Keras
Keras 3 API documentation
Layers API
Layer weight constraints

Layer weight constraints

Usage of constraints

Classes from the keras.constraints module allow setting constraints (eg. non-negativity) on model parameters during training. They are per-variable projection functions applied to the target variable after each gradient update (when using fit()).

The exact API will depend on the layer, but the layers Dense, Conv1D, Conv2D and Conv3D have a unified API.

These layers expose two keyword arguments:

  • kernel_constraint for the main weights matrix
  • bias_constraint for the bias.
from keras.constraints import max_norm
model.add(Dense(64, kernel_constraint=max_norm(2.)))

Available weight constraints

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Constraint class 

keras.constraints.Constraint()

Base class for weight constraints.

A Constraint instance works like a stateless function. Users who subclass this class should override the __call__() method, which takes a single weight parameter and return a projected version of that parameter (e.g. normalized or clipped). Constraints can be used with various Keras layers via the kernel_constraint or bias_constraint arguments.

Here’s a simple example of a non-negative weight constraint:

>>> class NonNegative(keras.constraints.Constraint):
...
...  def __call__(self, w):
...    return w * ops.cast(ops.greater_equal(w, 0.), dtype=w.dtype)

>>> weight = ops.convert_to_tensor((-1.0, 1.0))
>>> NonNegative()(weight)
[0.,  1.]

Usage in a layer:

>>> keras.layers.Dense(4, kernel_constraint=NonNegative())

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MaxNorm class 

keras.constraints.MaxNorm(max_value=2, axis=0)

MaxNorm weight constraint.

Constrains the weights incident to each hidden unit to have a norm less than or equal to a desired value.

Also available via the shortcut function keras.constraints.max_norm.

Arguments

  • max_value: the maximum norm value for the incoming weights.
  • axis: integer, axis along which to calculate weight norms. For instance, in a Dense layer the weight matrix has shape (input_dim, output_dim), set axis to 0 to constrain each weight vector of length (input_dim,). In a Conv2D layer with data_format="channels_last", the weight tensor has shape (rows, cols, input_depth, output_depth), set axis to [0, 1, 2] to constrain the weights of each filter tensor of size (rows, cols, input_depth).

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MinMaxNorm class 

keras.constraints.MinMaxNorm(min_value=0.0, max_value=1.0, rate=1.0, axis=0)

MinMaxNorm weight constraint.

Constrains the weights incident to each hidden unit to have the norm between a lower bound and an upper bound.

Arguments

  • min_value: the minimum norm for the incoming weights.
  • max_value: the maximum norm for the incoming weights.
  • rate: rate for enforcing the constraint: weights will be rescaled to yield (1 - rate) * norm + rate * norm.clip(min_value, max_value). Effectively, this means that rate=1.0 stands for strict enforcement of the constraint, while rate<1.0 means that weights will be rescaled at each step to slowly move towards a value inside the desired interval.
  • axis: integer, axis along which to calculate weight norms. For instance, in a Dense layer the weight matrix has shape (input_dim, output_dim), set axis to 0 to constrain each weight vector of length (input_dim,). In a Conv2D layer with data_format="channels_last", the weight tensor has shape (rows, cols, input_depth, output_depth), set axis to [0, 1, 2] to constrain the weights of each filter tensor of size (rows, cols, input_depth).

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NonNeg class 

keras.constraints.NonNeg()

Constrains the weights to be non-negative.

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UnitNorm class 

keras.constraints.UnitNorm(axis=0)

Constrains the weights incident to each hidden unit to have unit norm.

Arguments

  • axis: integer, axis along which to calculate weight norms. For instance, in a Dense layer the weight matrix has shape (input_dim, output_dim), set axis to 0 to constrain each weight vector of length (input_dim,). In a Conv2D layer with data_format="channels_last", the weight tensor has shape (rows, cols, input_depth, output_depth), set axis to [0, 1, 2] to constrain the weights of each filter tensor of size (rows, cols, input_depth).

Creating custom weight constraints

A weight constraint can be any callable that takes a tensor and returns a tensor with the same shape and dtype. You would typically implement your constraints as subclasses of keras.constraints.Constraint.

Here’s a simple example: a constraint that forces weight tensors to be centered around a specific value on average.

from keras import ops

class CenterAround(keras.constraints.Constraint):
  """Constrains weight tensors to be centered around `ref_value`."""

  def __init__(self, ref_value):
    self.ref_value = ref_value

  def __call__(self, w):
    mean = ops.mean(w)
    return w - mean + self.ref_value

  def get_config(self):
    return {'ref_value': self.ref_value}

Optionally, you an also implement the method get_config and the class method from_config in order to support serialization – just like with any Keras object. Note that we don’t have to implement from_config in the example above since the constructor arguments of the class the keys in the config returned by get_config are the same. In this case, the default from_config works fine.