Should I use setup or nn.compact?

In Flax’s module system (named Linen), submodules and variables (parameters or others) can be defined in two ways:

  1. Explicitly (using setup):

    Assign submodules or variables to self.<attr> inside a setup method. Then use the submodules and variables assigned to self.<attr> in setup from any “forward pass” method defined on the class. This resembles how modules are defined in PyTorch.

  2. In-line (using nn.compact):

    Write your network’s logic directly within a single “forward pass” method annotated with nn.compact. This allows you to define your whole module in a single method, and “co-locate” submodules and variables next to where they are used.

Both of these approaches are perfectly valid, behave the same way, and interoperate with all of Flax.

Here is a short example of a module defined in both ways, with exactly the same functionality.

Using setup

Using nn.compact

class MLP(nn.Module):
  def setup(self):
    # Submodule names are derived by the attributes you assign to. In this
    # case, "dense1" and "dense2". This follows the logic in PyTorch.
    self.dense1 = nn.Dense(32)
    self.dense2 = nn.Dense(32)

  def __call__(self, x):
    x = self.dense1(x)
    x = nn.relu(x)
    x = self.dense2(x)
    return x
class MLP(nn.Module):

  def __call__(self, x):
    x = nn.Dense(32, name="dense1")(x)
    x = nn.relu(x)
    x = nn.Dense(32, name="dense2")(x)
    return x

So, how would you decide which style to use? It can be a matter of taste, but here are some pros and cons:

Reasons to prefer using nn.compact:

  1. Allows defining submodules, parameters and other variables next to where they are used: less scrolling up/down to see how everything is defined.

  2. Reduces code duplication when there are conditionals or for loops that conditionally define submodules, parameters or variables.

  3. Code typically looks more like mathematical notation: y = self.param('W', ...) @ x + self.param('b', ...) looks similar to \(y=Wx+b\))

  4. If you are using shape inference, i.e. using parameters whose shape/value depend on shapes of the inputs (which are unknown at initialization), this is not possible using setup.

Reasons to prefer using setup:

  1. Closer to the PyTorch convention, thus easier when porting models from PyTorch

  2. Some people find it more natural to explicitly separate the definition of submodules and variables from where they are used

  3. Allows defining more than one “forward pass” method (see MultipleMethodsCompactError)