Keras: Implementing kullback-leilbler divergence as custom Keras objective function

[rsmichael]

Hi,

I want to train models where the gold standard data are discrete probability distributions, which come from many observations of different discrete outcomes with the same input. For my model, I want to minimize, kullback-leibler divergence, which measures the information gain from a predicted distribution, q, to a true distribution, p:

KL_div = \sum{i}p_i*log(p_i/q_i)

Where i in the index for all of the categorical possibilities.

I implemented a function that takes in vectors and computes the kullback-leibler divergence:

def kullback_leibler(y_pred, y_true):
    p = T.vector('p')
    q = T.vector('q')
    results, updates = theano.scan(lambda p,q: p*T.log(p/q), sequences = [p,q]) 
    get_kldiv = T.sum(theano.function(inputs = [p,q], outputs = -T.sum(results)))
    kldiv = get_kldiv(y_true,y_pred)
    return(kldiv)

Here's an example of the function computing kullback-leibler divergence of two discrete distributions with length 2:

kullback_leibler(np.array([0.1,0.2]).astype('float32'),np.array([0.3,0.4]).astype('float32'))
array(-0.606842577457428, dtype=float32)

When I try to compile my model with this objective, I get the following error:

model.compile(loss=kullback_leibler, optimizer='sgd')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 66, in compile_model
  File "/usr/local/lib/python2.7/site-packages/keras/models.py", line 332, in compile
    **kwargs)
  File "/usr/local/lib/python2.7/site-packages/keras/engine/training.py", line 578, in compile
    sample_weight, mask)
  File "/usr/local/lib/python2.7/site-packages/keras/engine/training.py", line 305, in weighted
    score_array = fn(y_true, y_pred)
  File "<stdin>", line 6, in kullback_leibler
  File "/usr/local/lib/python2.7/site-packages/theano/compile/function_module.py", line 786, in __call__
    allow_downcast=s.allow_downcast)
  File "/usr/local/lib/python2.7/site-packages/theano/tensor/type.py", line 86, in filter
    'Expected an array-like object, but found a Variable: '
TypeError: ('Bad input argument to theano function with name "<stdin>:5"  at index 0(0-based)', **'Expected an array-like object, but found a Variable: maybe you are trying to call a function on a (possibly shared) variable instead of a numeric array?')**

This suggests that instead of making a function that directly computes the kullback-leibler divergence, I instead need it to output a Theano variable. I tried to do this and I get no error in compiling my model, but I get nan as the result of my model.

Here's the function:

def kullback_leibler(y_pred,y_true):
    results, updates = theano.scan(lambda y_true,y_pred: y_true*T.log(y_true/y_pred), sequences =   [y_true,y_pred])
    return(T.sum(results, axis= - 1))

The call to fit my model:

model.fit(features_small[:10000], Y[:10000], nb_epoch=20, batch_size=1, verbose=1,
validation_data=(features_small[10000:20000], Y[10000:20000]))

And the output:

Train on 10000 samples, validate on 10000 samples
Epoch 1/20
  905/10000 [=>............................] - ETA: 13s - loss: nan

I'm not quite sure what's going wrong, but would greatly appreciate any help. I am also unsure: do keras objective functions also need to be able to pass in 2D tensors? I am not 100% sure how to do this from reading the Theano documentation.

Thanks for your help!

[fchollet]

You'll need to add fuzz factors to avoid dividing by zero.

[dakshvar22]

Hi @fchollet what exactly do you mean when you say fuzz factors?

Hi @rsmichael Did you find a workaround for this yet?

[rsmichael]

Yes, here's my function:

def kullback_leibler2(y_pred,y_true):
    eps = 0.0001
    results, updates = theano.scan(lambda y_true,y_pred: (y_true+eps)*(T.log(y_true+eps)-T.log(y_pred+eps)), 
    sequences = [y_true,y_pred])
    return(T.sum(results, axis= - 1))

You have to add fuzz factors (the "eps" variable in my function) to prevent kullback leibler divergence from taking extreme (or infinite values) when the predicted probability is very low.

[dakshvar22]

Thanks @rsmichael .. Thats's exactly what I thought.
Thanks again.

[brunoalano]

Thanks.

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