# tf.function

Compiles a function into a callable TensorFlow graph. (deprecated arguments) (deprecated arguments) (deprecated arguments)

`tf.function` constructs a `tf.types.experimental.PolymorphicFunction` that executes a TensorFlow graph (`tf.Graph`) created by trace-compiling the TensorFlow operations in `func`. More information on the topic can be found in Introduction to Graphs and tf.function.

See Better Performance with tf.function for tips on performance and known limitations.

#### Example usage:

````@tf.function`
`def f(x, y):`
`  return x ** 2 + y`
`x = tf.constant([2, 3])`
`y = tf.constant([3, -2])`
`f(x, y)`
`<tf.Tensor: ... numpy=array([7, 7], ...)>`
```

The trace-compilation allows non-TensorFlow operations to execute, but under special conditions. In general, only TensorFlow operations are guaranteed to run and create fresh results whenever the `PolymorphicFunction` is called.

## Features

`func` may use data-dependent Python control flow statements, including `if`, `for`, `while` `break`, `continue` and `return`:

````@tf.function`
`def f(x):`
`  if tf.reduce_sum(x) > 0:`
`    return x * x`
`  else:`
`    return -x // 2`
`f(tf.constant(-2))`
`<tf.Tensor: ... numpy=1>`
```

`func`'s closure may include `tf.Tensor` and `tf.Variable` objects:

````@tf.function`
`def f():`
`  return x ** 2 + y`
`x = tf.constant([-2, -3])`
`y = tf.Variable([3, -2])`
`f()`
`<tf.Tensor: ... numpy=array([7, 7], ...)>`
```

`func` may also use ops with side effects, such as `tf.print`, `tf.Variable` and others:

````v = tf.Variable(1)`
`@tf.function`
`def f(x):`
`  for i in tf.range(x):`
`    v.assign_add(i)`
`f(3)`
`v`
`<tf.Variable ... numpy=4>`
```
````l = []`
`@tf.function`
`def f(x):`
`  for i in x:`
`    l.append(i + 1)    # Caution! Will only happen once when tracing`
`f(tf.constant([1, 2, 3]))`
`l`
`[<tf.Tensor ...>]`
```

Instead, use TensorFlow collections like `tf.TensorArray`:

````@tf.function`
`def f(x):`
`  ta = tf.TensorArray(dtype=tf.int32, size=0, dynamic_size=True)`
`  for i in range(len(x)):`
`    ta = ta.write(i, x[i] + 1)`
`  return ta.stack()`
`f(tf.constant([1, 2, 3]))`
`<tf.Tensor: ..., numpy=array([2, 3, 4], ...)>`
```

## `tf.function` creates polymorphic callables

Internally, `tf.types.experimental.PolymorphicFunction` may contain multiple `tf.types.experimental.ConcreteFunction`s, each specialized to arguments with different data types or shapes, since TensorFlow can perform more optimizations on graphs of specific shapes, dtypes and values of constant arguments. `tf.function` treats any pure Python values as opaque objects (best thought of as compile-time constants), and builds a separate `tf.Graph` for each set of Python arguments that it encounters. For more information, see the tf.function guide

Executing a `PolymorphicFunction` will select and execute the appropriate `ConcreteFunction` based on the argument types and values.

To obtain an individual `ConcreteFunction`, use the `PolymorphicFunction.get_concrete_function` method. It can be called with the same arguments as `func` and returns a `tf.types.experimental.ConcreteFunction`. `ConcreteFunction`s are backed by a single `tf.Graph`:

````@tf.function`
`def f(x):`
`  return x + 1`
`isinstance(f.get_concrete_function(1).graph, tf.Graph)`
`True`
```

`ConcreteFunction`s can be executed just like `PolymorphicFunction`s, but their input is resticted to the types to which they're specialized.

## Retracing

`ConcreteFunctions` are built (traced) on the fly, as the `PolymorphicFunction` is called with new TensorFlow types or shapes, or with new Python values as arguments. When `PolymorphicFunction` builds a new trace, it is said that `func` is retraced. Retracing is a frequent performance concern for `tf.function` as it can be considerably slower than executing a graph that's already been traced. It is ideal to minimize the amount of retracing in your code.

````@tf.function`
`def f(x):`
`  return tf.abs(x)`
`f1 = f.get_concrete_function(1)`
`f2 = f.get_concrete_function(2)  # Slow - compiles new graph`
`f1 is f2`
`False`
`f1 = f.get_concrete_function(tf.constant(1))`
`f2 = f.get_concrete_function(tf.constant(2))  # Fast - reuses f1`
`f1 is f2`
`True`
```

Python numerical arguments should only be used when they take few distinct values, such as hyperparameters like the number of layers in a neural network.

## Input signatures

For Tensor arguments, `PolymorphicFunction`creates a new `ConcreteFunction` for every unique set of input shapes and datatypes. The example below creates two separate `ConcreteFunction`s, each specialized to a different shape:

````@tf.function`
`def f(x):`
`  return x + 1`
`vector = tf.constant([1.0, 1.0])`
`matrix = tf.constant([[3.0]])`
`f.get_concrete_function(vector) is f.get_concrete_function(matrix)`
`False`
```

An "input signature" can be optionally provided to `tf.function` to control this process. The input signature specifies the shape and type of each Tensor argument to the function using a `tf.TensorSpec` object. More general shapes can be used. This ensures only one `ConcreteFunction` is created, and restricts the `PolymorphicFunction` to the specified shapes and types. It is an effective way to limit retracing when Tensors have dynamic shapes.

````@tf.function(`
`    input_signature=[tf.TensorSpec(shape=None, dtype=tf.float32)])`
`def f(x):`
`  return x + 1`
`vector = tf.constant([1.0, 1.0])`
`matrix = tf.constant([[3.0]])`
`f.get_concrete_function(vector) is f.get_concrete_function(matrix)`
`True`
```

