Extending C with Python
Hello, I'm extending C with python (which is opposite way of what people usually do, extending python with C), I'm currently stuck in passing a C array to python layer, could anyone plz advise?
I have a C buffer in my C code and want to pass it to a python function. In the C code, I have:
npy_intp dims [2];
dims[0] = 10; dims[1] = 20; import_array(); npy_intp m=2; PyObject * py_dims = PyArray_SimpleNewFromData(1, &m, NPY_INT16 ,(void *)dims ); // I also tried NPY_INT PyObject_CallMethod(pInstance, method_name, "O", py_dims);
In the Python code, I want to just print that array:
def f(self, dims):
print ("np array:%d,%d"%(dims[0],dims[1]))
But it only prints the first number correctly, i.e., dims[0]. The second number is always 0.
Best, Jialin LBNL/NERSC
On Wed, Jan 31, 2018 at 3:25 PM, Jialin Liu jalnliu@lbl.gov wrote:
Hello, I'm extending C with python (which is opposite way of what people usually do, extending python with C), I'm currently stuck in passing a C array to python layer, could anyone plz advise?
I have a C buffer in my C code and want to pass it to a python function. In the C code, I have:
npy_intp dims [2];
dims[0] = 10; dims[1] = 20; import_array(); npy_intp m=2; PyObject * py_dims = PyArray_SimpleNewFromData(1, &m, NPY_INT16 ,(void *)dims ); // I also tried NPY_INT PyObject_CallMethod(pInstance, method_name, "O", py_dims);
In the Python code, I want to just print that array:
def f(self, dims):
print ("np array:%d,%d"%(dims[0],dims[1]))
But it only prints the first number correctly, i.e., dims[0]. The second number is always 0.
The correct typecode would be NPY_INTP.
Amazing! It works! Thank you Robert.
I've been stuck with this many days.
Best, Jialin LBNL/NERSC
On Tue, Jan 30, 2018 at 10:52 PM, Robert Kern robert.kern@gmail.com wrote:
On Wed, Jan 31, 2018 at 3:25 PM, Jialin Liu jalnliu@lbl.gov wrote:
Hello, I'm extending C with python (which is opposite way of what people usually do, extending python with C), I'm currently stuck in passing a C array to python layer, could anyone plz advise?
I have a C buffer in my C code and want to pass it to a python function. In the C code, I have:
npy_intp dims [2];
dims[0] = 10; dims[1] = 20; import_array(); npy_intp m=2; PyObject * py_dims = PyArray_SimpleNewFromData(1, &m, NPY_INT16 ,(void *)dims ); // I also tried NPY_INT PyObject_CallMethod(pInstance, method_name, "O", py_dims);
In the Python code, I want to just print that array:
def f(self, dims):
print ("np array:%d,%d"%(dims[0],dims[1]))
But it only prints the first number correctly, i.e., dims[0]. The second number is always 0.
The correct typecode would be NPY_INTP.
 Robert Kern
NumPyDiscussion mailing list NumPyDiscussion@python.org https://mail.python.org/mailman/listinfo/numpydiscussion
I'm guessing you could use Cython to make this easier. It's usually used for calling C from Python, but can do the sandwich in both directions...
Just a thought  it will help with some of that boilerplate code...
CHB
On Tue, Jan 30, 2018 at 10:57 PM, Jialin Liu jalnliu@lbl.gov wrote:
Amazing! It works! Thank you Robert.
I've been stuck with this many days.
Best, Jialin LBNL/NERSC
On Tue, Jan 30, 2018 at 10:52 PM, Robert Kern robert.kern@gmail.com wrote:
On Wed, Jan 31, 2018 at 3:25 PM, Jialin Liu jalnliu@lbl.gov wrote:
Hello, I'm extending C with python (which is opposite way of what people usually do, extending python with C), I'm currently stuck in passing a C array to python layer, could anyone plz advise?
I have a C buffer in my C code and want to pass it to a python function. In the C code, I have:
npy_intp dims [2];
dims[0] = 10; dims[1] = 20; import_array(); npy_intp m=2; PyObject * py_dims = PyArray_SimpleNewFromData(1, &m, NPY_INT16 ,(void *)dims ); // I also tried NPY_INT PyObject_CallMethod(pInstance, method_name, "O", py_dims);
In the Python code, I want to just print that array:
def f(self, dims):
print ("np array:%d,%d"%(dims[0],dims[1]))
But it only prints the first number correctly, i.e., dims[0]. The second number is always 0.
The correct typecode would be NPY_INTP.
 Robert Kern
NumPyDiscussion mailing list NumPyDiscussion@python.org https://mail.python.org/mailman/listinfo/numpydiscussion
NumPyDiscussion mailing list NumPyDiscussion@python.org https://mail.python.org/mailman/listinfo/numpydiscussion
On 31.01.2018 17:58, Chris Barker wrote:
I'm guessing you could use Cython to make this easier.
... or Boost.Python (http://boostorg.github.io/python), which has builtin support for NumPy (http://boostorg.github.io/python/doc/html/numpy/index.html), and supports both directions: extending Python with C++, as well as embedding Python into C++ applications.
Stefan
You can also check out pybind11, xtensor, and xtensorpython
The latter enables a highlevel numpylike API on the C++ side.
You can check out the numpy to xtensor cheat sheet:
http://xtensor.readthedocs.io/en/latest/numpy.html
Best,
Sylvain
On Thu, Feb 1, 2018 at 12:11 AM, Stefan Seefeld stefan@seefeld.name wrote:
On 31.01.2018 17:58, Chris Barker wrote:
I'm guessing you could use Cython to make this easier.
... or Boost.Python (http://boostorg.github.io/python), which has builtin support for NumPy (http://boostorg.github.io/ python/doc/html/numpy/index.html), and supports both directions: extending Python with C++, as well as embedding Python into C++ applications.
[image: Stefan]

