# Adding a New Op

PREREQUISITES:

* Some familiarity with C++.
* Must have [downloaded TensorFlow source](../../get_started/index.md#source),
  and be able to build it.

If you'd like to incorporate an operation that isn't covered by the existing
library, you can create a custom Op. To incorporate your custom Op, you'll need
to:

* Register the new Op in a C++ file. The Op registration is independent of the
  implementation, and describes the semantics of how the Op is invoked. For
  example, it defines the Op name, and specifies its inputs and outputs.
* Implement the Op in C++. This implementation is called a "kernel", and there
  can be multiple kernels for different architectures (e.g. CPUs, GPUs) or
  input / output types.
* Create a Python wrapper. This wrapper is the public API to create the Op. A
  default wrapper is generated from the Op registration, which can be used
  directly or added to.
* Optionally, write a function to compute gradients for the Op.
* Optionally, write a function that describes the input and output shapes
  for the Op.  This allows shape inference to work with your Op.
* Test the Op, typically in Python. If you define gradients, you can verify them   with the Python [`GradientChecker`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/kernel_tests/gradient_checker.py).

[TOC]

## Define the Op's interface {#define_interface}

You define the interface of an Op by registering it with the TensorFlow system.
In the registration, you specify the name of your Op, its inputs (types and
names) and outputs (types and names), as well as docstrings and
any [attrs](#Attrs) the Op might require.

To see how this works, suppose you'd like to create an Op that takes a tensor of
`int32`s and outputs a copy of the tensor, with all but the first element set to
zero. Create file [`tensorflow/core/user_ops`][user_ops]`/zero_out.cc` and
add a call to the `REGISTER_OP` macro that defines the interface for such an Op:

```c++
#include "tensorflow/core/framework/op.h"

REGISTER_OP("ZeroOut")
    .Input("to_zero: int32")
    .Output("zeroed: int32");
```

This `ZeroOut` Op takes one tensor `to_zero` of 32-bit integers as input, and
outputs a tensor `zeroed` of 32-bit integers.

> A note on naming: The name of the Op should be unique and CamelCase.  Names
> starting with an underscore (`_`) are reserved for internal use.

## Implement the kernel for the Op

After you define the interface, provide one or more implementations of the Op.
To create one of these kernels, create a class that extends `OpKernel` and
overrides the `Compute` method. The `Compute` method provides one `context`
argument of type `OpKernelContext*`, from which you can access useful things
like the input and output tensors.

Add your kernel to the file you created above. The kernel might look something
like this:

```c++
#include "tensorflow/core/framework/op_kernel.h"

using namespace tensorflow;

class ZeroOutOp : public OpKernel {
 public:
  explicit ZeroOutOp(OpKernelConstruction* context) : OpKernel(context) {}

  void Compute(OpKernelContext* context) override {
    // Grab the input tensor
    const Tensor& input_tensor = context->input(0);
    auto input = input_tensor.flat<int32>();

    // Create an output tensor
    Tensor* output_tensor = NULL;
    OP_REQUIRES_OK(context, context->allocate_output(0, input_tensor.shape(),
                                                     &output_tensor));
    auto output = output_tensor->template flat<int32>();

    // Set all but the first element of the output tensor to 0.
    const int N = input.size();
    for (int i = 1; i < N; i++) {
      output(i) = 0;
    }

    // Preserve the first input value if possible.
    if (N > 0) output(0) = input(0);
  }
};
```

After implementing your kernel, you register it with the TensorFlow system. In
the registration, you specify different constraints under which this kernel
will run. For example, you might have one kernel made for CPUs, and a separate
one for GPUs.

To do this for the `ZeroOut` op, add the following to `zero_out.cc`:

```c++
REGISTER_KERNEL_BUILDER(Name("ZeroOut").Device(DEVICE_CPU), ZeroOutOp);
```

Once you
[build and reinstall TensorFlow](../../get_started/os_setup.md#create-pip), the
Tensorflow system can reference and use the Op when requested.

## Generate the client wrapper
### The Python Op wrapper

Python op wrappers are created automatically in
`bazel-genfiles/tensorflow/python/ops/gen_user_ops.py` for all ops placed in the
[`tensorflow/core/user_ops`][user_ops] directory when you build Tensorflow.

> Note: The generated function will be given a snake\_case name (to comply with
> [PEP8](https://www.python.org/dev/peps/pep-0008/)).  So if your op is named
> `ZeroOut` in the C++ files, the python function will be called `zero_out`.

