np.reshape()

 np.arange(6).reshape(2, 3)
  # [0,1,2,3,4,5] → [[0,1,2],
  #                  [3,4,5]]

np.arange(12).reshape(3, -1)   # (12,) → (3, 4)
np.arange(12).reshape(-1, 3)   # (12,) → (4, 3)
np.arange(12).reshape(2, 2, -1) # (12,) → (2, 2, 3)

 arr.reshape(-1)  # flatten