Gradient clipping appears to choke on None

I'm trying to add gradient clipping to my graph. I used the approach recommended here: How to effectively apply gradient clipping in tensor flow?

    optimizer = tf.train.GradientDescentOptimizer(learning_rate)
    if gradient_clipping:
        gradients = optimizer.compute_gradients(loss)
        clipped_gradients = [(tf.clip_by_value(grad, -1, 1), var) for grad, var in gradients]
        opt = optimizer.apply_gradients(clipped_gradients, global_step=global_step)
    else:
        opt = optimizer.minimize(loss, global_step=global_step)

But when I turn on gradient clipping, I get the following stack trace:

<ipython-input-19-be0dcc63725e> in <listcomp>(.0)
     61         if gradient_clipping:
     62             gradients = optimizer.compute_gradients(loss)
---> 63             clipped_gradients = [(tf.clip_by_value(grad, -1., 1.), var) for grad, var in gradients]
     64             opt = optimizer.apply_gradients(clipped_gradients, global_step=global_step)
     65         else:

/home/armence/mlsandbox/venv/lib/python3.4/site-packages/tensorflow/python/ops/clip_ops.py in clip_by_value(t, clip_value_min, clip_value_max, name)
     51   with ops.op_scope([t, clip_value_min, clip_value_max], name,
     52                    "clip_by_value") as name:
---> 53     t = ops.convert_to_tensor(t, name="t")
     54 
     55     # Go through list of tensors, for each value in each tensor clip

/home/armence/mlsandbox/venv/lib/python3.4/site-packages/tensorflow/python/framework/ops.py in convert_to_tensor(value, dtype, name, as_ref)
    619     for base_type, conversion_func in funcs_at_priority:
    620       if isinstance(value, base_type):
--> 621         ret = conversion_func(value, dtype=dtype, name=name, as_ref=as_ref)
    622         if ret is NotImplemented:
    623           continue

/home/armence/mlsandbox/venv/lib/python3.4/site-packages/tensorflow/python/framework/constant_op.py in _constant_tensor_conversion_function(v, dtype, name, as_ref)
    178                                          as_ref=False):
    179   _ = as_ref
--> 180   return constant(v, dtype=dtype, name=name)
    181 
    182 

/home/armence/mlsandbox/venv/lib/python3.4/site-packages/tensorflow/python/framework/constant_op.py in constant(value, dtype, shape, name)
    161   tensor_value = attr_value_pb2.AttrValue()
    162   tensor_value.tensor.CopyFrom(
--> 163       tensor_util.make_tensor_proto(value, dtype=dtype, shape=shape))
    164   dtype_value = attr_value_pb2.AttrValue(type=tensor_value.tensor.dtype)
    165   const_tensor = g.create_op(

/home/armence/mlsandbox/venv/lib/python3.4/site-packages/tensorflow/python/framework/tensor_util.py in make_tensor_proto(values, dtype, shape)
    344   else:
    345     if values is None:
--> 346       raise ValueError("None values not supported.")
    347     # if dtype is provided, forces numpy array to be the type
    348     # provided if possible.

ValueError: None values not supported.

How do I solve this problem?

So, one option that seems to work is this:

    optimizer = tf.train.GradientDescentOptimizer(learning_rate)
    if gradient_clipping:
        gradients = optimizer.compute_gradients(loss)

        def ClipIfNotNone(grad):
            if grad is None:
                return grad
            return tf.clip_by_value(grad, -1, 1)
        clipped_gradients = [(ClipIfNotNone(grad), var) for grad, var in gradients]
        opt = optimizer.apply_gradients(clipped_gradients, global_step=global_step)
    else:
        opt = optimizer.minimize(loss, global_step=global_step)

It looks like compute_gradients returns None instead of a zero tensor when the gradient would be a zero tensor and tf.clip_by_value does not support a None value. So just don't pass None to it and preserve None values.