Selecting test and train sets in Keras for Python
Your method is not impossible, but it is impractical. A Neural Network learns from the training dataset. If you give the same data again in the test set, it will generate accurate predictions, which will have no relation with the network's predictive power.
I am trying to use Keras in Python for time series classification. 
I have made a simple CNN and was wondering if it is possible to randomly select a subsection of the full data set at each epoch to serve as the test set?
I am quite new to neural nets so if there are any reasons this is impossible or impractical I would appreciate clarity.