How does import_array in numpy C API work?

Question

I am trying to convert a c-style array in c++ to a numpy array and ran into problems when trying to use the "PyArray_SimpleNewFromData" function. It turns out I need to call

import_array()

Though I do not understand how to call this function. Whenever I try calling it I get compiler error which I do not manage to understand. For instance writing the following simple script:

#include <Python.h>
#include <numpy/arrayobject.h>

int main(){
    Py_Initialize();
    import_array();
    Py_Finalize();
    return 0;
}

produces the compiler error

error: return-statement with no value, in function returning 'int' [-fpermissive] import_array();

I looked at several examples, such as :

Numpy C-Api example gives a SegFault

PyArray_SimpleNewFromData example

https://codereview.stackexchange.com/questions/92266/sending-a-c-array-to-python-numpy-and-back

Numpy/CAPI error with import_array() when compiling multiple modules

But whatever I try (even when seemingly following those examples) I seem to run into the compiler error above. What am I missing or doing wrong? An explanation on how the import_array() function should be called would be very welcome. Thanks for the help!

Update:

I am using python 2.7.11, and I think it might be related to what is discussed here :

https://github.com/clemenscorny/brisk/issues/3

but I still have no idea how to fix it.

score 2 · Answer 1 · answered Apr 19 '19 at 10:49

Reading the source code of import_array() from numpy, it is a macro that mainly calls the _import_array() regular function, and then does some weird exception handling. If you call _import_array() instead of import_array(), the compilation error disappears. And, at least in my case, the behavior is then correct (i.e. no segfault when calling Numpy C API functions)

user5994461 · Answer 2 · 2023-04-05T13:55:51.340

It's a bug in numpy: import_array() is a macro that can return 0 or NULL or nothing (void).

It's impossible to use in strict mode where the compiler verifies the return type of functions. A function returning an integer can't return NULL. A function returning a pointer can't return 0. A function returning something can't return nothing.

There's an alternative macro to use instead import_array1(0)

int
numpy_init()
{
    import_array1(0);
    return 0;
}

See numpy source code __multiarray_api.h

#if PY_VERSION_HEX >= 0x03000000
#define NUMPY_IMPORT_ARRAY_RETVAL NULL
#else
#define NUMPY_IMPORT_ARRAY_RETVAL
#endif

#define import_array() {if (_import_array() < 0) {PyErr_Print(); PyErr_SetString(PyExc_ImportError, "numpy.core.multiarray failed to import"); return NUMPY_IMPORT_ARRAY_RETVAL; } }

#define import_array1(ret) {if (_import_array() < 0) {PyErr_Print(); PyErr_SetString(PyExc_ImportError, "numpy.core.multiarray failed to import"); return ret; } }

#define import_array2(msg, ret) {if (_import_array() < 0) {PyErr_Print(); PyErr_SetString(PyExc_ImportError, msg); return ret; } }

#endif

How does import_array in numpy C API work?

2 Answers2