tensorflow How to use TensorFlow Graph Collections? Collect variables from nested scopes

Example

Below is a single hidden layer Multilayer Perceptron (MLP) using nested scoping of variables.

def weight_variable(shape):
    return tf.get_variable(name="weights", shape=shape,
                           initializer=tf.zeros_initializer(dtype=tf.float32))

def bias_variable(shape):
    return tf.get_variable(name="biases", shape=shape,
                           initializer=tf.zeros_initializer(dtype=tf.float32))

def fc_layer(input, in_dim, out_dim, layer_name):
    with tf.variable_scope(layer_name):
        W = weight_variable([in_dim, out_dim])
        b = bias_variable([out_dim])
        linear = tf.matmul(input, W) + b
        output = tf.sigmoid(linear)

with tf.variable_scope("MLP"):
    x = tf.placeholder(dtype=tf.float32, shape=[None, 1], name="x")
    y = tf.placeholder(dtype=tf.float32, shape=[None, 1], name="y")
    fc1 = fc_layer(x, 1, 8, "fc1")
    fc2 = fc_layer(fc1, 8, 1, "fc2")

mse_loss = tf.reduce_mean(tf.reduce_sum(tf.square(fc2 - y), axis=1))

The MLP uses the the top level scope name MLP and it has two layers with their respective scope names fc1 and fc2. Each layer also has its own weights and biases variables.

The variables can be collected like so:

trainable_var_key = tf.GraphKeys.TRAINABLE_VARIABLES
all_vars = tf.get_collection(key=trainable_var_key, scope="MLP")
fc1_vars = tf.get_collection(key=trainable_var_key, scope="MLP/fc1")
fc2_vars = tf.get_collection(key=trainable_var_key, scope="MLP/fc2")
fc1_weight_vars = tf.get_collection(key=trainable_var_key, scope="MLP/fc1/weights")
fc1_bias_vars = tf.get_collection(key=trainable_var_key, scope="MLP/fc1/biases")

The values of the variables can be collected using the sess.run() command. For example if we would like to collect the values of the fc1_weight_vars after training, we could do the following:

sess = tf.Session()
# add code to initialize variables
# add code to train the network
# add code to create test data x_test and y_test

fc1_weight_vals = sess.run(fc1, feed_dict={x: x_test, y: y_test})
print(fc1_weight_vals)  # This should be an ndarray with ndim=2 and shape=[1, 8]