## Variables may only be created once

`tf.function` only allows creating new `tf.Variable` objects when it is called for the first time:

````class MyModule(tf.Module):`
`  def __init__(self):`
`    self.v = None`

`  @tf.function`
`  def __call__(self, x):`
`    if self.v is None:`
`      self.v = tf.Variable(tf.ones_like(x))`
`    return self.v * x`
```

In general, it is recommended to create `tf.Variable`s outside of `tf.function`. In simple cases, persisting state across `tf.function` boundaries may be implemented using a pure functional style in which state is represented by `tf.Tensor`s passed as arguments and returned as return values.

Contrast the two styles below:

````state = tf.Variable(1)`
`@tf.function`
`def f(x):`
`  state.assign_add(x)`
`f(tf.constant(2))  # Non-pure functional style`
`state`
`<tf.Variable ... numpy=3>`
```
````state = tf.constant(1)`
`@tf.function`
`def f(state, x):`
`  state += x`
`  return state`
`state = f(state, tf.constant(2))  # Pure functional style`
`state`
`<tf.Tensor: ... numpy=3>`
```

## Python operations execute only once per trace

`func` may contain TensorFlow operations mixed with pure Python operations. However, when the function is executed, only the TensorFlow operations will run. The Python operations run only once, at trace time. If TensorFlow operations depend on results from Python operations, those results will be frozen into the graph.

````@tf.function`
`def f(a, b):`
`  print('this runs at trace time; a is', a, 'and b is', b)`
`  return b`
`f(1, tf.constant(1))`
`this runs at trace time; a is 1 and b is Tensor("...", shape=(), dtype=int32)`
`<tf.Tensor: shape=(), dtype=int32, numpy=1>`
```
````f(1, tf.constant(2))`
`<tf.Tensor: shape=(), dtype=int32, numpy=2>`
```
````f(2, tf.constant(1))`
`this runs at trace time; a is 2 and b is Tensor("...", shape=(), dtype=int32)`
`<tf.Tensor: shape=(), dtype=int32, numpy=1>`
```
````f(2, tf.constant(2))`
`<tf.Tensor: shape=(), dtype=int32, numpy=2>`
```

`func` The function to be compiled. If `func` is None, `tf.function` returns a decorator that can be invoked with a single argument - `func`. In other words, `tf.function(input_signature=...)(func)` is equivalent to `tf.function(func, input_signature=...)`. The former can be used as decorator.
`input_signature` A possibly nested sequence of `tf.TensorSpec` objects specifying the shapes and dtypes of the Tensors that will be supplied to this function. If `None`, a separate function is instantiated for each inferred input signature. If input_signature is specified, every input to `func` must be a `Tensor`, and `func` cannot accept `**kwargs`.
`autograph` Whether autograph should be applied on `func` before tracing a graph. Data-dependent Python control flow statements require `autograph=True`. For more information, see the tf.function and AutoGraph guide.
`jit_compile` If `True`, compiles the function using XLA. XLA performs compiler optimizations, such as fusion, and attempts to emit more efficient code. This may drastically improve the performance. If set to `True`, the whole function needs to be compilable by XLA, or an `errors.InvalidArgumentError` is thrown. If `None` (default), compiles the function with XLA when running on TPU and goes through the regular function execution path when running on other devices. If `False`, executes the function without XLA compilation. Set this value to `False` when directly running a multi-device function on TPUs (e.g. two TPU cores, one TPU core and its host CPU). Not all functions are compilable, see a list of sharp corners.
`reduce_retracing` When True, `tf.function` attempts to reduce the amount of retracing, for example by using more generic shapes. This can be controlled for user objects by customizing their associated `tf.types.experimental.TraceType`.
`experimental_implements` If provided, contains a name of a "known" function this implements. For example "mycompany.my_recurrent_cell". This is stored as an attribute in inference function, which can then be detected when processing serialized function. See standardizing composite ops
for details. For an example of utilizing this attribute see this example The code above automatically detects and substitutes function that implements "embedded_matmul" and allows TFLite to substitute its own implementations. For instance, a tensorflow user can use this attribute to mark that their function also implements `embedded_matmul` (perhaps more efficiently!) by specifying it using this parameter: `@tf.function(experimental_implements="embedded_matmul")` This can either be specified as just the string name of the function or a NameAttrList corresponding to a list of key-value attributes associated with the function name. The name of the function will be in the 'name' field of the NameAttrList. To define a formal TF op for this function implements, try the experimental composite TF project.
`experimental_autograph_options` Optional tuple of `tf.autograph.experimental.Feature` values.
`experimental_attributes` Optional dictionary of attributes to include in the generated FunctionDefs.
`experimental_relax_shapes` Deprecated. Use `reduce_retracing` instead.
`experimental_compile` Deprecated alias to 'jit_compile'.
`experimental_follow_type_hints` Deprecated. Please use input_signature or reduce_retracing instead.

If `func` is not None, returns a `tf.types.experimental.PolymorphicFunction`. If `func` is None, returns a decorator that, when invoked with a single `func` argument, returns a `tf.types.experimental.PolymorphicFunction`.

`ValueError` when attempting to use `jit_compile=True`, but XLA support is not available.

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{ "last_modified": "Last updated 2024-01-23 UTC.", "state": "" }