python - 向下转换为 pybind11 派生类

Question

我正在使用 pybind11 的“Overriding virtual functions in Python”功能来创建继承自 C++ 抽象类的 Python 类。我有一个C++类(class)State它在 Python 中的子类为 MyState 。在这种情况下我有一些MyState丢失其类型信息的对象，Python 认为它是 State 。我需要将其向下转换回 MyState在 Python 代码中，我不知道执行此操作的好方法。

这是 C++ 示例代码:

#include <memory>

#include <pybind11/pybind11.h>

namespace py = pybind11;

// ========== State ==========

class State {
 public:
  virtual ~State() = default;

  virtual void dump() = 0;
};

using StatePtr = std::shared_ptr<State>;

class PyState : public State {
 public:
  using State::State;

  void dump() override {
    PYBIND11_OVERLOAD_PURE(void, State, dump);
  }
};

// ========== Machine ==========

class Machine {
 public:
  virtual ~Machine() = default;

  virtual StatePtr begin() = 0;

  virtual StatePtr step(const StatePtr&) = 0;
};

using MachinePtr = std::shared_ptr<Machine>;

class PyMachine : public Machine {
 public:
  using Machine::Machine;

  StatePtr begin() override {
    PYBIND11_OVERLOAD_PURE(StatePtr, Machine, begin);
  }

  StatePtr step(const StatePtr& state) override {
    PYBIND11_OVERLOAD_PURE(StatePtr, Machine, step, state);
  }
};

// ========== run ==========

void run(const MachinePtr& machine) {
  StatePtr state = machine->begin();
  for (int i = 0; i < 5; ++i) {
    state = machine->step(state);
    state->dump();
  }
}

// ========== pybind11 ==========

PYBIND11_MODULE(example, m) {
  py::class_<State, StatePtr, PyState>(m, "State").def(py::init<>());
  py::class_<Machine, MachinePtr, PyMachine>(m, "Machine")
      .def(py::init<>())
      .def("begin", &Machine::begin)
      .def("step", &Machine::step);
  m.def("run", &run, "Run the machine");
}

以及 Python 代码:

#!/usr/bin/env python3

from example import Machine, State, run


class MyState(State):
    def __init__(self, x):
        State.__init__(self)
        self.x = x

    def dump(self):
        print(self.x)


class MyMachine(Machine):
    def __init__(self):
        Machine.__init__(self)

    def begin(self):
        return MyState(0)

    def step(self, state):
        # problem: when called from C++, `state` is an `example.State`
        # instead of `MyState`. In order to access `state.x` we need
        # some way to downcast it...
        return MyState(state.x + 1)


machine = MyMachine()

print("running machine with python")
state = machine.begin()
for _ in range(5):
    state = machine.step(state)
    state.dump()

print("running machine with C++")
run(machine)  # error

错误消息:

running machine with python
1
2
3
4
5
running machine with C++
Traceback (most recent call last):
  File "<string>", line 38, in <module>
  File "<string>", line 36, in __run
  File "/usr/local/fbcode/platform007/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/fbcode/platform007/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/data/users/jcai/fbsource/fbcode/buck-out/dev/gen/experimental/jcai/pybind/run_example#link-tree/run_example.py", line 38, in <module>
    run(machine)  # error
  File "/data/users/jcai/fbsource/fbcode/buck-out/dev/gen/experimental/jcai/pybind/run_example#link-tree/run_example.py", line 26, in step
    return MyState(state.x + 1)
AttributeError: 'example.State' object has no attribute 'x'

我确实有一个巧妙的解决方法，我基本上保留了一张“沮丧的 map ”std::unordered_map<State*, py::object>并注册每个创建的 MyState用它。但我不想诉诸这样的事情。

Answer 1

我认为您可能遇到了这一系列问题:

https://github.com/pybind/pybind11/issues/1774

最终，因为您只是直接返回 MyState ，然后直接进入 C++，Python 解释器会失去对您的实例的跟踪，并继续进行垃圾收集 Python 部分对象的，这就是为什么你的对象最终变得有点 sliced .

潜在的解决方案:

• 存储对返回的 MyState 的引用，至少足够长的时间，以便 Python 解释器再次获取引用。
  • 例如将 return MyState(...) 更改为 self._stashed_state = MyState(...);返回 self._stashed_state
• 看看是否可以以某种方式在 C++ 中的 Python 版本的类上 incref(糟糕，但它会起作用)
• 查看上述问题中列出的解决方法(记不清所有问题)
• 使用我们的 pybind11 分支，它可以处理这个问题，但也会拖入其他内容:overview for RobotLocomotion/pybind11

您可能还想在现有问题之一上发帖，提及您也遇到过此问题(以便可以对其进行跟踪)。