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Base class for weight constraints.

A Constraint() instance works like a stateless function. Users who subclass the Constraint 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:

constraint_nonnegative <- Constraint("NonNegative",
  call = function(w) {
    w * op_cast(w >= 0, dtype = w$dtype)
weight <- op_convert_to_tensor(c(-1, 1))

## tf.Tensor([-0.  1.], shape=(2), dtype=float32)

Usage in a layer:

layer_dense(units = 4, kernel_constraint = constraint_nonnegative())

## <Dense name=dense, built=False>
##  signature: (*args, **kwargs)


  call = NULL,
  get_config = NULL,
  public = list(),
  private = list(),
  inherit = NULL,
  parent_env = parent.frame()



String, the name of the custom class. (Conventionally, CamelCase).



Applies the constraint to the input weight variable.

By default, the inputs weight variable is not modified. Users should override this method to implement their own projection function.


  • w: Input weight variable.

Returns: Projected variable (by default, returns unmodified inputs).



Function that returns a named list of the object config.

A constraint config is a named list (JSON-serializable) that can be used to reinstantiate the same object (via<constraint_class>, <config>)).

..., public

Additional methods or public members of the custom class.


Named list of R objects (typically, functions) to include in instance private environments. private methods will have all the same symbols in scope as public methods (See section "Symbols in Scope"). Each instance will have it's own private environment. Any objects in private will be invisible from the Keras framework and the Python runtime.


What the custom class will subclass. By default, the base keras class.


The R environment that all class methods will have as a grandparent.


A function that returns Constraint instances, similar to the builtin constraint functions like constraint_maxnorm().

Symbols in scope

All R function custom methods (public and private) will have the following symbols in scope:

  • self: The custom class instance.

  • super: The custom class superclass.

  • private: An R environment specific to the class instance. Any objects assigned here are invisible to the Keras framework.

  • __class__ and as.symbol(classname): the custom class type object.