Cannot pass arbitrary object as py::object

[abergmeier-dsfishlabs]

I have a function bar to which I want to allow to pass anything "stringy":

py::class_< Foo >( m, "Foo" )
  ...
  .def( "bar", [](Foo& foo, py::object pathy) {
            auto const path = py::str(pathy).cast<std::string>();
            return foo.bar(path.c_str());
        }
  )
  ...

Naively I thought I could pass arbitrary objects to py::object.
When I do foo_instance.bar( pathlib.Path("/path") ) it complains:

TypeError: Incompatible function arguments. The following argument types are supported:
1. (module.Foo, object) -> Ret
Invoked with: <module.Foo object at 0x7fc175b2e3b0>, /path

I seem to have assumed something wrong.
Is there a way of passing arbitrary objects to C++?

[jagerman]

py::object is working just as you expect—the error you're getting is misleading you here. (That's been fixed in master, which gives the more helpful error message RuntimeError: Unable to cast Python instance to C++ type (compile in debug mode for details) rather than an argument error; that change isn't in 1.8.1 however).

The real issue is that py::str doesn't convert the argument to a string, it simply attempts to cast it to a string. But that fails here, because what you are passing in obviously isn't a python string object, it's a pathlib.Path object.

What you want to do instead is explicitly call the __str__ method of that object, which will give you back the string you expect:

  .def( "bar", [](Foo& foo, py::object pathy) {
            std::string path = py::str(((py::object) pathy.attr("__str__"))());
            return foo.bar(path.c_str());
        }
  )

[aldanor]

Or just simply:

std::string path = pathy.str();

[jagerman]

Yes, that's much smarter (and better: it works as in Python for objects that define __repr__ but not __str__).

[abergmeier-dsfishlabs]

std::string path = pathy.str();

Works like a charm, thanks guys.

[wjakob]

I'll reopen this since it could serve as a nice example on providing a custom type_caster for an object that can be converted into a string. (i.e. a kind of filtered py::object)