...ich hab' noch einen Koffer in Berlin...
NumPyDiscussion mailing list NumPyDiscussion@python.org https://mail.python.org/mailman/listinfo/numpydiscussion
Thank you guys.
Best, Jialin
Sent from my iPhone
On Feb 3, 2018, at 8:11 AM, Sylvain Corlay sylvain.corlay@gmail.com wrote:
You can also check out pybind11, xtensor, and xtensorpython
The latter enables a highlevel numpylike API on the C++ side.
You can check out the numpy to xtensor cheat sheet:
http://xtensor.readthedocs.io/en/latest/numpy.html
Best,
Sylvain
On Thu, Feb 1, 2018 at 12:11 AM, Stefan Seefeld stefan@seefeld.name wrote:
On 31.01.2018 17:58, Chris Barker wrote: I'm guessing you could use Cython to make this easier.
... or Boost.Python (http://boostorg.github.io/python), which has builtin support for NumPy (http://boostorg.github.io/python/doc/html/numpy/index.html), and supports both directions: extending Python with C++, as well as embedding Python into C++ applications.
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...ich hab' noch einen Koffer in Berlin...
NumPyDiscussion mailing list NumPyDiscussion@python.org https://mail.python.org/mailman/listinfo/numpydiscussion
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With PyObject_CallMethod(pInstance, method_name, "O", py_dims);
Can I pass in a reference and modify its content in python?
Best, Jialin
On Sat, Feb 3, 2018 at 10:29 AM, Jialin Liu jalnliu@lbl.gov wrote:
Thank you guys.
Best, Jialin
Sent from my iPhone
On Feb 3, 2018, at 8:11 AM, Sylvain Corlay sylvain.corlay@gmail.com wrote:
You can also check out pybind11, xtensor, and xtensorpython
The latter enables a highlevel numpylike API on the C++ side.
You can check out the numpy to xtensor cheat sheet:
http://xtensor.readthedocs.io/en/latest/numpy.html
Best,
Sylvain
On Thu, Feb 1, 2018 at 12:11 AM, Stefan Seefeld stefan@seefeld.name wrote:
On 31.01.2018 17:58, Chris Barker wrote:
I'm guessing you could use Cython to make this easier.
... or Boost.Python (http://boostorg.github.io/python), which has builtin support for NumPy (http://boostorg.github.io/pyt hon/doc/html/numpy/index.html), and supports both directions: extending Python with C++, as well as embedding Python into C++ applications.
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participants (5)

Chris Barker

Jialin Liu

Robert Kern

Stefan Seefeld

Sylvain Corlay