Those ops are imported into
[`tensorflow/python/user_ops/user_ops.py`][python-user_ops] with the statement:

```python
from tensorflow.python.ops.gen_user_ops import *
```

You may optionally use your own function instead.  To do this, you first hide
the generated code for that op by adding its name to the `hidden` list in the
`"user_ops"` rule in
[`tensorflow/python/BUILD`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/BUILD):

```python
tf_gen_op_wrapper_py(
    name = "user_ops",
    hidden = [
        "Fact",
    ],
    require_shape_functions = False,
)
```

List your op next to `"Fact"`.  Next you add your replacement function to
[`tensorflow/python/user_ops/user_ops.py`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/user_ops/user_ops.py).
Typically your function will call the generated function to actually add the op
to the graph.  The hidden version of the generated function will be in the
`gen_user_ops` package and start with an underscore ("`_`").  For example:

```python
def my_fact():
    """Example of overriding the generated code for an Op."""
    return gen_user_ops._fact()
```

### The C++ Op wrapper

C++ op wrappers are created automatically for all ops placed in the
[`tensorflow/core/user_ops`][user_ops] directory, when you build Tensorflow. For
example, ops in `tensorflow/core/user_ops/zero_out.cc` will generate wrappers in
`bazel-genfiles/tensorflow/cc/ops/user_ops.{h,cc}`.

All generated wrappers for user ops are automatically
imported into [`tensorflow/cc/ops/standard_ops.h`][standard_ops-cc] with the
statement

```c++
#include "tensorflow/cc/ops/user_ops.h"
```

## Verify it works

A good way to verify that you've successfully implemented your Op is to write a
test for it. Create the file
`tensorflow/python/kernel_tests/zero_out_op_test.py` with the contents:

```python
import tensorflow as tf


class ZeroOutTest(tf.test.TestCase):
  def testZeroOut(self):
    with self.test_session():
      result = tf.user_ops.zero_out([5, 4, 3, 2, 1])
      self.assertAllEqual(result.eval(), [5, 0, 0, 0, 0])
```

Then run your test:

```sh
$ bazel test tensorflow/python:zero_out_op_test
```

## Validation {#Validation}

The example above assumed that the Op applied to a tensor of any shape.  What
if it only applied to vectors?  That means adding a check to the above OpKernel
implementation.

```c++
  void Compute(OpKernelContext* context) override {
    // Grab the input tensor
    const Tensor& input_tensor = context->input(0);

    OP_REQUIRES(context, TensorShapeUtils::IsVector(input_tensor.shape()),
                errors::InvalidArgument("ZeroOut expects a 1-D vector."));
    // ...
  }
```

This asserts that the input is a vector, and returns having set the
`InvalidArgument` status if it isn't.  The
[`OP_REQUIRES` macro][validation-macros] takes three arguments:

*   The `context`, which can either be an `OpKernelContext` or
    `OpKernelConstruction` pointer (see
    [`tensorflow/core/framework/op_kernel.h`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/op_kernel.h)),
    for its `SetStatus()` method.
*   The condition.  For example, there are functions for validating the shape
    of a tensor in
    [`tensorflow/core/public/tensor_shape.h`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/public/tensor_shape.h)
*   The error itself, which is represented by a `Status` object, see
    [`tensorflow/core/public/status.h`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/public/status.h). A
    `Status` has both a type (frequently `InvalidArgument`, but see the list of
    types) and a message.  Functions for constructing an error may be found in
    [`tensorflow/core/lib/core/errors.h`][validation-macros].

Alternatively, if you want to test whether a `Status` object returned from some
function is an error, and if so return it, use
[`OP_REQUIRES_OK`][validation-macros].  Both of these macros return from the
function on error.

## Op registration

### Attrs {#Attrs}

Ops can have attrs, whose values are set when the Op is added to a graph. These
are used to configure the Op, and their values can be accessed both within the
kernel implementation and in the types of inputs and outputs in the Op
registration. Prefer using an input instead of an attr when possible, since
inputs are more flexible.  They can change every step, be set using a feed, etc.
Attrs are used for things that can't be done with inputs: any configuration
that affects the signature (number or type of inputs or outputs) or that
can't change from step-to-step.

You define an attr when you register the Op, by specifying its name and type
using the `Attr` method, which expects a spec of the form:

```
<name>: <attr-type-expr>
```

where `<name>` begins with a letter and can be composed of alphanumeric
characters and underscores, and `<attr-type-expr>` is a type expression of the
form [described below](#attr-types)

For example, if you'd like the `ZeroOut` Op to preserve a user-specified index,
instead of only the 0th element, you can register the Op like so:

<code class="lang-c++"><pre>
REGISTER\_OP("ZeroOut")
    <b>.Attr("preserve\_index: int")</b>
    .Input("to\_zero: int32")
    .Output("zeroed: int32");
</pre></code>

Your kernel can then access this attr in its constructor via the `context`
parameter:

<code class="lang-c++"><pre>
class ZeroOutOp : public OpKernel {
 public:
  explicit ZeroOutOp(OpKernelConstruction\* context) : OpKernel(context) {<b>
    // Get the index of the value to preserve
    OP\_REQUIRES\_OK(context,
                   context-&gt;GetAttr("preserve\_index", &preserve\_index\_));
    // Check that preserve\_index is positive
    OP\_REQUIRES(context, preserve\_index_ &gt;= 0,
                errors::InvalidArgument("Need preserve\_index &gt;= 0, got ",
                                        preserve\_index_));
  </b>}
  void Compute(OpKernelContext\* context) override {
    // ...
  }
 <b>private:
  int preserve\_index\_;</b>
};
</pre></code>

which can then be used in the `Compute` method:

