User Defined Type Conversion

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 native-to-native, that may define the conversion of multiple Python types to C++’s “Complex” type.

Qt for Python 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 (!PyNumber_Check(pyReal))
        return false;
    Shiboken::AutoDecRef pyImag(PySequence_GetItem(pyIn, 1));
    if (!PyNumber_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_#. Qt for Python combines the conversion code for containers with the conversion defined (or automatically generated) for the containers.

<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, &amp;pos, &amp;key, &amp;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>

Note

The C++ containers std::list, std::vector, std::pair, std::map, std::span and std::unordered_map are built-in. To specify Opaque Containers, use the opaque-container element. container-type can still be specified to modify the built-in behavior. For this case, a number of pre-defined conversion templates are provided (see Predefined Templates).

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