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Use this to define a custom loss class. Note, in most cases you do not need to subclass Loss to define a custom loss: you can also pass a bare R function, or a named R function defined with custom_metric(), as a loss function to compile().


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



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


function(y_true, y_pred)

Method to be implemented by subclasses: Function that contains the logic for loss calculation using y_true, y_pred.

..., 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 Loss instances, similar to the builtin loss functions.


Example subclass implementation:

loss_custom_mse <- Loss(
  classname = "CustomMeanSquaredError",
  call = function(y_true, y_pred) {
    op_mean(op_square(y_pred - y_true), axis = -1)

# Usage in compile()
model <- keras_model_sequential(input_shape = 10) |> layer_dense(10)
model |> compile(loss = loss_custom_mse())

# Standalone usage
mse <- loss_custom_mse(name = "my_custom_mse_instance")

y_true <- op_arange(20) |> op_reshape(c(4, 5))
y_pred <- op_arange(20) |> op_reshape(c(4, 5)) * 2
(loss <- mse(y_true, y_pred))

## tf.Tensor(123.5, shape=(), dtype=float32)

loss2 <- (y_pred - y_true)^2 |>
  op_mean(axis = -1) |>

stopifnot(all.equal(as.array(loss), as.array(loss2)))

sample_weight <-array(c(.25, .25, 1, 1))
(weighted_loss <- mse(y_true, y_pred, sample_weight = sample_weight))

## tf.Tensor(112.8125, shape=(), dtype=float32)

weighted_loss2 <- (y_true - y_pred)^2 |>
  op_mean(axis = -1) |>
  op_multiply(sample_weight) |>


Methods defined by base Loss class:

  • initialize(name=NULL, reduction="sum_over_batch_size", dtype=NULL)


    • name

    • reduction: Valid values are one of {"sum_over_batch_size", "sum", NULL, "none"}

    • dtype

  • __call__(y_true, y_pred, sample_weight=NULL)

    Call the loss instance as a function, optionally with sample_weight.

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.