<code class="lang-c++"><pre>
  void Compute(OpKernelContext\* context) override {
    // ...
<br/>    <b>// Check that preserve\_index is in range
    OP\_REQUIRES(context, preserve\_index_ &lt; input.dimension(0),
                errors::InvalidArgument("preserve\_index out of range"));<br/>
    </b>// Set all the elements of the output tensor to 0
    const int N = input.size();
    for (int i = 0; i < N; i++) {
      output\_flat(i) = 0;
    }<br/>
    <b>// Preserve the requested input value
    output\_flat(preserve\_index\_) = input(preserve\_index\_);</b>
  }
</pre></code>

> To preserve [backwards compatibility](#backwards-compatibility), you should
> specify a [default value](#default-values-constraints) when adding an attr to
> an existing op:
>
> <code class="lang-c++"><pre>
> REGISTER\_OP("ZeroOut")
>     <b>.Attr("preserve\_index: int = 0")</b>
>     .Input("to\_zero: int32")
>     .Output("zeroed: int32");
> </pre></code>

### Attr types

The following types are supported in an attr:

* `string`: Any sequence of bytes (not required to be UTF8).
* `int`: A signed integer.
* `float`: A floating point number.
* `bool`: True or false.
* `type`: One of the (non-ref) values of [`DataType`][DataTypeString].
* `shape`: A [`TensorShapeProto`][TensorShapeProto].
* `tensor`: A [`TensorProto`][TensorProto].
* `list(<type>)`: A list of `<type>`, where `<type>` is one of the above types.
  Note that `list(list(<type>))` is invalid.

See also: [`op_def_builder.cc:FinalizeAttr`][FinalizeAttr] for a definitive list.

#### Default values & constraints

Attrs may have default values, and some types of attrs can have constraints. To
define an attr with constraints, you can use the following `<attr-type-expr>`s:

* `{'<string1>', '<string2>'}`: The value must be a string that has either the
  value `<string1>` or `<string2>`.  The name of the type, `string`, is implied
  when you use this syntax.  This emulates an enum:

  ```c++
  REGISTER_OP("EnumExample")
      .Attr("e: {'apple', 'orange'}");
  ```

* `{<type1>, <type2>}`: The value is of type `type`, and must be one of
  `<type1>` or `<type2>`, where `<type1>` and `<type2>` are supported
  [tensor types](../../resources/dims_types.md#data-types).  You don't specify
  that the type of the attr is `type`. This is implied when you have a list of
  types in `{...}`.  For example, in this case the attr `t` is a type that must
  be an `int32`, a `float`, or a `bool`:

  ```c++
  REGISTER_OP("RestrictedTypeExample")
      .Attr("t: {int32, float, bool}");
  ```

* There are shortcuts for common type constraints:
    * `numbertype`: Type `type` restricted to the numeric (non-string and
      non-bool) types.
    * `realnumbertype`: Like `numbertype` without complex types.
    * `quantizedtype`: Like `numbertype` but just the quantized number types.

    The specific lists of types allowed by these are defined by the functions
    (like `NumberTypes()`) in
    [`tensorflow/core/framework/types.h`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/types.h).
    In this example the attr `t` must be one of the numeric types:

    ```c++
    REGISTER_OP("NumberType")
        .Attr("t: numbertype");
    ```

    For this op:

    ```python
    tf.number_type(t=tf.int32)  # Valid
    tf.number_type(t=tf.bool)   # Invalid
    ```

* `int >= <n>`: The value must be an int whose value is greater than or equal to
  `<n>`, where `<n>` is a natural number.

  For example, the following Op registration specifies that the attr `a` must
  have a value that is at least `2`:

  ```c++
  REGISTER_OP("MinIntExample")
      .Attr("a: int >= 2");
  ```

* `list(<type>) >= <n>`: A list of type `<type>` whose length is greater than
  or equal to `<n>`.

  For example, the following Op registration specifies that the attr `a` is a
  list of types (either `int32` or `float`), and that there must be at least 3
  of them:

  ```c++
  REGISTER_OP("TypeListExample")
      .Attr("a: list({int32, float}) >= 3");
  ```

To set a default value for an attr (making it optional in the generated code),
add `= <default>` to the end, as in:

```c++
REGISTER_OP("AttrDefaultExample")
    .Attr("i: int = 0");
```

The supported syntax of the default value is what would be used in the proto
representation of the resulting GraphDef definition.

