Requesting Python API for variant types

[roastduck]

I see there is a support for std::variant-like types in pybind11, and variant types can be passed as arguments and return values of C++ functions. However, there seems no way to check a variant type in Python side.

It would be better if I can use a variant type like this:

x = foo() # A variant type returned from a C++ function
if type(x) == AnActualType: # Check the actual type
  bar(AnActualType(x)) # Cast it to the actual type and do something

[jiwaszki]

I don't see a problem in general, can you be more specific with description or give some reproducer of your case?

Try this sample:

#include <variant>

#include <pybind11/pybind11.h>
#include <pybind11/stl.h>

namespace py = pybind11;

class TestClass
{
public:
    TestClass(int value) : m_value(value) {}

    int get_value()
    {
        return m_value;
    }

private:
    int m_value;
};

PYBIND11_MODULE(mymodule, m)
{
    py::class_<TestClass, std::shared_ptr<TestClass>> cls(m, "TestClass");

    cls.def(py::init<int>());
    cls.def("get_value", &TestClass::get_value);

    m.def("foo", [](bool flag)
          {
              std::variant<int, TestClass> value;
              if (flag)
              {
                  value = TestClass(13);
              }
              else
              {
                  value = 42;
              }
              return value;
          });
}

In Python it works like a charm, no need to cast it. Returned type is already correct.

In [1]: import mymodule

In [2]: x = mymodule.foo(False)

In [3]: x
Out[3]: 42

In [4]: x = mymodule.foo(True)

In [5]: x
Out[5]: <mymodule.TestClass at 0x1031d9e70>

In [6]: type(mymodule.foo(False)) == int
Out[6]: True

In [7]: type(mymodule.foo(False)) == float
Out[7]: False

In [8]: type(mymodule.foo(True)) == int
Out[8]: False

In [9]: type(mymodule.foo(True)) == mymodule.TestClass
Out[9]: True

[roastduck]

It seems it was my mistake and I get it work now. Sorry for bothering.

[jiwaszki]

Don't worry, always ask questions when in trouble! That is why we provide support here 😃