텐서플로우 코드 에러 - FailedPreconditionError

Deep MNIST for Experts 튜토리얼 실행시 발생할 수 있는 텐서플로우 코드 에러

FailedPreconditionError 가 발생하는 경우

---

for i in range(20000):
    batch = mnist.train.next_batch(50)
    if i%100 == 0:
        train_accuracy = accuracy.eval(session=sess, feed_dict={x:batch[0], y_: batch[1], keep_prob: 1.0})
        print "step %d, training accuracy %g" % (i, train_accuracy)
    train_step.run(feed_dict={x: batch[0], y_: batch[1], keep_prob: 0.5})
 
-------------------------------------------------------------------------
FailedPreconditionError                 Traceback (most recent call last)
<ipython-input-21-3be1f0fe5d4d> in <module>()
      2     batch = mnist.train.next_batch(50)
      3     if i%100 == 0:
----> 4         train_accuracy = accuracy.eval(session=sess, feed_dict={x:batch[0], y_: batch[1], keep_prob: 1.0})
      5         print "step %d, training accuracy %g" % (i, train_accuracy)
      6     train_step.run(feed_dict={x: batch[0], y_: batch[1], keep_prob: 0.5})
 
/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/framework/ops.pyc in eval(self, feed_dict, session)
    553 
    554     """
--> 555     return _eval_using_default_session(self, feed_dict, self.graph, session)
    556 
    557 

/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/framework/ops.pyc in _eval_using_default_session(tensors, feed_dict, graph, session)
   3496                        "the tensor's graph is different from the session's "
   3497                        "graph.")
-> 3498   return session.run(tensors, feed_dict)
   3499 
   3500 

/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/client/session.pyc in run(self, fetches, feed_dict, options, run_metadata)
    370     try:
    371       result = self._run(None, fetches, feed_dict, options_ptr,
--> 372                          run_metadata_ptr)
    373       if run_metadata:
    374         proto_data = tf_session.TF_GetBuffer(run_metadata_ptr)

/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/client/session.pyc in _run(self, handle, fetches, feed_dict, options, run_metadata)
    634     try:
    635       results = self._do_run(handle, target_list, unique_fetches,
--> 636                              feed_dict_string, options, run_metadata)
    637     finally:
    638       # The movers are no longer used. Delete them.

/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/client/session.pyc in _do_run(self, handle, target_list, fetch_list, feed_dict, options, run_metadata)
    706     if handle is None:
    707       return self._do_call(_run_fn, self._session, feed_dict, fetch_list,
--> 708                            target_list, options, run_metadata)
    709     else:
    710       return self._do_call(_prun_fn, self._session, handle, feed_dict,

/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/client/session.pyc in _do_call(self, fn, *args)
    726         except KeyError:
    727           pass
--> 728       raise type(e)(node_def, op, message)
    729 
    730   def _extend_graph(self):

FailedPreconditionError: Attempting to use uninitialized value Variable_9
     [[Node: Variable_9/read = Identity[T=DT_FLOAT, _class=["loc:@Variable_9"], _device="/job:localhost/replica:0/task:0/cpu:0"](Variable_9)]]
Caused by op u'Variable_9/read', defined at:
  File "/root/anaconda/envs/tensorflow/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/ipykernel/__main__.py", line 3, in <module>
    app.launch_new_instance()
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/traitlets/config/application.py", line 658, in launch_instance
    app.start()
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/ipykernel/kernelapp.py", line 474, in start
    ioloop.IOLoop.instance().start()
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/zmq/eventloop/ioloop.py", line 177, in start
    super(ZMQIOLoop, self).start()
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tornado/ioloop.py", line 887, in start
    handler_func(fd_obj, events)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tornado/stack_context.py", line 275, in null_wrapper
    return fn(*args, **kwargs)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/zmq/eventloop/zmqstream.py", line 440, in _handle_events
    self._handle_recv()
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/zmq/eventloop/zmqstream.py", line 472, in _handle_recv
    self._run_callback(callback, msg)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/zmq/eventloop/zmqstream.py", line 414, in _run_callback
    callback(*args, **kwargs)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tornado/stack_context.py", line 275, in null_wrapper
    return fn(*args, **kwargs)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/ipykernel/kernelbase.py", line 276, in dispatcher
    return self.dispatch_shell(stream, msg)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/ipykernel/kernelbase.py", line 228, in dispatch_shell
    handler(stream, idents, msg)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/ipykernel/kernelbase.py", line 390, in execute_request
    user_expressions, allow_stdin)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/ipykernel/ipkernel.py", line 196, in do_execute
    res = shell.run_cell(code, store_history=store_history, silent=silent)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/ipykernel/zmqshell.py", line 501, in run_cell
    return super(ZMQInteractiveShell, self).run_cell(*args, **kwargs)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/IPython/core/interactiveshell.py", line 2717, in run_cell
    interactivity=interactivity, compiler=compiler, result=result)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/IPython/core/interactiveshell.py", line 2821, in run_ast_nodes
    if self.run_code(code, result):
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/IPython/core/interactiveshell.py", line 2881, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-12-3af896285c70>", line 2, in <module>
    b_fc2 = bias_variable([10])
  File "<ipython-input-2-3b31b89bc627>", line 7, in bias_variable
    return tf.Variable(initial)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/ops/variables.py", line 211, in __init__
    dtype=dtype)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/ops/variables.py", line 319, in _init_from_args
    self._snapshot = array_ops.identity(self._variable, name="read")
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/ops/gen_array_ops.py", line 831, in identity
    result = _op_def_lib.apply_op("Identity", input=input, name=name)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/ops/op_def_library.py", line 704, in apply_op
    op_def=op_def)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/framework/ops.py", line 2260, in create_op
    original_op=self._default_original_op, op_def=op_def)
  File "/root/anaconda/envs/tensorflow/lib/python2.7/site-packages/tensorflow/python/framework/ops.py", line 1230, in __init__
    self._traceback = _extract_stack()

FailedPreconditionError 는 변수가 초기화되지 전에 해당 변수를 읽을려고 할 때 발생한다. 텐서플로우에서는 모든 변수는 "initializer" 오퍼레이션을 통해 명시적으로 초기화되어야 한다. 트레이닝 반복문을 실행하기 전에 다음 구문을 실행함으로써 간편하게 모든 변수를 초기화할 수 있다.

tf.initialize_all_variables().run()

특정 세션을 지정하지 않고 오퍼레이션 실행이 가능하게 하는 tf.InteractiveSession 을 사용하는 경우는 위와 같고, non-interactive의 경우는 tf.Session을 사용하여 초기화한다.

init_op = tf.initialize_all_variables()
 
sess = tf.Session()
sess.run(init_op)