exception "access violation" with minimal example

[TomFD]

I'm getting an exception with access violation with the minimal example, change slightly to that below, see screenshot for place of exception (plot->first detail::get_array(X) -> PyList_New(v.size());

Any idea what is wrong?

Thank you in advance,
Thomas

#include
// according to: boostorg/system#32
#define HAVE_SNPRINTF
#include <Python.h>

#define WITHOUT_NUMPY
#include "C:\Users\tsch\OneDrive\Dokumente\fiberdesk wo code\fiberdesk code dependencies\matplotlib-cpp-master\matplotlibcpp.h"

int main()
{
std::cout << "Hello world";
namespace plt = matplotlibcpp;

plt::plot({ 1,3,2,4 });

return 0;

}

[TomFD]

One step further:

included

Py_SetPythonHome(L"C:\\Users\\tsch\\.conda\\envs\\TomsEnv");
Py_Initialize();

before plt::plot(...)

and then run into the error that matplotlib cannot be loaded:
throw std::runtime_error("Error loading module matplotlib!");

However, with the Anaconda promt, it works:

(base) C:\Users\tsch>conda activate TomsEnv
(TomsEnv) C:\Users\tsch>python
Python 3.7.7 (default, Mar 23 2020, 23:19:08) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.

import matplotlib

[lava]

Hm, I've seen similar issues recently - I think there are a few places in the code that are missing calls to Py_Initialize() that were never detected because just happened to work in older python versions.

Could you test whether the following patch fixes the issue?

@@ -990,6 +990,8 @@ bool plot(const std::vector<NumericX>& x, const std::vector<NumericY>& y, const
 {
     assert(x.size() == y.size());
 
+    detail::_interpreter::get();
+
     PyObject* xarray = detail::get_array(x);
     PyObject* yarray = detail::get_array(y);

[mdaley]

Ooh, that's interesting. I've just been having a problem with named_plot not working. I'm using Python 3.7.

My code is calling named plot is like this:

    for (pair<string, vector<double>> costByAlpha : costsByAlpha) {
        plt::named_plot(costByAlpha.first, iterations_v, costByAlpha.second);
    }

and I was finding the third line of named_plot to result in an access violation:

    PyDict_SetItemString(kwargs, "label", PyString_FromString(name.c_str())); 

If I change it to this, my code works and a nice graph gets displayed:

    PyDict_SetItem(kwargs, PyString_FromString("label"), PyString_FromString(name.c_str()));

But, also, just adding the statement you suggest makes it work too:

    detail::_interpreter::get();

[lava]

Ok, I just pushed a commit adding initialization of the interpreter to all user-facing functions, so hopefully this should be resolved now.

[TomFD]

Sorry for my late reply. It is not working for me. Good to see that it helped others. I'm now getting always an error, because the basic module 'matplotlib' is not loaded, despite setting the right path, initializing python etc. I'm still investigating it ...

-- I just want to code and not waste my time in getting it running. --