how to explain this inhomogeneous numpy.ndarray?

Why do I see a inhomogeneous numpy ndarray?

This is the output of a test script.

Quote:

type(train_data) <class 'numpy.ndarray'> train_data.shape (25000,) train_data[0] [1, 14, 22, 16, 43, 2, 2, 2, 2, 65, 2, 2, 66, 2, 4, 2, 36, 2, 5, 25, 2, 43, 2, 2, 50, 2, 2, 9, 35, 2, 2, 5, 2, 4, 2, 2, 2, 2, 2, 2, 39, 4, 2, 2, 2, 17, 2, 38, 13, 2, 4, 2, 50, 16, 6, 2, 2, 19, 14, 22, 4, 2, 2, 2, 4, 22, 71, 87, 12, 16, 43, 2, 38, 76, 15, 13, 2, 4, 22, 17, 2, 17, 12, 16, 2, 18, 2, 5, 62, 2, 12, 8, 2, 8, 2, 5, 4, 2, 2, 16, 2, 66, 2, 33, 4, 2, 12, 16, 38, 2, 5, 25, 2, 51, 36, 2, 48, 25, 2, 33, 6, 22, 12, 2, 28, 77, 52, 5, 14, 2, 16, 82, 2, 8, 4, 2, 2, 2, 15, 2, 4, 2, 7, 2, 5, 2, 36, 71, 43, 2, 2, 26, 2, 2, 46, 7, 4, 2, 2, 13, 2, 88, 4, 2, 15, 2, 98, 32, 2, 56, 26, 2, 6, 2, 2, 18, 4, 2, 22, 21, 2, 2, 26, 2, 5, 2, 30, 2, 18, 51, 36, 28, 2, 92, 25, 2, 4, 2, 65, 16, 38, 2, 88, 12, 16, 2, 5, 16, 2, 2, 2, 32, 15, 16, 2, 19, 2, 32] len(train_data[0]) 218 train_data[1] [1, 2, 2, 2, 2, 78, 2, 5, 6, 2, 2, 2, 2, 26, 4, 2, 8, 2, 2, 14, 2, 20, 13, 2, 2, 2, 2, 5, 2, 2, 2, 21, 14, 69, 2, 8, 30, 23, 7, 4, 2, 2, 93, 4, 2, 9, 2, 2, 5, 2, 4, 2, 9, 35, 2, 4, 2, 9, 2, 2, 4, 2, 9, 4, 2, 2, 19, 4, 2, 5, 89, 29, 2, 46, 37, 4, 2, 9, 45, 43, 38, 2, 2, 2, 4, 2, 26, 2, 5, 2, 11, 2, 2, 4, 2, 9, 2, 2, 7, 2, 2, 2, 2, 2, 2, 2, 2, 15, 2, 2, 68, 2, 2, 15, 2, 2, 2, 98, 5, 4, 2, 9, 43, 2, 2, 15, 2, 2, 5, 2, 2, 11, 2, 2, 2, 50, 9, 2, 2, 2, 5, 2, 2, 2, 2, 5, 4, 2, 2, 2, 2, 18, 2, 32, 2, 2, 14, 9, 6, 2, 78, 22, 2, 64, 2, 9, 8, 2, 2, 23, 4, 2, 15, 16, 4, 2, 5, 28, 6, 52, 2, 2, 33, 89, 78, 2, 16, 2, 95] len(train_data[1]) 189