Here are examples for how to specify a default for all types:

```c++
REGISTER_OP("AttrDefaultExampleForAllTypes")
   .Attr("s: string = 'foo'")
   .Attr("i: int = 0")
   .Attr("f: float = 1.0")
   .Attr("b: bool = true")
   .Attr("ty: type = DT_INT32")
   .Attr("sh: shape = { dim { size: 1 } dim { size: 2 } }")
   .Attr("te: tensor = { dtype: DT_INT32 int_val: 5 }")
   .Attr("l_empty: list(int) = []")
   .Attr("l_int: list(int) = [2, 3, 5, 7]");
```

Note in particular that the values of type `type` use [the `DT_*` names
for the types](../../resources/dims_types.md#data-types).

### Polymorphism {#Polymorphism}
#### Type Polymorphism {#type-polymorphism}

For ops that can take different types as input or produce different output
types, you can specify [an attr](#attrs) in
[an input or output type](#inputs-outputs) in the Op registration.  Typically
you would then register an `OpKernel` for each supported type.

For instance, if you'd like the `ZeroOut` Op to work on `float`s
in addition to `int32`s, your Op registration might look like:

<code class="lang-c++"><pre>
REGISTER\_OP("ZeroOut")
    <b>.Attr("T: {float, int32}")</b>
    .Input("to\_zero: <b>T</b>")
    .Output("zeroed: <b>T</b>");
</pre></code>

Your Op registration now specifies that the input's type must be `float`, or
`int32`, and that its output will be the same type, since both have type `T`.

> A note on naming:{#naming} Inputs, outputs, and attrs generally should be
> given snake\_case names.  The one exception is attrs that are used as the type
> of an input or in the type of an input. Those attrs can be inferred when the
> op is added to the graph and so don't appear in the op's function.  For
> example, this last definition of ZeroOut will generate a Python function that
> looks like:
>
> ```python
> def zero_out(to_zero, name=None):
>   """...
>   Args:
>     to_zero: A `Tensor`. Must be one of the following types:
>         `float32`, `int32`.
>     name: A name for the operation (optional).
>
>   Returns:
>     A `Tensor`. Has the same type as `to_zero`.
>   """
> ```
>
> If `to_zero` is passed an `int32` tensor, then `T` is automatically set to
> `int32` (well, actually `DT_INT32`). Those inferred attrs are given
> Capitalized or CamelCase names.
>
> Compare this with an op that has a type attr that determines the output
> type:
>
> ```c++
> REGISTER_OP("StringToNumber")
>     .Input("string_tensor: string")
>     .Output("output: out_type")
>     .Attr("out_type: {float, int32}");
>     .Doc(R"doc(
> Converts each string in the input Tensor to the specified numeric type.
> )doc");
> ```
>
> In this case, the user has to specify the output type, as in the generated
> Python:
>
> ```python
> def string_to_number(string_tensor, out_type=None, name=None):
>   """Converts each string in the input Tensor to the specified numeric type.
>
>   Args:
>     string_tensor: A `Tensor` of type `string`.
>     out_type: An optional `tf.DType` from: `tf.float32, tf.int32`.
>       Defaults to `tf.float32`.
>     name: A name for the operation (optional).
>
>   Returns:
>     A `Tensor` of type `out_type`.
>   """
> ```

<code class="lang-c++"><pre>
\#include "tensorflow/core/framework/op_kernel.h"<br/>
class ZeroOut<b>Int32</b>Op : public OpKernel {
  // as before
};<br/>
class ZeroOut<b>Float</b>Op : public OpKernel {
 public:
  explicit ZeroOut<b>Float</b>Op(OpKernelConstruction\* context)
      : OpKernel(context) {}<br/>
  void Compute(OpKernelContext\* context) override {
    // Grab the input tensor
    const Tensor& input\_tensor = context-&gt;input(0);
    auto input = input\_tensor.flat&lt;<b>float</b>&gt;();<br/>
    // Create an output tensor
    Tensor* output = NULL;
    OP\_REQUIRES\_OK(context,
                   context-&gt;allocate\_output(0, input_tensor.shape(), &output));
    auto output\_flat = output-&gt;template flat&lt;<b>float</b>&gt;();<br/>
    // Set all the elements of the output tensor to 0
    const int N = input.size();
    for (int i = 0; i &lt; N; i++) {
      output\_flat(i) = 0;
    }<br/>
    // Preserve the first input value
    if (N &gt; 0) output\_flat(0) = input(0);
  }
};<br/><b>
// Note that TypeConstraint&lt;int32&gt;("T") means that attr "T" (defined
// in the Op registration above) must be "int32" to use this template
// instantiation.</b>
REGISTER\_KERNEL\_BUILDER(
    Name("ZeroOut")
    .Device(DEVICE\_CPU)
    <b>.TypeConstraint&lt;int32&gt;("T"),</b>
    ZeroOutOp<b>Int32</b>);
<b>REGISTER\_KERNEL\_BUILDER(
    Name("ZeroOut")
    .Device(DEVICE\_CPU)
    .TypeConstraint&lt;float&gt;("T"),
    ZeroOutFloatOp);
</b></pre></code>

