Embedding Python into C++ calling two functions and passing array around

Question

I'm trying to embedding python functions into my c++ code for doing some symbolic and algebraic operations.

My idea: I want to create a matrix with symbolic variables depending on some input variables. This matrix I want to use afterwards in another function. This function is generating some coordinates out of the symbolic matrix. And i want then to pass this results to my c++ code back.

My problem: I have a problem to get this matrix out of python to my c++ code and back to the other function as input. Because i want to pass the symbolic matrix via c++ to the other python function which should generate out of this matrix the results. Just to try my python code and the integration into c++ i wrote the code below.

Edit: To make my question more specific I wrote the whole work I want to do with this functions in Python. To clarify I need one function which is generating the symbolic matrix and the other has to calculate the values. And for the calculation the values are coming from the inside of my C++ code.

Here the example written in Python:

from sympy import *
import numpy as np


def main():
    start_pos = [1,2,3]
    end_pos = [4,5,6]

    generic_function = generate_symbolic_transformation(start_pos,end_pos)
    calculate_symbolic_transformation(generic_function,90,0.5);

# Calculation of the symbolic transformation depending on the input
def calculate_symbolic_transformation(generic_function,thetay_value,k_value):
    thetay = symbols('thetay')
    k = symbols('k')
    transf_matrix = MatrixSymbol('tm',4,4)
    transf_matrix = Matrix(transf_matrix)
    transf_matrix = sympify(generic_function)
    transf_matrix = transf_matrix.subs([(thetay,thetay_value),(k,k_value)])
    print transf_matrix
    return 1

# Generation of the symbolic transformation depending on the input
def generate_symbolic_transformation(Start_pos_coords,End_pos_coords):

    # Symbolic startposition
    Start_pos = MatrixSymbol('S',3,1)
    Start_pos = Matrix(Start_pos)
    Start_pos[0] = Start_pos_coords[0]
    Start_pos[1] = Start_pos_coords[1]
    Start_pos[2] = Start_pos_coords[2]

    print Start_pos

    # Symbolic endposition
    End_pos = MatrixSymbol('E',3,1)
    End_pos = Matrix(End_pos)
    End_pos[0] = End_pos_coords[0]
    End_pos[1] = End_pos_coords[1]
    End_pos[2] = End_pos_coords[2]

    print End_pos

    # Symbolic rotation matric
    R = MatrixSymbol('R',3,3)

    # Symbolic transformation matric
    T = MatrixSymbol('T',4,4)

    # Necessary symbolic variabls
    k = symbols('k')
    thetax = symbols('thetax')
    thetay = symbols('thetay')
    thetaz = symbols('thetaz')

    # For rotation of EulerAngles RzRyRx:
    Rx = MatrixSymbol('Rx',3,3)
    Ry = MatrixSymbol('Ry',3,3)
    Rz = MatrixSymbol('Rz',3,3)

    # Filling Rx rotation matric
    #   |   1           0                   0         |
    #   |   0       -cos(thetax)      sin(thetax)     |
    #   |   0        sin(thetax)      cos(thetax)     |

    Rx = Matrix(Rx)
    Rx[0,0] = 1
    Rx[0,1] = 0
    Rx[0,2] = 0
    Rx[1,0] = 0
    Rx[1,1] = cos(thetax)
    Rx[1,2] = -sin(thetax)
    Rx[2,0] = 0
    Rx[2,1] = sin(thetax)
    Rx[2,2] = cos(thetax)

    # Filling Ry rotation matric
    #   |    cos(thetay)        0      sin(thetay)     |
    #   |          0            1           0          |
    #   |   -sin(thetay)        0      cos(thetay)     |

    Ry = Matrix(Ry)
    Ry[0,0] = cos(thetay)
    Ry[0,1] = 0
    Ry[0,2] = sin(thetay)
    Ry[1,0] = 0
    Ry[1,1] = 1
    Ry[1,2] = 0
    Ry[2,0] = -sin(thetay)
    Ry[2,1] = 0
    Ry[2,2] = cos(thetay)

    # Filling Rz rotation matric
    #   |    cos(thetaz)   -sin(thetaz)      0     |
    #   |    sin(thetaz)    cos(thetaz)      0     |
    #   |          0            0            1     |

    Rz = Matrix(Rz)
    Rz[0,0] = cos(thetaz)
    Rz[0,1] = -sin(thetaz)
    Rz[0,2] = 0
    Rz[1,0] = sin(thetaz)
    Rz[1,1] = cos(thetaz)
    Rz[1,2] = 0
    Rz[2,0] = 0
    Rz[2,1] = 0
    Rz[2,2] = 1

    # Generating the rotation matric
    R = Rz*Ry*Rx

    # Generating the linear translation
    # Symbolic 3D line function
    Translation = MatrixSymbol('Tl',3,1)
    Translation = Start_pos + k * (End_pos-Start_pos)

    # Integrate it into the transformation matric
    #   |    R      T    |
    #   |   000     1    |
    T = Matrix(T)

    i=0
    for r in range(4):
        for c in range(4):
            if (c < 3 and r < 3):
                T[r,c] = R[r,c]
            elif (c == 3 and r < 3):
                T[r,c] = Translation[i]
                ++i
            elif (c < 3 and r == 3):
                T[r,c] = 0
            else:
                T[r,c] = 1

    ## Save the created matrics with symbolic variables into global object
    T = T.subs([(thetax,0),(thetaz,0)])
    return T

if __name__ == "__main__":
    main()

Answer 1

There are several errors here:

1. The Python function generate_symbolic_transformation throws an exception, which makes resultObj be NULL . This propagates further and leads to the crash.

2. Even if resultObj was not NULL , it would not have been returned properly to its caller, as the last two lines of CallPlugIn_generate_symbolic_transformation make sure that a value is only returned it it is NULL .

But these are specific problems. I would also make a few general suggestion that might help you catch problems early, saving time and effort:

1. Handle Python errors. At the bare minimum, if a Python C/API function returns NULL , print the error. For example:

if (!resultObj) { PyErr_Print(); }

This would have resulted in an error message similar to:

Traceback (most recent call last): File "/home/sterin/ClionProjects/numpy1/numpy_test.py", line 15, in generate_symbolic_transformation T = T.subs([(thetax,0),(thetaz,0)]) NameError: global name 'thetax' is not defined

Helping you to catch the error much earlier.

2. Make sure no NULL values are provided as PyObject* arguments to Python C/API calls (unless specifically allowed). This would have caught the place where the NULL result have been used.