Lambdify works with Python, but throws an exception with Cython

Question

My website runs this Python script that would be way more optimized if Cython is used. Recently I needed to add Sympy with Lambdify, and this is not going well with Cython.

So I stripped the problem to a minimum working example. In the code, I have a dictionary with string keys with values that are lists. I would like to use these keys as variables. In the following simplified example, there's only 1 variable, but generally I need more. Please check the following example:

import numpy as np
from sympy.parsing.sympy_parser import parse_expr
from sympy.utilities.lambdify import lambdify, implemented_function
from sympy import S, Symbol
from sympy.utilities.autowrap import ufuncify


def CreateMagneticFieldsList(dataToSave,equationString,DSList):

    expression  = S(equationString)
    numOfElements = len(dataToSave["MagneticFields"])

    #initialize the magnetic field output array
    magFieldsArray    = np.empty(numOfElements)
    magFieldsArray[:] = np.NaN

    lam_f = lambdify(tuple(DSList),expression,modules='numpy')
    try:
        # pass
        for i in range(numOfElements):
            replacementList = np.zeros(len(DSList))


            for j in range(len(DSList)):
                replacementList[j] = dataToSave[DSList[j]][i]

            try:
                val = np.double(lam_f(*replacementList))

            except:
                val = np.nan
            magFieldsArray[i] = val
    except:
        print("Error while evaluating the magnetic field expression")
    return magFieldsArray


list={"MagneticFields":[1,2,3,4,5]}

out=CreateMagneticFieldsList(list,"MagneticFields*5.1",["MagneticFields"])

print(out)

Let's call this test.py. This works very well. Now I would like to cythonize this, so I use the following script:

#!/bin/bash

cython --embed -o test.c test.py
gcc -pthread -fPIC -fwrapv -Ofast -Wall -L/lib/x86_64-linux-gnu/ -lpython3.4m -I/usr/include/python3.4 -o test.exe test.c

Now if I execute ./test.exe, it throws an exception! Here's the exception:

Traceback (most recent call last):
  File "test.py", line 42, in init test (test.c:1811)
    out=CreateMagneticFieldsList(list,"MagneticFields*5.1",["MagneticFields"])
  File "test.py", line 19, in test.CreateMagneticFieldsList (test.c:1036)
    lam_f = lambdify(tuple(DSList),expression,modules='numpy')
  File "/usr/local/lib/python3.4/dist-packages/sympy/utilities/lambdify.py", line 363, in lambdify
    callers_local_vars = inspect.currentframe().f_back.f_locals.items()
AttributeError: 'NoneType' object has no attribute 'f_locals'

So the question is: How can I get lambdify to work with Cython?

Notes: I would like to point out that I have Debian Jessie, and that's why I'm using Python 3.4. Also I would like to point out that I don't have any problem with Cython when not using lambdify. Also I would like to point out that Cython is updated to the last version with pip3 install cython --upgrade.

Lambdify works with Python, but throws an exception with Cython

2 Answers