), there’s no need to pass in an extra 'dropout' PRNG key—just the 'params' key like in a “simpler” model.

  4. During the forward pass with flax.linen.apply(), pass in rngs={'dropout': dropout_key}.

Check out a full example below.

Why this works#

  • Internally, flax.linen.Dropout makes use of flax.linen.Module.make_rng to create a key for dropout (check out the source code).

  • Every time make_rng is called (in this case, it’s done implicitly in Dropout), you get a new PRNG key split from the main/root PRNG key.

  • make_rng still guarantees full reproducibility.


The dropout stochastic regularization technique randomly removes hidden and visible units in a network. Dropout is a random operation, requiring a PRNG state, and Flax (like JAX) uses Threefry PRNG that is splittable.

Note: Recall that JAX has an explicit way of giving you PRNG keys: you can fork the main PRNG state (such as key = jax.random.PRNGKey(seed=0)) into multiple new PRNG keys with key, subkey = jax.random.split(key). Refresh your memory in 🔪 JAX - The Sharp Bits 🔪 Randomness and PRNG keys.

Flax provides an implicit way of handling PRNG key streams via Flax Module’s flax.linen.Module.make_rng helper function. It allows the code in Flax Modules (or its sub-Modules) to “pull PRNG keys”. make_rng guarantees to provide a unique key each time you call it.

Note: Recall that flax.linen.Module is the base class for all neural network modules. All layers and models are subclassed from it.


Remember that each of the Flax PRNG streams has a name. The example below uses the 'params' stream for initializing parameters, as well as the 'dropout' stream. The PRNG key provided to flax.linen.init() is the one that seeds the 'params' PRNG key stream. To draw PRNG keys during the forward pass (with dropout), provide a PRNG key to seed that stream ('dropout') when you call Module.apply().

# Setup.
import jax
import jax.numpy as jnp
import flax.linen as nn
# Randomness.
seed = 0
root_key = jax.random.PRNGKey(seed=seed)
main_key, params_key, dropout_key = jax.random.split(key=root_key, num=3)

# A simple network.
class MyModel(nn.Module):
  num_neurons: int
  training: bool
  def __call__(self, x):
    x = nn.Dense(self.num_neurons)(x)
    # Set the dropout layer with a rate of 50% .
    # When the `deterministic` flag is `True`, dropout is turned off.
    x = nn.Dropout(rate=0.5, deterministic=not self.training)(x)
    return x

# Instantiate `MyModel` (you don't need to set `training=True` to
# avoid performing the forward pass computation).
my_model = MyModel(num_neurons=3, training=False)

x = jax.random.uniform(key=main_key, shape=(3, 4, 4))

# Initialize with `flax.linen.init()`.
# The `params_key` is equivalent to a dictionary of PRNGs.
# (Here, you are providing only one PRNG key.) 
variables = my_model.init(params_key, x)

# Perform the forward pass with `flax.linen.apply()`.
y = my_model.apply(variables, x, rngs={'dropout': dropout_key})
WARNING:jax._src.lib.xla_bridge:No GPU/TPU found, falling back to CPU. (Set TF_CPP_MIN_LOG_LEVEL=0 and rerun for more info.)

Real-life examples: