chainer.FunctionAdapter¶

class
chainer.
FunctionAdapter
(function)[source]¶ Adapter class to wrap Function with FunctionNode.
While
FunctionNode
provides the interface of newstyle differentiable functions, the oldstyleFunction
can still be used for the backward compatibility. This class provides an adapter of there interface; it addsFunctionNode
interface to anyFunction
object by delegation.Note
The ownership of
FunctionAdapter
andFunction
is a bit tricky. At the initialization,FunctionAdapter
is owned by theFunction
object. Once the function is applied to variables, the ownership is reversed; the adapter becomes the owner of theFunction
object and theFunction
object changes the reference to a weak one.Parameters: function (Function) – The function object to wrap. New in version 3.0.0.
Methods

add_hook
(hook, name=None)[source]¶ Registers a function hook.
Parameters:  hook (FunctionHook) – Function hook to be registered.
 name (str) – Name of the function hook. The name must be unique
among function hooks registered to this function. If
None
, the default name of the function hook is used.

apply
(inputs)[source]¶ Computes output variables and grows the computational graph.
Basic behavior is expressed in the documentation of
FunctionNode
.Note
If the
data
attribute of input variables exist on a GPU device, that device is made current before callingforward()
, so implementors do not need to take care of device selection in most cases.Parameters: inputs – Tuple of input variables. Each element can be either Variable
,numpy.ndarray
, orcupy.ndarray
. If the element is an ndarray, it is automatically wrapped withVariable
.Returns: A tuple of output Variable
objects.

backward
(target_input_indexes, grad_outputs)[source]¶ Computes gradients w.r.t. specified inputs given output gradients.
This method is used to compute one step of the backpropagation corresponding to the forward computation of this function node. Given the gradients w.r.t. output variables, this method computes the gradients w.r.t. specified input variables. Note that this method does not need to compute any input gradients not specified by
target_input_indices
.Unlike
Function.backward()
, gradients are given asVariable
objects and this method itself has to return input gradients asVariable
objects. It enables the function node to return the input gradients with the full computational history, in which case it supports differentiable backpropagation or higherorder differentiation.The default implementation returns
None
s, which means the function is not differentiable.Parameters:  target_input_indexes (tuple of int) – Sorted indices of the input variables w.r.t. which the gradients are required. It is guaranteed that this tuple contains at least one element.
 grad_outputs (tuple of
Variable
s) – Gradients w.r.t. the output variables. If the gradient w.r.t. an output variable is not given, the corresponding element isNone
.
Returns: Tuple of variables that represent the gradients w.r.t. specified input variables. The length of the tuple can be same as either
len(target_input_indexes)
or the number of inputs. In the latter case, the elements not specified bytarget_input_indexes
will be discarded.See also
backward_accumulate()
provides an alternative interface that allows you to implement the backward computation fused with the gradient accumulation.

backward_accumulate
(target_input_indexes, grad_outputs, grad_inputs)[source]¶ Computes gradients w.r.t. specified inputs and accumulates them.
This method provides a way to fuse the backward computation and the gradient accumulations in the case that the multiple functions are applied to the same variable.
Users have to override either of this method or
backward()
. It is often simpler to implementbackward()
and is recommended if you do not need to provide efficient gradient accumulation.Parameters:  target_input_indexes (tuple of int) – Sorted indices of the input variables w.r.t. which the gradients are required. It is guaranteed that this tuple contains at least one element.
 grad_outputs (tuple of Variable) – Gradients w.r.t. the output
variables. If the gradient w.r.t. an output variable is not
given, the corresponding element is
None
.  grad_inputs (tuple of Variable) – Gradients w.r.t. the input
variables specified by
target_input_indexes
. These values are computed by other computation paths. If there is no gradient value existing for the variable, the corresponding element isNone
. See also the note below.
Returns: Tuple of variables that represent the gradients w.r.t. specified input variables. Unlike
backward()
, the length of the tuple must be same as that oftarget_input_indices
.Note
Gradient variables in
grad_outputs
are distinct, even if a variable is passed to multiple input arguments of the function. This is an implementationdetail convention to avoid the complication of correctly accumulating gradients in such a case.Usually, only the first position of
grad_inputs
corresponding to these input arguments may contain the gradient variable corresponding to that input variable, and other entries are set toNone
. This is not the case with thelazy_grad_sum
feature. This behavior might be changed in a future version.

