tensorflow v1 GradientTape: AttributeError: 'NoneType' object has no attribute 'eval' - tensorflow

Question

I want to compute the gradient of the distance between the NSynth WaveNet encoding of two sine waves.
This is tensorflow v1.
I am working with code based upon https://github.com/magenta/magenta/blob/master/magenta/models/nsynth/wavenet/fastgen.py
A minimal example of my bug is in this colab notebook: https://colab.research.google.com/drive/1oTEU8QAaOs0K1A0KHrAdt7kA7MkadNDr?usp=sharing
Here is the code:
# Commented out IPython magic to ensure Python compatibility.
# %tensorflow_version 1.x
!pip3 install -q magenta
!wget -c http://download.magenta.tensorflow.org/models/nsynth/wavenet-ckpt.tar && tar xvf wavenet-ckpt.tar
checkpoint_path = './wavenet-ckpt/model.ckpt-200000'
import math
from magenta.models.nsynth.wavenet import fastgen
import tensorflow as tf
session_config = tf.ConfigProto(allow_soft_placement=True)
session_config.gpu_options.allow_growth = True
sess = tf.Session(config=session_config)
pi = 3.1415926535897
SR = 16000
sample_length = 64000
DURATION_SECONDS = sample_length / SR
def sine(hz):
time = tf.linspace(0.0, DURATION_SECONDS, sample_length)
return tf.constant(0.5) * tf.cos(2.0 * pi * time * hz)
net = fastgen.load_nsynth(batch_size=2, sample_length=sample_length)
saver = tf.train.Saver()
saver.restore(sess, checkpoint_path)
"""We have two sine waves at 440 and 660 Hz. We use the encoder to generate two (125, 16) encodings:"""
twosines = tf.stack([sine(440), sine(660)]).eval(session=sess)
print(sess.run(net["encoding"], feed_dict={net["X"]: twosines}).shape)
"""Compute the distance between the two sine waves"""
distencode = tf.reduce_mean(tf.abs(net["encoding"][0] - net["encoding"][1]))
print(sess.run(distencode, feed_dict={net["X"]: twosines}))
"""I don't know why the following code doesn't work, but if I did I could solve the real task....
"""
net["X"] = twosines
distencode.eval(session=sess)
"""Here is the code that I need to work. I want to compute the gradient of the distance between the NSynth encoding of two sine waves:"""
fp = tf.constant(660.0)
newsines = tf.stack([sine(440), sine(fp)])
with tf.GradientTape() as g:
g.watch(fp)
dd_dfp = g.gradient(distencode, fp)
print(dd_dfp.eval(session=sess))
The last block, which I want to evaluate, gets the following error:
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-12-b5b8cdd00b24> in <module>()
4 g.watch(fp)
5 dd_dfp = g.gradient(distencode, fp)
----> 6 print(dd_dfp.eval(session=sess))
AttributeError: 'NoneType' object has no attribute 'eval'
I believe I need to define the operations to be executed within this block. However, I am using a pretrained model that I am just computing the distance over, so I am not sure how to define execution in that block.
The second-to-last block, which would help me fix the last block, gives the following error:
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-10-c3411dcbfa2c> in <module>()
3 with tf.GradientTape() as g:
4 g.watch(fp)
----> 5 dd_dfp = g.gradient(distencode, g)
6 print(dd_dfp.eval(session=sess))
/tensorflow-1.15.2/python3.6/tensorflow_core/python/eager/backprop.py in gradient(self, target, sources, output_gradients, unconnected_gradients)
997 flat_sources = [_handle_or_self(x) for x in flat_sources]
998 for t in flat_sources_raw:
--> 999 if not t.dtype.is_floating:
1000 logging.vlog(
1001 logging.WARN, "The dtype of the source tensor must be "
AttributeError: 'GradientTape' object has no attribute 'dtype'
---------------------------------------------------------------------------
InvalidArgumentError Traceback (most recent call last)
/tensorflow-1.15.2/python3.6/tensorflow_core/python/client/session.py in _do_call(self, fn, *args)
1364 try:
-> 1365 return fn(*args)
1366 except errors.OpError as e:
8 frames
InvalidArgumentError: 2 root error(s) found.
(0) Invalid argument: You must feed a value for placeholder tensor 'Placeholder' with dtype float and shape [2,64000]
[[{{node Placeholder}}]]
[[Mean/_759]]
(1) Invalid argument: You must feed a value for placeholder tensor 'Placeholder' with dtype float and shape [2,64000]
[[{{node Placeholder}}]]
0 successful operations.
0 derived errors ignored.
During handling of the above exception, another exception occurred:
InvalidArgumentError Traceback (most recent call last)
/tensorflow-1.15.2/python3.6/tensorflow_core/python/client/session.py in _do_call(self, fn, *args)
1382 '\nsession_config.graph_options.rewrite_options.'
1383 'disable_meta_optimizer = True')
-> 1384 raise type(e)(node_def, op, message)
1385
1386 def _extend_graph(self):
InvalidArgumentError: 2 root error(s) found.
