PySide Shiboken Type Converters

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PySide Shiboken Type Converters

In the process of creating Python bindings of a C++ library, most of the C++ classes will have wrappers representing them in Python land. But there may be other classes that are very simple and/or have a Python type as a direct counter part. (Example: a “Complex” class, that represents complex numbers, has a Python equivalent in the “complex” type.) Such classes, instead of getting a Python wrapper, normally have conversions rules, from Python to C++ and vice-versa.

<br />// C++ class<br />struct Complex {<br /> Complex(double real, double imag);<br /> double real() const;<br /> double imag() const;<br />};

// Converting from C++ to Python using the CPython API:<br />PyObject* pyCpxObj = PyComplex_FromDoubles(complex.real(), complex.imag());

// Converting from Python to C+'':<br />double real = PyComplex_RealAsDouble(pyCpxObj);<br />double imag = PyComplex_ImagAsDouble(pyCpxObj);<br />Complex cpx(real, imag);<br />

For the user defined conversion code to be inserted in the proper places, the “<conversion-rule>” tag must be used.

<br /><primitive-type name="Complex" target-lang-api-name="PyComplex"><br /> <include file-name="complex.h" location="global"/>
<br /> <conversion-rule>
<br /> <native-to-target><br /> return PyComplex_FromDoubles(%in.real(), %in.imag());<br /> </native-to-target>
<br /> <target-to-native><br /> <!— The 'check' attribute can be derived from the 'type' attribute,<br /> it is defined here to test the CHECKTYPE type system variable. —><br /> <add-conversion type="PyComplex" check="%CHECKTYPE[Complex](%in)"><br /> double real = PyComplex_RealAsDouble(%in);<br /> double imag = PyComplex_ImagAsDouble(%in);<br /> %out = %OUTTYPE (real, imag);<br /> </add-conversion><br /> </target-to-native>
<br /> </conversion-rule>
<br /></primitive-type><br />

The details will be given later, but the gist of it are the tags <native-to-target>, which has only one conversion from C+ to Python, and <target-to-native>, that may define the conversion of multiple Python types to C+’s “Complex” type.
https://raw.github.com/LnDn/PySide-Media/master/Media/BindingGenerator/converter.png
Shiboken expects the code for <native-to-target>, to directly return the Python result of the conversion, and the added conversions inside the <target-to-native> must attribute the Python to C+ conversion result to the %out variable.

Expanding on the last example, if the binding developer want a Python 2-tuple of numbers to be accepted by wrapped C++ functions with “Complex” arguments, an <add-conversion> tag and a custom check must be added. Here’s how to do it:

<br /><!— Code injection at module level. —><br /><inject-code class="native" position="beginning"><br />static bool Check2TupleOfNumbers(PyObject* pyIn) {<br /> if (!PySequence_Check(pyIn) || !(PySequence_Size(pyIn) == 2))<br /> return false;<br /> Shiboken::AutoDecRef pyReal(PySequence_GetItem(pyIn, 0));<br /> if (!SbkNumber_Check(pyReal))<br /> return false;<br /> Shiboken::AutoDecRef pyImag(PySequence_GetItem(pyIn, 1));<br /> if (!SbkNumber_Check(pyImag))<br /> return false;<br /> return true;<br />}<br /></inject-code>

<primitive-type name="Complex" target-lang-api-name="PyComplex"><br /> <include file-name="complex.h" location="global"/>

<conversion-rule>

<native-to-target><br /> return PyComplex_FromDoubles(%in.real(), %in.imag());<br /> </native-to-target>

<target-to-native>

<add-conversion type="PyComplex"><br /> double real = PyComplex_RealAsDouble(%in);<br /> double imag = PyComplex_ImagAsDouble(%in);<br /> %out = %OUTTYPE (real, imag);<br /> </add-conversion>

<add-conversion type="PySequence" check="Check2TupleOfNumbers(%in)"><br /> Shiboken::AutoDecRef pyReal(PySequence_GetItem(%in, 0));<br /> Shiboken::AutoDecRef pyImag(PySequence_GetItem(%in, 1));<br /> double real = %CONVERTTOCPP[double](pyReal);<br /> double imag = %CONVERTTOCPP[double](pyImag);<br /> %out = %OUTTYPE (real, imag);<br /> </add-conversion>

</target-to-native>

</conversion-rule>

</primitive-type><br />

Container Conversions

Converters for <container-type> are pretty much the same as for other type, except that they make use of the type system variables %INTYPE_# and %OUTTYPE_#. Shiboken combines the conversion code for containers with the conversion defined (or automatically generated) for the containees.

