in java, FlatBuffers will generate name() method for enums, which is very helpful.
public static final String[] names = { "SUCCESS", "TIME_OUT", "UNKNOWN_ERROR", };
public static String name(int e) { return names[e]; }
so does it in c++
inline const char **EnumNamesResultCode() {
static const char *names[] = { "SUCCESS", "TIME_OUT", "UNKNOWN_ERROR", nullptr };
return names;
}
inline const char *EnumNameResultCode(ResultCode e) { return EnumNamesResultCode()[static_cast<int>(e)]; }
but in python, there is no such method. it just generate a pure class with some constants.
class ResultCode(object):
SUCCESS = 0
TIME_OUT = 1
UNKNOWN_ERROR = 2
how to generate name() method in python?
in other words, how can I get enum names in python?
If the class has a bunch of members, you can dir() the class and get the members. You probably want to filter out things that start with __. You can then create a mapping of the class[member_name] where member_name is what you want.
Related
I am creating a C# library with some reusable code and was trying to create a method inside a method. I have a method like this:
public static void Method1()
{
// Code
}
What I would like to do is this:
public static void Method1()
{
public static void Method2()
{
}
public static void Method3()
{
}
}
Then I could choose either Method1.Method2 or Method1.Method3. Obviously the compiler isn't happy about this, any help is much appreciated. Thanks.
If by nested method, you mean a method that is only callable within that method (like in Delphi) you could use delegates.
public static void Method1()
{
var method2 = new Action(() => { /* action body */ } );
var method3 = new Action(() => { /* action body */ } );
//call them like normal methods
method2();
method3();
//if you want an argument
var actionWithArgument = new Action<int>(i => { Console.WriteLine(i); });
actionWithArgument(5);
//if you want to return something
var function = new Func<int, int>(i => { return i++; });
int test = function(6);
}
Yes, when C# 7.0 is released, Local Functions will allow you to do that. You will be able to have a method, inside a method as:
public int GetName(int userId)
{
int GetFamilyName(int id)
{
return User.FamilyName;
}
string firstName = User.FirstName;
var fullName = firstName + GetFamilyName(userId);
return fullName;
}
Note that public (and similar modifiers) are not supported C# programming guide:
Because all local functions are private, including an access modifier, such as the private keyword, generates compiler error CS0106, "
This answer was written before C# 7 came out. With C# 7 you can write local methods.
No, you can't do that. You could create a nested class:
public class ContainingClass
{
public static class NestedClass
{
public static void Method2()
{
}
public static void Method3()
{
}
}
}
You'd then call:
ContainingClass.NestedClass.Method2();
or
ContainingClass.NestedClass.Method3();
I wouldn't recommend this though. Usually it's a bad idea to have public nested types.
Can you tell us more about what you're trying to achieve? There may well be a better approach.
You can define delegates within your method with complete code and call them if you want.
public class MyMethods
{
public void Method1()
{
// defining your methods
Action method1 = new Action( () =>
{
Console.WriteLine("I am method 1");
Thread.Sleep(100);
var b = 3.14;
Console.WriteLine(b);
}
);
Action<int> method2 = new Action<int>( a =>
{
Console.WriteLine("I am method 2");
Console.WriteLine(a);
}
);
Func<int, bool> method3 = new Func<int, bool>( a =>
{
Console.WriteLine("I am a function");
return a > 10;
}
);
// calling your methods
method1.Invoke();
method2.Invoke(10);
method3.Invoke(5);
}
}
There is always an alternative of using a nested class within a class that will not be visible from outside and calling its methods, like:
public class SuperClass
{
internal static class HelperClass
{
internal static void Method2() {}
}
public void Method1 ()
{
HelperClass.Method2();
}
}
As of C# 7.0 you can do that:
public static void SlimShady()
{
void Hi([CallerMemberName] string name = null)
{
Console.WriteLine($"Hi! My name is {name}");
}
Hi();
}
This is called local functions, that is just what you were looking for.
I took the example from here, but further informatin can be found here and here.
Why you don't use classes?
public static class Helper
{
public static string MethodA()
{
return "A";
}
public static string MethodA()
{
return "A";
}
}
Now you can acces MethodA via
Helper.MethodA();
Older thread, but C# does have the concept of nested functions
Func<int> getCalcFunction(int total, bool useAddition)
{
int overallValue = 0;
if (useAddition)
{
Func<int> incrementer = new Func<int>(() =>
{
overallValue += total;
return overallValue;
});
return incrementer;
}
else
{
Func<int> decrementer = new Func<int>(() =>
{
overallValue -= total;
return overallValue;
});
return decrementer;
}
}
private void CalcTotals()
{
Func<int> decrem = getCalcFunction(30, false);
int a = decrem(); //result = -30
a = decrem(); //result = -60
Func<int> increm = getCalcFunction(30, true);
int b = increm(); //result = 30
b = increm(); //result = 60
}
Your nearly there
public static void Method1()
should be
public static class Method1{}
Don't you want to use nested class instead?
That's said, you seem to not respect the Single Responsibility Principle because you want a single method do more than one thing at a time.
Why don't you just Run a method within another
public void M1()
{
DO STUFF
}
public void M1()
{
DO STUFF
M1();
}
I want to check if an object is an instance of a certain class. In Python I can do this with instanceof. In C/C++, I found a function named PyObject_IsInstance. But it seems not to work like isinstance.
In detail (also described as sample codes below):
In C++ I defined my custom type My. The type definition is MyType and the object definition is MyObject.
Add MyType to the exported module with name My.