(0) Invalid argument: You must feed a value for placeholder tensor 'Placeholder' with dtype float and shape [2,64000]
[[node Placeholder (defined at /tensorflow-1.15.2/python3.6/tensorflow_core/python/framework/ops.py:1748) ]]
[[Mean/_759]]
(1) Invalid argument: You must feed a value for placeholder tensor 'Placeholder' with dtype float and shape [2,64000]
[[node Placeholder (defined at /tensorflow-1.15.2/python3.6/tensorflow_core/python/framework/ops.py:1748) ]]
0 successful operations.
0 derived errors ignored.
Original stack trace for 'Placeholder':
File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
"__main__", mod_spec)
File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
exec(code, run_globals)
File "/usr/local/lib/python3.6/dist-packages/ipykernel_launcher.py", line 16, in <module>
app.launch_new_instance()
File "/usr/local/lib/python3.6/dist-packages/traitlets/config/application.py", line 664, in launch_instance
app.start()
File "/usr/local/lib/python3.6/dist-packages/ipykernel/kernelapp.py", line 499, in start
self.io_loop.start()
File "/usr/local/lib/python3.6/dist-packages/tornado/platform/asyncio.py", line 132, in start
self.asyncio_loop.run_forever()
File "/usr/lib/python3.6/asyncio/base_events.py", line 438, in run_forever
self._run_once()
File "/usr/lib/python3.6/asyncio/base_events.py", line 1451, in _run_once
handle._run()
File "/usr/lib/python3.6/asyncio/events.py", line 145, in _run
self._callback(*self._args)
File "/usr/local/lib/python3.6/dist-packages/tornado/ioloop.py", line 758, in _run_callback
ret = callback()
File "/usr/local/lib/python3.6/dist-packages/tornado/stack_context.py", line 300, in null_wrapper
return fn(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 548, in <lambda>
self.io_loop.add_callback(lambda : self._handle_events(self.socket, 0))
File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 462, in _handle_events
self._handle_recv()
File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 492, in _handle_recv
self._run_callback(callback, msg)
File "/usr/local/lib/python3.6/dist-packages/zmq/eventloop/zmqstream.py", line 444, in _run_callback
callback(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/tornado/stack_context.py", line 300, in null_wrapper
return fn(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/ipykernel/kernelbase.py", line 283, in dispatcher
return self.dispatch_shell(stream, msg)
File "/usr/local/lib/python3.6/dist-packages/ipykernel/kernelbase.py", line 233, in dispatch_shell
handler(stream, idents, msg)
File "/usr/local/lib/python3.6/dist-packages/ipykernel/kernelbase.py", line 399, in execute_request
user_expressions, allow_stdin)
File "/usr/local/lib/python3.6/dist-packages/ipykernel/ipkernel.py", line 208, in do_execute
res = shell.run_cell(code, store_history=store_history, silent=silent)
File "/usr/local/lib/python3.6/dist-packages/ipykernel/zmqshell.py", line 537, in run_cell
return super(ZMQInteractiveShell, self).run_cell(*args, **kwargs)
File "/usr/local/lib/python3.6/dist-packages/IPython/core/interactiveshell.py", line 2718, in run_cell
interactivity=interactivity, compiler=compiler, result=result)
File "/usr/local/lib/python3.6/dist-packages/IPython/core/interactiveshell.py", line 2822, in run_ast_nodes
if self.run_code(code, result):
File "/usr/local/lib/python3.6/dist-packages/IPython/core/interactiveshell.py", line 2882, in run_code
exec(code_obj, self.user_global_ns, self.user_ns)
File "<ipython-input-5-5120c8282e75>", line 1, in <module>
net = fastgen.load_nsynth(batch_size=2, sample_length=sample_length)
File "/tensorflow-1.15.2/python3.6/magenta/models/nsynth/wavenet/fastgen.py", line 64, in load_nsynth
x = tf.placeholder(tf.float32, shape=[batch_size, sample_length])
File "/tensorflow-1.15.2/python3.6/tensorflow_core/python/ops/array_ops.py", line 2619, in placeholder
return gen_array_ops.placeholder(dtype=dtype, shape=shape, name=name)
File "/tensorflow-1.15.2/python3.6/tensorflow_core/python/ops/gen_array_ops.py", line 6669, in placeholder
"Placeholder", dtype=dtype, shape=shape, name=name)
File "/tensorflow-1.15.2/python3.6/tensorflow_core/python/framework/op_def_library.py", line 794, in _apply_op_helper
op_def=op_def)
File "/tensorflow-1.15.2/python3.6/tensorflow_core/python/util/deprecation.py", line 507, in new_func
return func(*args, **kwargs)
File "/tensorflow-1.15.2/python3.6/tensorflow_core/python/framework/ops.py", line 3357, in create_op
attrs, op_def, compute_device)
File "/tensorflow-1.15.2/python3.6/tensorflow_core/python/framework/ops.py", line 3426, in _create_op_internal
op_def=op_def)
File "/tensorflow-1.15.2/python3.6/tensorflow_core/python/framework/ops.py", line 1748, in __init__
self._traceback = tf_stack.extract_stack()
Thank you.