> To preserve [backwards compatibility](#backwards-compatibility), you should
> specify a [default value](#default-values-constraints) when adding an attr to
> an existing op:
>
> <code class="lang-c++"><pre>
> REGISTER\_OP("ZeroOut")
>   <b>.Attr("T: {float, int32} = DT_INT32")</b>
>   .Input("to\_zero: T")
>   .Output("zeroed: T")
> </pre></code>

Lets say you wanted to add more types, say `double`:

<code class="lang-c++"><pre>
REGISTER\_OP("ZeroOut")
    <b>.Attr("T: {float, <b>double,</b> int32}")</b>
    .Input("to\_zero: <b>T</b>")
    .Output("zeroed: <b>T</b>");
</pre></code>

Instead of writing another `OpKernel` with redundant code as above, often you
will be able to use a C++ template instead.  You will still have one kernel
registration (`REGISTER\_KERNEL\_BUILDER` call) per overload.

<code class="lang-c++"><pre>
<b>template &lt;typename T&gt;</b>
class ZeroOutOp : public OpKernel {
 public:
  explicit ZeroOutOp(OpKernelConstruction\* context) : OpKernel(context) {}<br/>
  void Compute(OpKernelContext\* context) override {
    // Grab the input tensor
    const Tensor& input\_tensor = context-&gt;input(0);
    auto input = input\_tensor.flat<b>&lt;T&gt;</b>();<br/>
    // Create an output tensor
    Tensor* output = NULL;
    OP\_REQUIRES\_OK(context,
                   context-&gt;allocate\_output(0, input_tensor.shape(), &output));
    auto output\_flat = output-&gt;template flat<b>&lt;T&gt;</b>();<br/>
    // Set all the elements of the output tensor to 0
    const int N = input.size();
    for (int i = 0; i &lt; N; i++) {
      output\_flat(i) = 0;
    }<br/>
    // Preserve the first input value
    if (N &gt; 0) output\_flat(0) = input(0);
  }
};<br/>
// Note that TypeConstraint&lt;int32&gt;("T") means that attr "T" (defined
// in the Op registration above) must be "int32" to use this template
// instantiation.</b>
REGISTER\_KERNEL\_BUILDER(
    Name("ZeroOut")
    .Device(DEVICE\_CPU)
    .TypeConstraint&lt;int32&gt;("T"),
    <b>ZeroOutOp&lt;int32&gt;</b>);
REGISTER\_KERNEL\_BUILDER(
    Name("ZeroOut")
    .Device(DEVICE\_CPU)
    .TypeConstraint&lt;float&gt;("T"),
    <b>ZeroOutOp&lt;float&gt;</b>);
<b>REGISTER\_KERNEL\_BUILDER(
    Name("ZeroOut")
    .Device(DEVICE\_CPU)
    .TypeConstraint&lt;double&gt;("T"),
    ZeroOutOp&lt;double&gt;);
</b></pre></code>

If you have more than a couple overloads, you can put the registration in a
macro.

```c++
#include "tensorflow/core/framework/op_kernel.h"

#define REGISTER_KERNEL(type)                                       \
  REGISTER_KERNEL_BUILDER(                                          \
      Name("ZeroOut").Device(DEVICE_CPU).TypeConstraint<type>("T"), \
      ZeroOutOp<type>)

REGISTER_KERNEL(int32);
REGISTER_KERNEL(float);
REGISTER_KERNEL(double);

#undef REGISTER_KERNEL
```

Depending on the list of types you are registering the kernel for, you may be
able to use a macro provided by
[`tensorflow/core/framework/register_types.h`][register_types]:

```c++
#include "tensorflow/core/framework/op_kernel.h"
#include "tensorflow/core/framework/register_types.h"

REGISTER_OP("ZeroOut")
    .Attr("T: realnumbertype")
    .Input("to_zero: T")
    .Output("zeroed: T");

template <typename T>
class ZeroOutOp : public OpKernel { ... };

#define REGISTER_KERNEL(type)                                       \
  REGISTER_KERNEL_BUILDER(                                          \
      Name("ZeroOut").Device(DEVICE_CPU).TypeConstraint<type>("T"), \
      ZeroOutOp<type>)

TF_CALL_REAL_NUMBER_TYPES(REGISTER_KERNEL);

#undef REGISTER_KERNEL
```

#### List Inputs and Outputs {#list-input-output}

In addition to being able to accept or produce different types, ops can consume
or produce a variable number of tensors.

In the next example, the attr `T` holds a *list* of types, and is used as the
type of both the input `in` and the output `out`.  The input and output are
lists of tensors of that type (and the number and types of tensors in the output
are the same as the input, since both have type `T`).

```c++
REGISTER_OP("PolymorphicListExample")
    .Attr("T: list(type)")
    .Input("in: T")
    .Output("out: T");
```

You can also place restrictions on what types can be specified in the list. In
this next case, the input is a list of `float` and `double` tensors. The Op
accepts, for example, input types `(float, double, float)` and in that case the
output type would also be `(float, double, float)`.

```c++
REGISTER_OP("ListTypeRestrictionExample")
    .Attr("T: list({float, double})")
    .Input("in: T")
    .Output("out: T");
```

If you want all the tensors in a list to be of the same type, you might do
something like:

```c++
REGISTER_OP("IntListInputExample")
    .Attr("N: int")
    .Input("in: N * int32")
    .Output("out: int32");
```

This accepts a list of `int32` tensors, and uses an `int` attr `N` to
specify the length of the list.

