为什么我用python运行一个可视化MINST数据的文件，系统总是提示‘X_train is not defined’?

import matplotlib.pyplot as plt

fig, ax = plt.subplots(
nrows=2,
ncols=5,
sharex=True,
sharey=True, )

ax = ax.flatten()
for i in range(10):
img = X_train[y_train == i][0].reshape(28, 28)
ax[i].imshow(img, cmap='Greys', interpolation='nearest')

ax[0].set_xticks([])
ax[0].set_yticks([])
plt.tight_layout()
plt.show()