Bayesian RNN / GRU for Molecules


I am trying to construct a RNN/GRU to predict a label for molecular data. Each molecule can be represented as a SMILES (simplified molecular-input line-entry system) string which (as of now) I am attempting to encode it using one-hot encoding which converts each molecule into a 120 x 56 matrix, and each molecule has a continuous label. I am following a GRU implementation from here.

I am wondering whether it is sensible to flatten the input dimension to a 1 by 120x56 vector. However, when I run my code, I get this error RuntimeError: maximum recursion depth exceeded while calling a Python object.

I am wondering whether this is a TensorFlow memory problem, as I have only just started using TensorFlow. I am also wondering whether an Embedding layer is appropriate for this. Thank you in advance for any help!

import edward as ed
import numpy as np
import tensorflow as tf
from edward.models import *
from edward.util import Progbar
from keras.layers import Embedding, Dense

H = 5
D = 2
V = 10
E = 2
batch_size = 10
M = 10
nb_steps = 120*56


X_train = np.ones([N, nb_steps ], dtype=np.int32)
y_train = np.ones([N, 1 ], dtype=np.int32)

with tf.variable_scope('model', reuse=True):

    # Weights in GRU
    Wfo = Normal(loc=tf.zeros([D, H]), scale=tf.ones([D, H]))
    Wro = Normal(loc=tf.zeros([H, H]), scale=tf.ones([H, H]))

    Wff = Normal(loc=tf.zeros([D, H]), scale=tf.ones([D, H]))
    Wrf = Normal(loc=tf.zeros([H, H]), scale=tf.ones([H, H]))

    Wfy = Normal(loc=tf.zeros([D, H]), scale=tf.ones([D, H]))
    Wry = Normal(loc=tf.zeros([H, H]), scale=tf.ones([H, H]))

    qWfo = Normal(loc=tf.Variable(tf.random_normal([D, H])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([D, H]))))
    qWro = Normal(loc=tf.Variable(tf.random_normal([H, H])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([H, H]))))

    qWff = Normal(loc=tf.Variable(tf.random_normal([D, H])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([D, H]))))
    qWrf = Normal(loc=tf.Variable(tf.random_normal([H, H])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([H, H]))))

    qWfy = Normal(loc=tf.Variable(tf.random_normal([D, H])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([D, H]))))
    qWry = Normal(loc=tf.Variable(tf.random_normal([H, H])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([H, H]))))
    # Placeholders 
    y_ph = tf.placeholder(tf.float32, [batch_size, 1], name='y_ph')

    x = tf.placeholder(tf.int32, [batch_size, nb_steps ], name='x')     
    # GRU cell
    def gru_cell(hprev, xt):
        #  output gate
        #import pdb; pdb.set_trace()
        i_o = tf.sigmoid(tf.matmul(xt,Wfo) + tf.matmul(hprev,Wro) )
        #  forget gate
        i_f = tf.sigmoid(tf.matmul(xt,Wff) + tf.matmul(hprev,Wrf) )
        #  intermediate
        y = tf.tanh(tf.matmul(xt,Wfy) + tf.matmul( (i_f*hprev),Wry) )
        # new state
        return (1-i_o)*y + (i_o*hprev)
    # Embedding (?)
    x_ = Embedding(V, D, name='Embedding')(x)
    # Initialise hidden state
    h = tf.zeros(shape=(batch_size, H)) # initial state

    for t in range(nb_steps-1):
        h = gru_cell(h, x_[:,t,:])
        print('h : ',h)
    # Variational Inference
    W1 = Normal(loc=tf.zeros([D, 1]), scale=tf.ones([D, 1]))
    W2 = Normal(loc=tf.zeros([H, D]), scale=tf.ones([H, D]))
    qW1 = Normal(loc=tf.Variable(tf.random_normal([D, 1])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([D, 1]))))
    qW2 = Normal(loc=tf.Variable(tf.random_normal([H, D])), scale=tf.nn.softplus(tf.Variable(tf.random_normal([H, D]))))
    def fhw(h_in):
        fhw = tf.matmul(tf.sigmoid(tf.matmul(h_in, W2)), W1)    
        return fhw
    y = Normal(loc=fhw(h), scale=0.1 * tf.ones([batch_size,1]))

# Inference
inference = ed.KLqp({W1: qW1, W2: qW2, 
                     Wfo: qWfo, Wro: qWro, 
                     Wff: qWff, Wrf: qWrf, 
                     Wfy: qWfy, Wry: qWry}, data={y: y_ph})

optimizer = tf.train.RMSPropOptimizer(0.01, epsilon=1.0)
inference.initialize(optimizer=optimizer,scale={y: len(X_train) / batch_size}) # always redefine inference before
sess = ed.get_session()

n_epoch = 1
n_iter_per_epoch = 1

for epoch in range(n_epoch):
  avg_loss = 0.0

  pbar = Progbar(n_iter_per_epoch)
  for t in range(1, n_iter_per_epoch + 1):
    batch = np.random.randint(0, len(X_train)-1, batch_size)
    info_dict = inference.update({x: X_train[batch], y_ph: y_train[batch]})
    avg_loss  += info_dict['loss']
  # Print a lower bound to the average marginal likelihood for an
  # image.
  avg_loss = avg_loss / n_iter_per_epoch
  avg_loss = avg_loss / batch_size
  print("log p(x) >= {:0.3f}".format(avg_loss))

## Results should give ones
X_test = np.ones([10, nb_steps])
y_test = np.ones([10, nb_steps])

test1 ={W1: qW1.sample(), W2: qW2.sample(),
                 Wfo: qWfo.sample(), Wro: qWro.sample(),
                 Wff: qWff.sample(), Wrf: qWrf.sample(),
                 Wfy: qWfy.sample(), Wry: qWry.sample()},{x: X_test})

y_post = ed.copy(y, {W1: qW1, W2: qW2,
                     Wfo: qWfo, Wro: qWro,
                     Wff: qWff, Wrf: qWrf,
                     Wfy: qWfy, Wry: qWry})
y_out =, feed_dict={x:X_test})

print('MSE : ',np.mean(np.square(y_out-y_test)))

That’s interesting. Do you know the recursion appears. E.g., have you tried limiting n_steps to 2 or 3? I’m surprised you get a recursion error given it’s a finite for loop.

Not strange and if you try to Google the issue you’d learn that

> python -c "import sys; print(sys.getrecursionlimit())"

And no, it is not a good idea to flatten the matrix. RNN’s forget really fast so perhaps try with 56 steps and an input of 120.

Update: so I’ve changed it to 120 steps, which is far below 1000, but it stills says that I am reaching the recursion limit. Small steps for up to 20 still work. However, for my case, my input is of a fixed sequence length of 120 so I should iterate over each character. Any suggestions?