<br /><container-type name="std::map" type="map"><br /> <include file-name="map" location="global"/>

<conversion-rule>

<native-to-target><br /> PyObject* %out = PyDict_New();<br /> %INTYPE::const_iterator it = %in.begin();<br /> for (; it != %in.end(); +''it) {<br /> %INTYPE_0 key = it->first;<br /> %INTYPE_1 value = it->second;<br /> PyDict_SetItem(%out,<br /> %CONVERTTOPYTHON[%INTYPE_0](key),<br /> %CONVERTTOPYTHON[%INTYPE_1](value));<br /> }<br /> return %out;<br /> </native-to-target>
<br /> <target-to-native>
<br /> <add-conversion type="PyDict"><br /> PyObject* key;<br /> PyObject* value;<br /> Py_ssize_t pos = 0;<br /> while (PyDict_Next(%in, &amp;amp;pos, &amp;amp;key, &amp;amp;value)) {<br /> %OUTTYPE_0 cppKey = %CONVERTTOCPP[%OUTTYPE_0](key);<br /> %OUTTYPE_1 cppValue = %CONVERTTOCPP[%OUTTYPE_1](value);<br /> %out.insert(%OUTTYPE::value_type(cppKey, cppValue));<br /> }<br /> </add-conversion>
<br /> </target-to-native><br /> </conversion-rule><br /></container-type><br />

h2. Variables & Functions
h3. %in
Variable replaced by the C+ input variable.

%out

Variable replaced by the C++ output variable. Needed to convey the result of a Python to C++ conversion.

%INTYPE

Used in Python to C++ conversions. It is replaced by the name of type for which the conversion is being defined. Don’t use the type’s name directly.

%INTYPE_#

Replaced by the name of the #th type used in a container.

%OUTTYPE

Used in Python to C++ conversions. It is replaced by the name of type for which the conversion is being defined. Don’t use the type’s name directly.

%OUTTYPE_#

Replaced by the name of the #th type used in a container.

%CHECKTYPE[CPPTYPE]

Replaced by a Shiboken type checking function for a Python variable. The C++ type is indicated by CPPTYPE.

Converting The Old Converters

If you use Shiboken for your bindings, and has defined some type conversions using the Shiboken::Converter template, then you must update your converters to the new scheme.

Previously your conversion rules were declared in one line, like this:

<br /><primitive-type name="Complex" target-lang-api-name="PyComplex"><br /> <include file-name="complex.h" location="global"/><br /> <conversion-rule file="complex_conversions.h"/><br /></primitive-type><br />

And implemented in a separate C++ file, like this:

<br />namespace Shiboken {<br />template<> struct Converter<Complex><br />{<br /> static inline bool checkType(PyObject* pyObj) {<br /> return PyComplex_Check(pyObj);<br /> }<br /> static inline bool isConvertible(PyObject* pyObj) {<br /> return PyComplex_Check(pyObj);<br /> }<br /> static inline PyObject* toPython(void* cppobj) {<br /> return toPython('''reinterpret_cast<Complex'''>(cppobj));<br /> }<br /> static inline PyObject* toPython(const Complex&amp;amp; cpx) {<br /> return PyComplex_FromDoubles(cpx.real(), cpx.imag());<br /> }<br /> static inline Complex toCpp(PyObject* pyobj) {<br /> double real = PyComplex_RealAsDouble(pyobj);<br /> double imag = PyComplex_ImagAsDouble(pyobj);<br /> return Complex(real, imag);<br /> }<br />};<br />}<br />