Session, Tensor, Variable OP

Posted by Pinkerbot on Sat, 20 Jun 2020 03:37:48 +0200

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2.3 Sessions, Tensors, Variables OP

Learning Objectives

  • target
    • applicationSess.runOr eval runs the diagram program and gets the tensor value
    • Apply feed_dict mechanism to populate data at runtime
    • Applying placeholder implementation to create placeholders
    • Knowing the common TensorFlow creation tensors
    • Knowing common tensor mathematical operations
    • Explain the identity of numpy's array and tensor
    • Explain the two shape-changing characteristics of a tensor
    • Apply set_Shapes andTf.reshapeImplement modification of tensor shape
    • applicationTf.matmulModification of Matrix Operations to Implement Tensors
    • applicationTf.castType of implementation tensor
    • Explain the special role of variable op
    • Explains the role of the trainable parameter of the variable op
    • Apply global_variables_initializer implements the initialization of variable op
  • application
    • nothing
  • Content Preview

2.3.1 Sessions

A class that runs TensorFlow operation.Sessions can be opened in two ways

  • tf.Session: For use in a complete program
  • tf.InteractiveSession: TensorFlow in an interactive context, such as a shell

1 TensorFlow usesTf.SessionClasses represent connections between client programs (usually Python programs, but also provide similar interfaces in other languages) and the C++ runtime

2Tf.SessionObjects use the distributed TensorFlow runtime to provide access to devices on the local computer and remote devices. __init__(target='', graph=None, config=None)

Resources that a session may have, such asTf.Variable,Tf.QueueBaseandTf.ReaderBase.Releasing these resources is important when they are no longer needed.Therefore, you need to callTf.Session.closeMethod in a session, or use the session as a context manager.The following two examples serve the same purpose:

def session_demo():
    //Session Demo

    a_t = tf.constant(10)
    b_t = tf.constant(20)
    # Direct use of this symbolic operator for calculation is not advocated
    # More commonly, tensorflow functions are used to calculate
    # c_t = a_t + b_t
    c_t = tf.add(a_t, b_t)
    print("tensorflow Implement addition operations:\n", c_t)

    # Open Session
    # Traditional session definitions
    # sess = tf.Session()
    # sum_t =
    # print("sum_t:\n", sum_t)
    # sess.close()

    # Open Session
    with tf.Session() as sess:
        # sum_t =
        # Want to execute multiple tensor s simultaneously
        print([a_t, b_t, c_t]))
        # A convenient way to get tensor values
        # print("sum_in sess"T:\n', c_T.eval())
        # Diagram Properties of Sessions
        print("Diagram properties of a session:\n", sess.graph)

    return None
  • target: If this parameter is left blank (the default setting), the session will only use devices on the local computer.You can specify the grpc://web address to specify the address of the TensorFlow server, which allows the session to access all devices on the computer controlled by that server.
  • graph: by default, NEWTf.SessionBind to the current default diagram.
  • config: This parameter allows you to specify aTf.ConfigProtoIn order to control the behavior of the session.For example, the ConfigProto protocol is used to print device usage information
# Run session and print device information
sess = tf.Session(config=tf.ConfigProto(allow_soft_placement=True,

Sessions can allocate different resources to run on different devices.


device_type: type of device (e.g. CPU, GPU, TPU)

run() of a session

  • run(fetches,feed_dict=None, options=None, run_metadata=None)
    • through the use to run the operation
    • fetches: a single operation, or a list, tuple (other types that are not tensorflow do not work)
    • feed_dict: The parameter allows the caller to override the value of the tensor in the graph and assign values at runtime
      • andTf.placeholderWhen used together, the shape of the value is checked for compatibility with the placeholder.

UseTf.operation.eval() Operations can also be run, but need to be run in a session

# Create a graph
a = tf.constant(5.0)
b = tf.constant(6.0)
c = a * b

# Create Session
sess = tf.Session()

# Calculate C value
print(c.eval(session=sess)) feed operation

  • placeholder provides placeholders, run through feed_dict Specifies Parameter
def session_run_demo():
    //run methods for sessions
    # Define placeholders
    a = tf.placeholder(tf.float32)
    b = tf.placeholder(tf.float32)
    sum_ab = tf.add(a, b)
    print("sum_ab:\n", sum_ab)
    # Open Session
    with tf.Session() as sess:
        print("Results of placeholders:\n",, feed_dict={a: 3.0, b: 4.0}))
    return None

Note the error reported at runtime:

RuntimeError: If this Session is in an invalid state (e.g. closed).
TypeError: If fetches or feed_The dict key is of an inappropriate type.
ValueError: If fetches or feed_dict key is invalid or refers to a key that Tensor does not exist.