This can be made [type polymorphic](#type-polymorphism) as well.  In the next
example, the input is a list of tensors (with length `"N"`) of the same (but
unspecified) type (`"T"`), and the output is a single tensor of matching type:

```c++
REGISTER_OP("SameListInputExample")
    .Attr("N: int")
    .Attr("T: type")
    .Input("in: N * T")
    .Output("out: T");
```

By default, tensor lists have a minimum length of 1. You can change that default
using
[a `">="` constraint on the corresponding attr](#default-values-constraints).
In this next example, the input is a list of at least 2 `int32` tensors:

```c++
REGISTER_OP("MinLengthIntListExample")
    .Attr("N: int >= 2")
    .Input("in: N * int32")
    .Output("out: int32");
```

The same syntax works with `"list(type)"` attrs:

```c++
REGISTER_OP("MinimumLengthPolymorphicListExample")
    .Attr("T: list(type) >= 3")
    .Input("in: T")
    .Output("out: T");
```

### Inputs and Outputs

To summarize the above, an Op registration can have multiple inputs and outputs:

```c++
REGISTER_OP("MultipleInsAndOuts")
    .Input("y: int32")
    .Input("z: float")
    .Output("a: string")
    .Output("b: int32");
```

Each input or output spec is of the form:

```
<name>: <io-type-expr>
```

where `<name>` begins with a letter and can be composed of alphanumeric
characters and underscores. `<io-type-expr>` is one of the following type
expressions:

* `<type>`, where `<type>` is a supported input type (e.g. `float`, `int32`,
  `string`). This specifies a single tensor of the given type.

  See
  [the list of supported Tensor types](../../resources/dims_types.md#data-types).

  ```c++
  REGISTER_OP("BuiltInTypesExample")
      .Input("integers: int32")
      .Input("complex_numbers: scomplex64");
  ```

* `<attr-type>`, where `<attr-type>` is the name of an [Attr](#attrs) with type
  `type` or `list(type)` (with a possible type restriction). This syntax allows
  for [polymorphic ops](#Polymorphism).

  ```c++
  REGISTER_OP("PolymorphicSingleInput")
      .Attr("T: type")
      .Input("in: T);

  REGISTER_OP("RestrictedPolymorphicSingleInput")
      .Attr("T: {int32, int64}")
      .Input("in: T);
  ```

  Referencing an attr of type `list(type)` allows you to accept a sequence of
  tensors.

  ```c++
  REGISTER_OP("ArbitraryTensorSequenceExample")
      .Attr("T: list(type)")
      .Input("in: T")
      .Output("out: T");

  REGISTER_OP("RestrictedTensorSequenceExample")
      .Attr("T: list({int32, int64})")
      .Input("in: T")
      .Output("out: T");
  ```

  Note that the number and types of tensors in the output `out` is the same as
  in the input `in`, since both are of type `T`.

* For a sequence of tensors with the same type: `<number> * <type>`, where
  `<number>` is the name of an [Attr](#attrs) with type `int`.  The `<type>` can
  either be
  [a specific type like `int32` or `float`](../../resources/dims_types.md#data-types),
  or the name of an attr with type `type`.  As an example of the first, this
  Op accepts a list of `int32` tensors:

  ```c++
  REGISTER_OP("Int32SequenceExample")
      .Attr("NumTensors: int")
      .Input("in: NumTensors * int32")
  ```

  Whereas this Op accepts a list of tensors of any type, as long as they are all
  the same:

  ```c++
  REGISTER_OP("SameTypeSequenceExample")
      .Attr("NumTensors: int")
      .Attr("T: type")
      .Input("in: NumTensors * T")
  ```

* For a reference to a tensor: `Ref(<type>)`, where `<type>` is one of the
  previous types.

> A note on naming: Any attr used in the type of an input will be inferred.  By
> convention those inferred attrs use capital names (like `T` or `N`).
> Otherwise inputs, outputs, and attrs have names like function parameters
> (e.g. `num_outputs`).  For more details, see the
> [earlier note on naming](#naming).

For more details, see
[`tensorflow/core/framework/op_def_builder.h`][op_def_builder].

### Backwards compatibility

In general, changes to specifications must be backwards-compatible: changing the
specification of an Op must not break prior serialized `GraphDef` protocol
buffers constructed from older specfications.

There are several ways to preserve backwards-compatibility.

