python代码：生成一个8×8分块对角矩阵，其中包括3个分块矩阵，前两个分块矩阵等于12乘以2阶单位矩阵，第三个分块等于3乘以4阶单位矩阵

以下是生成该分块对角矩阵的 Python 代码：

import numpy as np

def block_diagonal_matrix(block_sizes, block_values):
    """
    Create a block diagonal matrix with different values for each block.

    Parameters:
    block_sizes: list of integers, sizes of each block
    block_values: list of 2D arrays, values for each block

    Returns:
    The block diagonal matrix as a 2D array.
    """
    n = sum(block_sizes)
    result = np.zeros((n, n))
    start = 0
    for size, value in zip(block_sizes, block_values):
        result[start:start+size, start:start+size] = value * np.eye(size)
        start += size
    return result

block_sizes = [2, 2, 4]
block_values = [12*np.eye(2), 12*np.eye(2), 3*np.eye(4)]

result = block_diagonal_matrix(block_sizes, block_values)
print(result)

该代码会输出如下的分块对角矩阵：

array([[12.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0., 12.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0., 12.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0., 12.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  3.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  3.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  3.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  3.]])

其中，前两个分块矩阵等于12乘以2阶单位矩阵，第三个分块等于3乘以4阶单位矩阵。