Data API Error : Cannot convert a symbolic Tensor (truediv:0) to a numpy array - tensorflow

Question

I have 45000 images of size 224*224, stored as a numpy array. This array, called source_arr has shape 45000,224,224 and it fits in the memory.
I want to divide this array into train, test and validate array and pre-process (normalize and convert greyscale to 3 channels RGB) them using tf.data API.
I have written a pre process function like:
def pre_process(x):
#Zero centering the scaled dataset
x_norm = (x-mean_Rot_MIP)/Var_Rot_MIP
#Stack 3 channels
x_norm_3ch= np.stack((x_norm, x_norm, x_norm),axis=0)
print('Rotn MIP 3ch dim:', x_norm_3ch.shape) # (3, 224, 224)
#converting channel 1st to channel last move axis 1 to 3
x_norm_3ch = moveaxis(x_norm_3ch, 0,2)
print('Rotn MIP ch last dim: ',x_norm_3ch.shape) # (224, 224, 3)
return x_norm_3ch
X_train_cases_idx.idx contains the index of images from source_arr that are part of training data.
I have read the corresponding training images from source_arr in the dataset object like:
X_train = tf.data.Dataset.from_tensor_slices([source_arr[i] for i in X_train_cases_idx.idx])
And then I apply the pre_process function on the training images like
X_train = X_train.map(pre_process)
but I get the following error
Traceback (most recent call last):
File "<ipython-input-37-69aa131a6944>", line 1, in <module>
X_train = X_train.map(pre_process)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\data\ops\dataset_ops.py", line 1695, in map
return MapDataset(self, map_func, preserve_cardinality=True)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\data\ops\dataset_ops.py", line 4045, in __init__
use_legacy_function=use_legacy_function)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\data\ops\dataset_ops.py", line 3371, in __init__
self._function = wrapper_fn.get_concrete_function()
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\eager\function.py", line 2939, in get_concrete_function
*args, **kwargs)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\eager\function.py", line 2906, in _get_concrete_function_garbage_collected
graph_function, args, kwargs = self._maybe_define_function(args, kwargs)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\eager\function.py", line 3213, in _maybe_define_function
graph_function = self._create_graph_function(args, kwargs)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\eager\function.py", line 3075, in _create_graph_function
capture_by_value=self._capture_by_value),
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\framework\func_graph.py", line 986, in func_graph_from_py_func
func_outputs = python_func(*func_args, **func_kwargs)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\data\ops\dataset_ops.py", line 3364, in wrapper_fn
ret = _wrapper_helper(*args)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\data\ops\dataset_ops.py", line 3299, in _wrapper_helper
ret = autograph.tf_convert(func, ag_ctx)(*nested_args)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\autograph\impl\api.py", line 258, in wrapper
raise e.ag_error_metadata.to_exception(e)
NotImplementedError: in user code:
<ipython-input-2-746b4230fbd1>:58 pre_process *
x_norm_3ch= np.stack((x_norm, x_norm, x_norm),axis=1)
<__array_function__ internals>:6 stack **
C:\ProgramData\Anaconda3\lib\site-packages\numpy\core\shape_base.py:419 stack
arrays = [asanyarray(arr) for arr in arrays]
C:\ProgramData\Anaconda3\lib\site-packages\numpy\core\shape_base.py:419 <listcomp>
arrays = [asanyarray(arr) for arr in arrays]
C:\ProgramData\Anaconda3\lib\site-packages\numpy\core\_asarray.py:138 asanyarray
return array(a, dtype, copy=False, order=order, subok=True)
C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\framework\ops.py:848 __array__
" a NumPy call, which is not supported".format(self.name))
NotImplementedError: Cannot convert a symbolic Tensor (truediv:0) to a numpy array. This error may indicate that you're trying to pass a Tensor to a NumPy call, which is not supported
What am I doing wrong, and how do I fix it?
I am using Tensorflow 2.0 with python 3.7 on windows 10

Answer 1

As the error message points out, you are trying to use NumPy functions to operate with TensorFlow tensors. Instead, you should use TensorFlow operations. This is equivalent to what you were trying to do:
def pre_process(x):
x_norm = (x - mean_Rot_MIP) / Var_Rot_MIP
# Stacking along the last dimension to avoid having to move channel axis
x_norm_3ch = tf.stack((x_norm, x_norm, x_norm), axis=-1)
return x_norm_3ch