When writing TensorFlow programs, the primary goal of program delivery and operation isTf.Tensor

2.3.2 Tensor

TensorFlow's tensor is an n-dimensional array of typeTf.Tensor.Tensor has two important properties

  • Type:Data type
  • shape: shape (order)

Types of Tensors

Order of Tensor

Shapes are of order 0, 1, 2....

tensor1 = tf.constant(4.0)
tensor2 = tf.constant([1, 2, 3, 4])
linear_squares = tf.constant([[4], [9], [16], [25]], dtype=tf.int32)

# 0 dimension: () 1 dimension: (10,) 2 dimension: (3, 4) 3 dimension: (3, 4, 5)

2.3.3 Instructions to Create Tensors

  • Fixed Value Tensor

  • Random Value Tensor

  • Other Special Ops to create tensors
    • tf.Variable
    • tf.placeholder

Transformation of 2.3.4 Tensor Type Change Shape Change

TensorFlow's tensor has two shape transformations, dynamic and static

  • tf.reshape
  • tf.set_shape

Dynamic and static shapes must conform to the following rules

  • Static Shape
    • When converting a static shape, 1-D to 1-D, 2-D to 2-D, cannot change shape across orders
    • Static shapes cannot be set again for tensors of static shapes that have already been fixed
  • dynamic shape
    • tf.reshape() When creating a new tensor dynamically, the number of elements of the tensor must match
def tensor_demo():
    //Introduction to Tensors
    a = tf.constant(value=30.0, dtype=tf.float32, name="a")
    b = tf.constant([[1, 2], [3, 4]], dtype=tf.int32, name="b")
    a2 = tf.constant(value=30.0, dtype=tf.float32, name="a2")
    c = tf.placeholder(dtype=tf.float32, shape=[2, 3, 4], name="c")
    sum = tf.add(a, a2, name="my_add")
    print(a, a2, b, c)
    # Get Tensor Properties
    print("a Graph properties:\n", a.graph)
    print("b Name:\n",
    print("a2 Shape:\n", a2.shape)
    print("c Data type:\n", c.dtype)
    print("sum Of op:\n", sum.op)

    # Get static shape
    print("b The static shape of:\n", b.get_shape())

    # Define placeholders
    a_p = tf.placeholder(dtype=tf.float32, shape=[None, None])
    b_p = tf.placeholder(dtype=tf.float32, shape=[None, 10])
    c_p = tf.placeholder(dtype=tf.float32, shape=[3, 2])
    # Get static shape
    print("a_p The static shape of the is:\n", a_p.get_shape())
    print("b_p The static shape of the is:\n", b_p.get_shape())
    print("c_p The static shape of the is:\n", c_p.get_shape())

    # Shape Update
    # a_p.set_shape([2, 3])
    # Static shape cannot be modified once it has been fixed
    # b_p.set_shape([10, 3])
    # c_p.set_shape([2, 3])

    # The part of a static shape that is already fixed includes its order, and if it is fixed, the shape cannot be updated across the order.
    # Use dynamic shapes if you want to change shapes across orders
    # a_p.set_shape([1, 2, 3])
    # Get static shape
    print("a_p The static shape of the is:\n", a_p.get_shape())
    print("b_p The static shape of the is:\n", b_p.get_shape())
    print("c_p The static shape of the is:\n", c_p.get_shape())

    # dynamic shape
    # c_p_r = tf.reshape(c_p, [1, 2, 3])
    c_p_r = tf.reshape(c_p, [2, 3])
    # Dynamic shape, when changing, cannot change the total number of elements
    # c_p_r2 = tf.reshape(c_p, [3, 1])
    print("Results of dynamic shapes:\n", c_p_r)
    # print("Result of dynamic shape 2:\n", c_p_r2)
    return None

Mathematical operations of 2.3.5 tensors

  • Arithmetic Operators
  • Basic Mathematical Functions
  • Matrix operations
  • reduce operation
  • Sequence Index Operation

Refer to:

These API s are used, as described in the documentation

2.3.6 Variables

The TensorFlow variable is the best way to represent a shared persistent state handled by a program.Variables pass throughTf.VariableOP class to operate on.Variable characteristics:

  • Storage persistence
  • Modifiable Value
  • Can be assigned to be trained Creating variables

  • tf.Variable(initial_value=None,trainable=True,collections=None,name=None)
    • initial_value:Initialized value
    • trainable: Is it trained
    • collections: The new variable will be added to the collection of listed graphs, defaulting to [GraphKeys.GLOBAL_VARIABLES], if the trainable is a True variable, is also added to the graphics collectionGraphKeys.TRAINABLE_VARIABLES
  • Variables need to be explicitly initialized to run values
def variable_demo():
    //Presentation of variables
    # Define Variables
    a = tf.Variable(initial_value=30)
    b = tf.Variable(initial_value=40)
    sum = tf.add(a, b)

    # initialize variable
    init = tf.global_variables_initializer()

    # Open Session
    with tf.Session() as sess:
        # Variable Initialization

    return None UseTf.variable_Scope() modifies the namespace of a variable

Namespace name is added before OP name

with tf.variable_scope("name"):
    var = tf.Variable(name='var', initial_value=[4], dtype=tf.float32)
    var_double = tf.Variable(name='var', initial_value=[4], dtype=tf.float32)

<tf.Variable 'name/var:0' shape=() dtype=float32_ref>
<tf.Variable 'name/var_1:0' shape=() dtype=float32_ref>


Topics: Session Python shell