tf.function

TensorFlow 1 version View source on GitHub

Compiles a function into a callable TensorFlow graph.

tf.function(
    func=None, input_signature=None, autograph=True, experimental_implements=None,
    experimental_autograph_options=None, experimental_relax_shapes=False,
    experimental_compile=None
)

Used in the notebooks

Used in the guide Used in the tutorials

tf.function constructs a callable that executes a TensorFlow graph (tf.Graph) created by trace-compiling the TensorFlow operations in func, effectively executing func as a TensorFlow graph.

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], ...)>

Features

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

@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 is polymorphic

Internally, tf.function can build more than one graph, to support arguments with different data types or shapes, since TensorFlow can build more efficient graphs that are specialized on shapes and dtypes. tf.function also treats any pure Python value as opaque objects, and builds a separate graph for each set of Python arguments that it encounters.

To obtain an individual graph, use the get_concrete_function method of the callable created by tf.function. It can be called with the same arguments as func and returns a special tf.Graph object:

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

@tf.function 
def f(x): 
  return tf.abs(x) 
f1 = f.get_concrete_function(1) 
f2 = f.get_concrete_function(2)  # Slow - builds 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, tf.function instantiates a separate graph for every unique set of input shapes and datatypes. The example below creates two separate graphs, 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 the graphs traced. 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 is useful to avoid creating multiple graphs when Tensors have dynamic shapes. It also restricts the dhape and datatype of Tensors that can be used:

@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 stateful objects like tf.Variable outside of tf.function and passing them as arguments.

Args:

  • 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 control flow requires autograph=True. For more information, see the tf.function and AutoGraph guide.
  • 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 https://github.com/tensorflow/community/blob/master/rfcs/20190610-standardizing-composite_ops.md for details. For an example of utilizing this attribute see: https://github.com/tensorflow/tensorflow/blob/master/tensorflow/compiler/mlir/lite/transforms/prepare_composite_functions_tf.cc 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 flag.
@tf.function(experimental_implements="embedded_matmul"):
def embedding_matmul(a, b):
   # custom implementation here
  • experimental_autograph_options: Optional tuple of tf.autograph.experimental.Feature values.
  • experimental_relax_shapes: When True, tf.function may generate fewer, graphs that are less specialized on input shapes.
  • experimental_compile: If True, the function is always compiled by XLA. XLA may be more efficient in some cases (e.g. TPU, XLA_GPU, dense tensor computations).

Returns:

If func is not None, returns a callable that will execute the compiled function (and return zero or more tf.Tensor objects). If func is None, returns a decorator that, when invoked with a single func argument, returns a callable equivalent to the case above.