1. Any new attrs added to an operation must have default values defined, and
   with that default value the Op must have the original behavior. To change an
   operation from not polymorphic to polymorphic, you *must* give a default
   value to the new type attr to preserve the original signature by default. For
   example, if your operation was:

   ```c++
   REGISTER_OP("MyGeneralUnaryOp")
       .Input("in: float")
       .Output("out: float");
   ```

   you can make it polymorphic in a backwards-compatible way using:

   ```c++
   REGISTER_OP("MyGeneralUnaryOp")
       .Input("in: T")
       .Output("out: T")
       .Attr("T: numerictype = DT_FLOAT");
   ```

2. You can safely make a constraint on an attr less restrictive.  For example,
   you can change from `{int32, int64}` to `{int32, int64, float}` or `type`.
   Or you may change from `{"apple", "orange"}` to `{"apple", "banana",
   "orange"}` or `string`.

3. You can change single inputs / outputs into list inputs / outputs, as long as
   the default for the list type matches the old signature.

4. You can add a new list input / output, if it defaults to empty.

5. Namespace any new Ops you create, by prefixing the Op names with something
   unique to your project. This avoids having your Op colliding with any Ops
   that might be included in future versions of Tensorflow.

6. Plan ahead! Try to anticipate future uses for the Op. Some signature changes
   can't be done in a compatible way (for example, making a list of the same
   type into a list of varying types).

The full list of safe and unsafe changes can be found in
[`tensorflow/core/framework/op_compatibility_test.cc`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/op_compatibility_test.cc).
If you cannot make your change to an operation backwards compatible, then create
a new operation with a new name with the new semantics.

Also note that while these changes can maintain `GraphDef` compatibility, the
generated Python code may change in a way that isn't compatible with old
callers.  The Python API may be kept compatible by careful changes in a
hand-written Python wrapper, by keeping the old signature except possibly adding
new optional arguments to the end.  Generally incompatible changes may only be
made when TensorFlow's changes major versions.

## GPU Support {#mult-archs}

You can implement different OpKernels and register one for CPU and another for
GPU, just like you can [register kernels for different types](#Polymorphism).
There are several examples of kernels with GPU support in
[`tensorflow/core/kernels/`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/kernels/).
Notice some kernels have a CPU version in a `.cc` file, a GPU version in a file
ending in `_gpu.cu.cc`, and some code shared in common in a `.h` file.

For example, the [`pad` op](../../api_docs/python/array_ops.md#pad) has
everything but the GPU kernel in [`tensorflow/core/kernels/pad_op.cc`][pad_op].
The GPU kernel is in
[`tensorflow/core/kernels/pad_op_gpu.cu.cc`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/kernels/pad_op_gpu.cu.cc),
and the shared code is a templated class defined in
[`tensorflow/core/kernels/pad_op.h`](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/kernels/pad_op.h).
One thing to note, even when the GPU kernel version of `pad` is used, it still
needs its `"paddings"` input in CPU memory.  To mark that inputs or outputs are
kept on the CPU, add a `HostMemory()` call to the kernel registration, e.g.:

```c++
#define REGISTER_GPU_KERNEL(T)                         \
  REGISTER_KERNEL_BUILDER(Name("Pad")                  \
                              .Device(DEVICE_GPU)      \
                              .TypeConstraint<T>("T")  \
                              .HostMemory("paddings"), \
                          PadOp<GPUDevice, T>)
```

## Implement the gradient in Python

Given a graph of ops, TensorFlow uses automatic differentiation
(backpropagation) to add new ops representing gradients with respect to the
existing ops (see
[Gradient Computation](../../api_docs/python/train.md#gradient-computation)).
To make automatic differentiation work for new ops, you must register a gradient
function which computes gradients with respect to the ops' inputs given
gradients with respect to the ops' outputs.

Mathematically, if an op computes \\(y = f(x)\\) the registered gradient op
converts gradients \\(\partial / \partial y\\) with respect to \\(y\\) into
gradients \\(\partial / \partial x\\) with respect to \\(x\\) via the chain
rule:

$$\frac{\partial}{\partial x}
    = \frac{\partial}{\partial y} \frac{\partial y}{\partial x}
    = \frac{\partial}{\partial y} \frac{\partial f}{\partial x}.$$

In the case of `ZeroOut`, only one entry in the input affects the output, so the
gradient with respect to the input is a sparse "one hot" tensor.  This is
expressed as follows:

```python
from tensorflow.python.framework import ops
from tensorflow.python.ops import array_ops
from tensorflow.python.ops import sparse_ops

@ops.RegisterGradient("ZeroOut")
def _zero_out_grad(op, grad):
  """The gradients for `zero_out`.

  Args:
    op: The `zero_out` `Operation` that we are differentiating, which we can use
      to find the inputs and outputs of the original op.
    grad: Gradient with respect to the output of the `zero_out` op.

  Returns:
    Gradients with respect to the input of `zero_out`.
  """
  to_zero = op.inputs[0]
  shape = array_ops.shape(to_zero)
  index = array_ops.zeros_like(shape)
  first_grad = array_ops.reshape(grad, [-1])[0]
  to_zero_grad = sparse_ops.sparse_to_dense(index, shape, first_grad, 0)
  return [to_zero_grad]  # List of one Tensor, since we have one input
```

