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。
最佳答案
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
- 例如将
- 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)。
关于python - 向下转换为 pybind11 派生类,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/56367837/