check_type_forward
(in_types)[source]¶ Checks types of input data before forward propagation.
This method is called before
forward()
and validates the types of input variables using the type checking utilities.Parameters: in_types (TypeInfoTuple) – The type information of input variables for forward()
.

delete_hook
(name)[source]¶ Unregisters the function hook.
Parameters: name (str) – The name of the function hook to be unregistered.

forward
(inputs)[source]¶ Computes the output arrays from the input arrays.
It delegates the procedure to
forward_cpu()
orforward_gpu()
by default. Which of them this method selects is determined by the type of input arrays. Implementations ofFunctionNode
must implement either CPU/GPU methods or this method.Parameters: inputs – Tuple of input array(s). Returns: Tuple of output array(s). Warning
Implementations of
FunctionNode
must take care that the return value must be a tuple even if it returns only one array.

forward_cpu
(inputs)[source]¶ Computes the output arrays from the input NumPy arrays.
Parameters: inputs – Tuple of input numpy.ndarray
objects.Returns: Tuple of output arrays. Each element can be NumPy or CuPy arrays. Warning
Implementation of
FunctionNode
must take care that the return value must be a tuple even if it returns only one array.

forward_gpu
(inputs)[source]¶ Computes the output arrays from the input CuPy arrays.
Parameters: inputs – Tuple of input cupy.ndarray
objects.Returns: Tuple of output arrays. Each element can be NumPy or CuPy arrays. Warning
Implementation of
FunctionNode
must take care that the return value must be a tuple even if it returns only one array.

get_retained_inputs
()[source]¶ Returns a tuple of retained input variables.
This method is used to retrieve the input variables retained in
forward()
.Returns: A tuple of retained input variables, if available. Otherwise return None.

get_retained_outputs
()[source]¶ Returns a tuple of retained output variables.
This method is used to retrieve the output variables retained in
forward()
.Returns: A tuple of retained output variables, if available. Otherwise return None. Note
This method does a tricky thing to support the case of an output node garbagecollected before this method is called; in this case, this method creates a fresh variable node that acts as an output node of the function node.

retain_inputs
(indexes)[source]¶ Lets specified input variable nodes keep data arrays.
By calling this method from
forward()
, the function node can specify which inputs are required for backprop. The input variables with retained arrays can then be obtained by callingget_retained_inputs()
from insidebackward()
.Unlike
Function
, the function node DOES NOT keep input arrays by default. If you want to keep some or all input arrays, do not forget to call this method.Note that this method must not be called from the outside of
forward()
.Parameters: indexes (iterable of int) – Indexes of input variables that the function will require for backprop.

retain_outputs
(indexes)[source]¶ Lets specified output variable nodes keep data arrays.
By calling this method from
forward()
, the function node can specify which outputs are required for backprop. If this method is not called, no output variables will be marked to keep their data array at the point of returning fromapply()
. The output variables with retained arrays can then be obtained by callingget_retained_outputs()
from insidebackward()
.Note
It is recommended to use this method if the function requires some or all output arrays in backprop. The function can also use output arrays just by keeping references to them directly, although it might affect the performance of later function applications on the output variables.
Note that this method must not be called from the outside of
forward()
.Parameters: indexes (iterable of int) – Indexes of output variables that the function will require for backprop.
Attributes

inputs
= None¶

label
¶ Short text that represents the function.
The default implementation returns its type name. Each function should override it to give more information.

lazy_grad_sum
= False¶

local_function_hooks
¶ Ordered dictionary of registered function hooks.
Contrary to
chainer.thread_local.function_hooks
, which registers its elements to all functions, Function hooks in this property is specific to this function.

output_data
¶ A tuple of the retained output arrays.
This property is mainly used by
Function
. Users basically do not have to use this property; useget_retained_outputs()
instead.

outputs
= None¶

rank
= 0¶

stack
= None¶
