Simple matrix multiplication in Python

matmul.py

def matmul(mat1, mat2, mat3):
    r""" Function to multiply mat1 and mat2 
    returns mat3 
    Parameters
    ---------
    mat1: np.array, matrix A 
    mat2: np.array, matrix B 
    mat3: np.array, empty matrix C 
    
    Return 
    ------
    mat3: np.array, matmul between A & B
    """ 
    for i in range(mat1.shape):
        for j in range(mat2.shape):
            mat3[i][j] = 0.0 
            for k in range(mat3.shape):
                mat3[i][j] += mat1[i][k]*mat2[k][j]
    return mat3