The IndexError: from training MNIST data...

Code:
y_vec = np.zeros((len(y), self.y_dim), dtype=np.float)
for i, label in enumerate(y):
y_vec[i,y[i]] = 1.0

return X/255.,y_vec

IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices

Start with the shown MNIST data-preparation code and reproduce the training run that raises the IndexError. Inspect the types and values of y and the indexing expression, then verify the training path completes and the one-hot labels have the expected shape.

Written by the indexing model from the issue text.