Details about registering gradient functions with
[`ops.RegisterGradient`](../../api_docs/python/framework.md#RegisterGradient):

* For an op with one output, the gradient function will take an
  [`Operation`](../../api_docs/python/framework.md#Operation) `op` and a
  [`Tensor`](../../api_docs/python/framework.md#Tensor) `grad` and build new ops
  out of the tensors
  [`op.inputs[i]`](../../api_docs/python/framework.md#Operation.inputs),
  [`op.outputs[i]`](../../api_docs/python/framework.md#Operation.outputs), and `grad`.  Information
  about any attrs can be found via
  [`op.get_attr`](../../api_docs/python/framework.md#Operation.get_attr).

* If the op has multiple outputs, the gradient function will take `op` and
  `grads`, where `grads` is a list of gradients with respect to each output.
  The result of the gradient function must be a list of `Tensor` objects
  representing the gradients with respect to each input.

* If there is no well-defined gradient for some input, such as for integer
  inputs used as indices, the corresponding returned gradient should be
  `None`.  For example, for an op taking a floating point tensor `x` and an
  integer index `i`, the gradient function would `return [x_grad, None]`.

* If there is no meaningful gradient for the op at all, use
  `ops.NoGradient("OpName")` to disable automatic differentiation.

Note that at the time the gradient function is called, only the data flow graph
of ops is available, not the tensor data itself.  Thus, all computation must be
performed using other tensorflow ops, to be run at graph execution time.

## Implement a shape function in Python

The TensorFlow Python API has a feature called "shape inference" that provides
information about the shapes of tensors without having to execute the
graph. Shape inference is supported by "shape functions" that are registered for
each op type, and perform two roles: asserting that the shapes of the inputs are
compatible, and specifying the shapes for the outputs. A shape function is a
Python function that takes an
[`Operation`](../../api_docs/python/framework.md#Operation) as input, and
returns a list of
[`TensorShape`](../../api_docs/python/framework.md#TensorShape) objects (one per
output of the op). To register a shape function, apply the
[`tf.RegisterShape` decorator](../../api_docs/python/framework.md#RegisterShape)
to a shape function. For example, the
[`ZeroOut` op defined above](#define_interface) would have a shape function like
the following:

```python
@tf.RegisterShape("ZeroOut"):
def _zero_out_shape(op):
  """Shape function for the ZeroOut op.

  This is the unconstrained version of ZeroOut, which produces an output
  with the same shape as its input.
  """
  return [op.inputs[0].get_shape()]
```

A shape function can also constrain the shape of an input. For the version of
[`ZeroOut` with a vector shape constraint](#Validation), the shape function
would be as follows:

```python
@tf.RegisterShape("ZeroOut"):
def _zero_out_shape(op):
  """Shape function for the ZeroOut op.

  This is the constrained version of ZeroOut, which requires the input to
  have rank 1 (a vector).
  """
  input_shape = op.inputs[0].get_shape().with_rank(1)
  return [input_shape]
```

If your op is [polymorphic with multiple inputs](#Polymorphism), use the
properties of the operation to determine the number of shapes to check:

```
@tf.RegisterShape("IntListInputExample")
def _int_list_input_example_shape(op):
  """Shape function for the "IntListInputExample" op.

  All inputs and the output are matrices of the same size.
  """
  output_shape = tf.TensorShape(None)
  for input in op.inputs:
    output_shape = output_shape.merge_with(input.get_shape().with_rank(2))
  return [output_shape]
```

Since shape inference is an optional feature, and the shapes of tensors may vary
dynamically, shape functions must be robust to incomplete shape information for
any of the inputs. The [`merge_with`](../../api_docs/python/framework.md)
method allows the caller to assert that two shapes are the same, even if either
or both of them do not have complete information. Shape functions are defined
for all of the
[standard Python ops](https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/ops/),
and provide many different usage examples.

[core-array_ops]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/ops/array_ops.cc
[python-user_ops]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/user_ops/user_ops.py
[tf-kernels]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/kernels/
[user_ops]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/user_ops/
[pad_op]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/kernels/pad_op.cc
[standard_ops-py]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/ops/standard_ops.py
[standard_ops-cc]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/cc/ops/standard_ops.h
[python-BUILD]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/BUILD
[validation-macros]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/lib/core/errors.h
[op_def_builder]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/op_def_builder.h
[register_types]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/register_types.h
[FinalizeAttr]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/op_def_builder.cc#FinalizeAttr
[DataTypeString]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/types.cc#DataTypeString
[python-BUILD]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/python/BUILD
[types-proto]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/types.proto
[TensorShapeProto]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/tensor_shape.proto
[TensorProto]:https://tensorflow.googlesource.com/tensorflow/+/master/tensorflow/core/framework/tensor.proto
