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.

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

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

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

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

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

 <conversion-rule>

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

 <target-to-native>
 <!— The 'check' attribute can be derived from the 'type' attribute,
 it is defined here to test the CHECKTYPE type system variable. —>
 <add-conversion type="PyComplex" check="%CHECKTYPE[Complex](%in)">
 double real = PyComplex_RealAsDouble(%in);
 double imag = PyComplex_ImagAsDouble(%in);
 %out = %OUTTYPE (real, imag);
 </add-conversion>
 </target-to-native>

 </conversion-rule>

</primitive-type>

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.

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:

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

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

<conversion-rule>

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

<target-to-native>

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

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

</target-to-native>

</conversion-rule>

</primitive-type>

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.

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

<conversion-rule>

<native-to-target>
 PyObject* %out = PyDict_New();
 %INTYPE::const_iterator it = %in.begin();
 for (; it != %in.end(); +''it) {
 %INTYPE_0 key = it->first;
 %INTYPE_1 value = it->second;
 PyDict_SetItem(%out,
 %CONVERTTOPYTHON[%INTYPE_0](key),
 %CONVERTTOPYTHON[%INTYPE_1](value));
 }
 return %out;
 </native-to-target>

 <target-to-native>

 <add-conversion type="PyDict">
 PyObject* key;
 PyObject* value;
 Py_ssize_t pos = 0;
 while (PyDict_Next(%in, &pos, &key, &value)) {
 %OUTTYPE_0 cppKey = %CONVERTTOCPP[%OUTTYPE_0](key);
 %OUTTYPE_1 cppValue = %CONVERTTOCPP[%OUTTYPE_1](value);
 %out.insert(%OUTTYPE::value_type(cppKey, cppValue));
 }
 </add-conversion>

 </target-to-native>
 </conversion-rule>
</container-type>

Variables & Functions

%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:

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

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

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