To use TensorFlow you need to understand how TensorFlow:
- Represents computations as graphs.
- Executes graphs in the context of
Sessions. - Represents data as tensors.
- Maintains state with
Variables. - Uses feeds and fetches to get data into and out of arbitrary operations.
TensorFlow is a programming system in which you represent computations as
graphs. Nodes in the graph are called ops (short for operations). An op takes
zero or more Tensors, performs some computation, and produces zero or more
Tensors. In TensorFlow terminology, a Tensor is a typed multi-dimensional
array. For example, you can represent a mini-batch of images as a 4-D array of
floating point numbers with dimensions [batch, height, width, channels].
A TensorFlow graph is a description of computations. To compute anything,
a graph must be launched in a Session. A Session places the graph ops onto
Devices, such as CPUs or GPUs, and provides methods to execute them. These
methods return tensors produced by ops as numpy
ndarray objects in Python, and as tensorflow::Tensor instances in C and
C++.
TensorFlow programs are usually structured into a construction phase that assembles a graph, and an execution phase that uses a session to execute ops in the graph.
For example, it is common to create a graph to represent and train a neural network in the construction phase, and then repeatedly execute a set of training ops in the graph in the execution phase.
TensorFlow can be used from C, C++, and Python programs. It is presently much easier to use the Python library to assemble graphs, as it provides a large set of helper functions not available in the C and C++ libraries.
The session libraries have equivalent functionalities for the three languages.
To build a graph, start by defining ops that do not need any input, such as
constant, and pass their output to other ops that do computation.
The op constructors in the Python library return objects that represent the output of the constructed ops. You can pass these as inputs to other op constructors.
The TensorFlow Python library has a default graph to which op constructors add nodes. The default graph is sufficient for many applications. See the Graph class documentation for how to explicitly manage multiple graphs.
import tensorflow as tf
# Create a constant op that produces a 1x2 matrix. The op is
# added as a node to the default graph.
#
# The value returned by the constructor represents the output
# of the constant op.
matrix1 = tf.constant([[3., 3.]])
# Create another constant that produces a 2x1 matrix.
matrix2 = tf.constant([[2.],[2.]])
# Create a matmul op that takes 'matrix1' and 'matrix2' as inputs.
# The returned value, 'product', represents the result of the matrix
# multiplication.
product = tf.matmul(matrix1, matrix2)The default graph now has three nodes: two constant ops and one matmul
op. To actually multiply the matrices and get the result of the multiplication,
you must launch the graph in a Session.
After constructing a graph, you can launch it by creating a Session object.
The Session launches the default graph, unless a different graph is specified
in the constructor. See the
Session class for
the complete API.
# Launch the default graph.
sess = tf.Session()
# To run the matmul op we call the session 'run()' method, passing 'product'
# which represents the output of the matmul op. This indicates to the call
# that we want to get the output of the matmul op back.
#
# All inputs needed by the op are run automatically by the session. They
# typically are run in parallel.
#
# The call 'run(product)' thus causes the execution of three ops in the
# graph: the two constants and matmul.
#
# The output of the matmul is returned in 'result' as a numpy `ndarray` object.
result = sess.run(product)
print(result)
# ==> [[ 12.]]
# Close the Session when we're done.
sess.close()Sessions should be closed to release resources. To manage resources more
easily, use a with statement. Each Session implements a context manager that
calls close() when exiting the block.
with tf.Session() as sess:
result = sess.run([product])
print(result)The TensorFlow implementation translates the graph definition into executable operations distributed across available compute resources, such as the CPU or one of your computer's GPU cards. In general you do not have to specify CPUs or GPUs explicitly. TensorFlow uses your first GPU, if you have one, for as many operations as possible.
If you have more than one GPU available on your machine, to use a GPU beyond
the first you must assign ops to it explicitly. Use with...Device statements
to specify which CPU or GPU to use for operations:
with tf.Session() as sess:
with tf.device("/gpu:1"):
matrix1 = tf.constant([[3., 3.]])
matrix2 = tf.constant([[2.],[2.]])
product = tf.matmul(matrix1, matrix2)
...Devices are specified with strings. The currently supported devices are:
"/cpu:0": The CPU of your machine."/gpu:0": The GPU of your machine, if you have one."/gpu:1": The second GPU of your machine, etc.
See Using GPUs for more information about GPUs and TensorFlow.
