sample.py

from sklearn.model_selection import train_test_split
X_train, X_cv, y_train, y_cv = train_test_split(df_features, df_label, 
                                                test_size = 0.2,
                                                random_state = 1212)

X_train = np.array(X_train).reshape(33600, 784) #(33600, 784)
X_cv = np.array(X_cv).reshape(8400, 784) #(8400, 784)