Output of loss is None - tensorflow

Question

I have to finetune VGG.There are five convolutional layers and then three fully connected layers. Output from the last fully connected layer is the input of the loss function. Following is my code:
class vgg16:
def __init__(self, imgs1,imgs2, weights=None, sess=None):
self.imgs1 = imgs1
self.imgs2 = imgs2
with tf.variable_scope("siamese") as scope:
self.o1 = self.convlayers(imgs1)
self.fc_layers()
self.loss()
if weights is not None and sess is not None:
self.load_weights(weights, sess)
scope.reuse_variables()
self.o2 = self.convlayers(imgs2)
self.fc_layers()
self.loss()
if weights is not None and sess is not None:
self.load_weights(weights, sess)
#create loss function
def convlayers(self,imgs):
....
# conv1_2
with tf.name_scope('conv1_2') as scope:
......
# pool1
..
)
.....
# pool5
self.pool5 = tf.nn.max_pool(self.conv5_3,
ksize=[1, 2, 2, 1],
strides=[1, 2, 2, 1],
padding='SAME',
name='pool4')
def fc_layers(self):
# fc1
with tf.name_scope('fc1') as scope:
....
# fc2
with tf.name_scope('fc2') as scope:
...
# fc3
with tf.name_scope('fc3') as scope:
fc3w = tf.Variable(tf.truncated_normal([4096, 1000],
dtype=tf.float32,
stddev=1e-1), name='weights')
fc3b = tf.Variable(tf.constant(1.0, shape=[1000], dtype=tf.float32),
trainable=True, name='biases')
self.fc3l = tf.nn.bias_add(tf.matmul(self.fc2, fc3w), fc3b)
def load_weights(self, weight_file, sess):
weights = np.load(weight_file)
keys = sorted(weights.keys())
for i, k in enumerate(keys):
print i, k, np.shape(weights[k])
sess.run(self.parameters[i].assign(weights[k]))
def loss(self):
loss=tf.nn.l2_loss(self.fc3l)
self.train_step = tf.train.GradientDescentOptimizer(0.5).minimize(loss)
if __name__ == '__main__':
sess = tf.Session()
imgs1 = tf.placeholder(tf.float32, [None, 224, 224, 3])#jis size ka bhi imaeg hai usko 224x224 may kar diya or RGB chaeay hmay
imgs2 = tf.placeholder(tf.float32, [None, 224, 224, 3])
vgg = vgg16(imgs1,imgs2, 'vgg16_weights.npz', sess)
img1 = imread('laska.png', mode='RGB')
img1 = imresize(img1, (224, 224))
img2 = imread('laska2.jpg', mode='RGB')
img2 = imresize(img2,(224, 224))
prob = sess.run(vgg.train_step, feed_dict={vgg.imgs1: [img1],vgg.imgs2: [img2]})
print('loss is:')
print(prob)
The problem is that the output of prob is None. Kindly indicate what I am doing wrong.
PS: I am following siamese architecture. Input to both branches are different images here.

Answer 1

The op self.train_step does not return anything, it just calculates gradients and updates variables. See here.
What you need to do is to save reference to loss tensor in your vgg16 class like this:
self.loss=tf.nn.l2_loss(self.fc3l)
and then execute both train_step and loss operations in single sess.run:
_, loss_value = sess.run([vgg.train_step, vgg.loss], feed_dict=...)
print('loss is:')
print(loss_value)