To create a TensorFlow cluster, launch a TensorFlow server on each of the machines in the cluster. When you instantiate a Session in your client, you pass it the network location of one of the machines in the cluster:
with tf.Session("grpc://example.org:2222") as sess:
# Calls to sess.run(...) will be executed on the cluster.
...This machine becomes the master for the session. The master distributes the graph across other machines in the cluster (workers), much as the local implementation distributes the graph across available compute resources within a machine.
You can use "with tf.device():" statements to directly specify workers for particular parts of the graph:
with tf.device("/job:ps/task:0"):
weights = tf.Variable(...)
biases = tf.Variable(...)See the Distributed TensorFlow How To for more information about distributed sessions and clusters.
The Python examples in the documentation launch the graph with a
Session and use the
Session.run() method to execute
operations.
For ease of use in interactive Python environments, such as
IPython you can instead use the
InteractiveSession class,
and the Tensor.eval() and
Operation.run() methods. This
avoids having to keep a variable holding the session.
# Enter an interactive TensorFlow Session.
import tensorflow as tf
sess = tf.InteractiveSession()
x = tf.Variable([1.0, 2.0])
a = tf.constant([3.0, 3.0])
# Initialize 'x' using the run() method of its initializer op.
x.initializer.run()
# Add an op to subtract 'a' from 'x'. Run it and print the result
sub = tf.subtract(x, a)
print(sub.eval())
# ==> [-2. -1.]
# Close the Session when we're done.
sess.close()TensorFlow programs use a tensor data structure to represent all data -- only tensors are passed between operations in the computation graph. You can think of a TensorFlow tensor as an n-dimensional array or list. A tensor has a static type, a rank, and a shape. To learn more about how TensorFlow handles these concepts, see the Rank, Shape, and Type reference.
Variables maintain state across executions of the graph. The following example shows a variable serving as a simple counter. See Variables for more details.
# Create a Variable, that will be initialized to the scalar value 0.
state = tf.Variable(0, name="counter")
# Create an Op to add one to `state`.
one = tf.constant(1)
new_value = tf.add(state, one)
update = tf.assign(state, new_value)
# Variables must be initialized by running an `init` Op after having
# launched the graph. We first have to add the `init` Op to the graph.
init_op = tf.global_variables_initializer()
# Launch the graph and run the ops.
with tf.Session() as sess:
# Run the 'init' op
sess.run(init_op)
# Print the initial value of 'state'
print(sess.run(state))
# Run the op that updates 'state' and print 'state'.
for _ in range(3):
sess.run(update)
print(sess.run(state))
# output:
# 0
# 1
# 2
# 3The assign() operation in this code is a part of the expression graph just
like the add() operation, so it does not actually perform the assignment
until run() executes the expression.
You typically represent the parameters of a statistical model as a set of Variables. For example, you would store the weights for a neural network as a tensor in a Variable. During training you update this tensor by running a training graph repeatedly.
To fetch the outputs of operations, execute the graph with a run() call on
the Session object and pass in the tensors to retrieve. In the previous
example we fetched the single node state, but you can also fetch multiple
tensors:
input1 = tf.constant([3.0])
input2 = tf.constant([2.0])
input3 = tf.constant([5.0])
intermed = tf.add(input2, input3)
mul = tf.multiply(input1, intermed)
with tf.Session() as sess:
result = sess.run([mul, intermed])
print(result)
# output:
# [array([ 21.], dtype=float32), array([ 7.], dtype=float32)]All the ops needed to produce the values of the requested tensors are run once (not once per requested tensor).
The examples above introduce tensors into the computation graph by storing them
in Constants and Variables. TensorFlow also provides a feed mechanism for
patching a tensor directly into any operation in the graph.
A feed temporarily replaces the output of an operation with a tensor value.
You supply feed data as an argument to a run() call. The feed is only used for
the run call to which it is passed. The most common use case involves
designating specific operations to be "feed" operations by using
tf.placeholder() to create them:
input1 = tf.placeholder(tf.float32)
input2 = tf.placeholder(tf.float32)
output = input1 * input2
with tf.Session() as sess:
print(sess.run([output], feed_dict={input1:[7.], input2:[2.]}))
# output:
# [array([ 14.], dtype=float32)]A placeholder() operation generates an error if you do not supply a feed for
it. See the
MNIST fully-connected feed tutorial
(source code)
for a larger-scale example of feeds.