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tdegeustdegeus
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Adding example
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‎docs/source/examples.rst

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@@ -98,3 +98,53 @@ Then we can test the module:
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we should compile and run the example from the same folder.
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To install, please consult
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`this pybind11 / CMake example <https://github.com/pybind/cmake_example>`_.
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Fall-back cast
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==============
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The previous example showed you how to design your module to be flexible in accepting data.
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From C++ we used ``xt::xarray<double>``,
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whereas for the Python API we used ``xt::pyarray<double>`` to operate directly on the memory
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of a NumPy array from Python (without copying the data).
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Sometimes, you might not have the flexibility to design your module's methods
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with template parameters.
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This might occur when you want to ``override`` functions
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(though it is recommended to use CRTP to still use templates).
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In this case we can still bind the module in Python using *xtensor-python*,
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however, we have to copy the data from a (NumPy) array.
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This means that although the following signatures are quite different when used from C++,
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as follows:
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1. *Constant reference*: read from the data, without copying it.
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.. code-block:: cpp
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void foo(const xt::xarray<double>& a);
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2. *Reference*: read from and/or write to the data, without copying it.
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.. code-block:: cpp
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void foo(xt::xarray<double>& a);
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3. *Copy*: copy the data.
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.. code-block:: cpp
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void foo(xt::xarray<double> a);
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The Python will all cases result in a copy to a temporary variable
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(though the last signature will lead to a copy to a temporary variable, and another copy to ``a``).
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On the one hand, this is more costly than when using ``xt::pyarray`` and ``xt::pyxtensor``,
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on the other hand, it means that all changes you make to a reference, are made to the temporary
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copy, and are thus lost.
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Still, it might be a convenient way to create Python bindings, using a minimal effort.
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Consider this example:
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:download:`main.cpp <examples/copy_cast/main.cpp>`
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.. literalinclude:: examples/copy_cast/main.cpp
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:language: cpp

‎docs/source/examples/copy_cast/main.cpp

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@@ -4,12 +4,14 @@
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#define FORCE_IMPORT_ARRAY
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#include <xtensor-python/pyarray.hpp>
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double sum_of_sines(xt::pyarray<double>& m)
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template <class T>
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double sum_of_sines(T& m)
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{
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auto sines = xt::sin(m); // sines does not actually hold values.
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return std::accumulate(sines.begin(), sines.end(), 0.0);
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}
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// In the Python API this a reference to a temporary variable
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double sum_of_cosines(const xt::xarray<double>& m)
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{
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auto cosines = xt::cos(m); // cosines does not actually hold values.
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{
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xt::import_numpy();
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m.doc() = "Test module for xtensor python bindings";
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m.def("sum_of_sines", sum_of_sines, "Sum the sines of the input values");
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m.def("sum_of_sines", sum_of_sines<xt::pyarray<double>>, "Sum the sines of the input values");
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m.def("sum_of_cosines", sum_of_cosines, "Sum the cosines of the input values");
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}

‎include/xtensor-python/xtensor_type_caster_base.hpp

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@@ -26,8 +26,7 @@ namespace pybind11
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template <typename T, xt::layout_type L>
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struct xtensor_get_buffer
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{
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template <typename H>
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static auto get(H src)
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static auto get(handle src)
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{
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return array_t<T, array::c_style | array::forcecast>::ensure(src);
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}
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template <typename T>
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struct xtensor_get_buffer<T, xt::layout_type::column_major>
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{
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template <typename H>
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static auto get(H src)
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static auto get(handle src)
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{
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return array_t<T, array::f_style>::ensure(src);
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}
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template <class T, xt::layout_type L>
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struct xtensor_check_buffer<xt::xarray<T, L>>
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{
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template <typename H>
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static auto get(H src)
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static auto get(handle src)
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{
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auto buf = xtensor_get_buffer<T, L>::get(src);
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return buf;
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template <class T, std::size_t N, xt::layout_type L>
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struct xtensor_check_buffer<xt::xtensor<T, N, L>>
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{
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template <typename H>
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static auto get(H src)
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static auto get(handle src)
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{
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auto buf = xtensor_get_buffer<T, L>::get(src);
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if (buf.ndim() != N) {
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template <class CT, class S, xt::layout_type L, class FST>
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struct xtensor_check_buffer<xt::xstrided_view<CT, S, L, FST>>
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{
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template <typename H>
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static auto get(H /*src*/)
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static auto get(handle /*src*/)
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{
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return false;
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}
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template <class EC, xt::layout_type L, class SC, class Tag>
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struct xtensor_check_buffer<xt::xarray_adaptor<EC, L, SC, Tag>>
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{
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template <typename H>
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static auto get(H /*src*/)
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static auto get(handle src)
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{
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return false;
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auto buf = xtensor_get_buffer<EC, L>::get(src);
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return buf;
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}
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};
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template <class EC, std::size_t N, xt::layout_type L, class Tag>
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struct xtensor_check_buffer<xt::xtensor_adaptor<EC, N, L, Tag>>
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{
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template <typename H>
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static auto get(H /*src*/)
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static auto get(handle /*src*/)
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{
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return false;
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}

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