In Python, create a new instance my = My(), and isinstance(my, My) returns True.
While in C++ we use PyObject_IsInstance(my, (PyObject*)&MyType) to check my, and this returns 0, which means my is not an instance of the class defined by MyType.
Full C++ code:
#define PY_SSIZE_T_CLEAN
#include <python3.6/Python.h>
#include <python3.6/structmember.h>
#include <stddef.h>
typedef struct {
PyObject_HEAD
int num;
} MyObject;
static PyTypeObject MyType = []{
PyTypeObject ret = {
PyVarObject_HEAD_INIT(NULL, 0)
};
ret.tp_name = "cpp.My";
ret.tp_doc = NULL;
ret.tp_basicsize = sizeof(MyObject);
ret.tp_itemsize = 0;
ret.tp_flags = Py_TPFLAGS_DEFAULT;
ret.tp_new = PyType_GenericNew;
return ret;
}();
// check if obj is an instance of MyType
static PyObject *Py_fn_checkMy(PyObject *obj) {
if (PyObject_IsInstance(obj, (PyObject *)&MyType)) Py_RETURN_TRUE;
else Py_RETURN_FALSE;
}
static PyMethodDef modmethodsdef[] = {
{ "checkMy", (PyCFunction)Py_fn_checkMy, METH_VARARGS, NULL },
{ NULL }
};
static PyModuleDef moddef = []{
PyModuleDef ret = {
PyModuleDef_HEAD_INIT
};
ret.m_name = "cpp";
ret.m_doc = NULL;
ret.m_size = -1;
return ret;
}();
PyMODINIT_FUNC
PyInit_cpp(void)
{
PyObject *mod;
if (PyType_Ready(&MyType) < 0)
return NULL;
mod = PyModule_Create(&moddef);
if (mod == NULL)
return NULL;
Py_INCREF(&MyType);
PyModule_AddObject(mod, "My", (PyObject *)&MyType);
PyModule_AddFunctions(mod, modmethodsdef);
return mod;
}
Compile this into cpp.so, and test it in Python:
>>> import cpp
>>> isinstance(cpp.My(), cpp.My)
True
>>> cpp.checkMy(cpp.My())
False
METH_VARARGS
This is the typical calling convention, where the methods have the type PyCFunction. The function expects two PyObject* values. The first one is the self object for methods; for module functions, it is the module object. The second parameter (often called args) is a tuple object representing all arguments. This parameter is typically processed using PyArg_ParseTuple() or PyArg_UnpackTuple().
The function signature of Py_fn_checkMy does not match this. It should take two arguments. The first is the module, and this is what you are checking against MyType. The second argument (which you don't actually accept) is a tuple containing the object you passed. You should extract the argument from the tuple and check the type of this.
You'd probably be better using METH_O to specify a single argument instead of extracting arguments from a tuple:
static PyObject *Py_fn_checkMy(PyObject *self, PyObject *obj) {
if (PyObject_IsInstance(obj, (PyObject *)&MyType)) Py_RETURN_TRUE;
else Py_RETURN_FALSE;
}
static PyMethodDef modmethodsdef[] = {
{ "checkMy", (PyCFunction)Py_fn_checkMy, METH_O, NULL },
{ NULL }
};
I'm having an issue where a python class, which is derived from a c++ base class using pybind11, is being immediately destructed (garbage collected). I would like C++ to take ownership of the dynamically allocated object, but I can't seem to make that happen. I've tried keep_alive, passing shared_ptr<> as py::class_ template argument, and py::return_value_policy... nothing is working. I suspect this is just user error.
This is a simplification of the real issue I'm having with a much larger code base that is architected similarly. Changing the architecture is not an option, so making this example work is critical for me.
I have two c++ classes that I have created python interfaces for using pybind11. Class A and B both have virtual methods, so they have corresponding trampoline classes to support inheritance. The user calls the B::Run() function which results in a dynamically allocated (via new) A object to be created. When I create specializations of these two classes in python, as shown below.... Segmentation fault because the B::aBase is destroyed immediately after B::Run being called.
Any Ideas how to fix this? Thanks in advance!
class A
{
public:
A(){};
virtual ~A()
{
std::cout << "In A::~A()\n";
};
virtual char* SayHello()
{
char* x = "\n\nHello from Class A\n\n";
return x;
}
};
class ATramploline : public A
{
public:
using A::A;
char* SayHello() override
{
PYBIND11_OVERLOAD( char*,A,SayHello,);
}
};
class B
{
public:
B()
{
std::cout << "In Class B Constructor\n";
}
void Run()
{
aBase = AllocateAnAClass();
std::cout << aBase->SayHello();
}
virtual ~B()
{
fprintf(stderr,"About to delete aBase");
delete aBase;
}
A* aBase;
virtual A* AllocateAnAClass()
{
return new A;
}
};
class BTramploline : public B
{
public:
using B::B;
A* AllocateAnAClass() override
{
PYBIND11_OVERLOAD( A*,B,AllocateAnAClass,);
}
};
PYBIND11_MODULE(TestModule,m)
{
py::class_<A,ATramploline>(m,"A")
.def(py::init<>(),py::return_value_policy::reference_internal)
.def("SayHello",&A::SayHello);
py::class_<B,BTramploline>(m,"B")
.def(py::init<>())
.def("Run",&B::Run)
.def("AllocateAnAClass",&B::AllocateAnAClass,py::return_value_policy::reference_internal);
}
#!/usr/bin/python3
from TestModule import A,B
class MyA(A):
def __init__(self):
super().__init__()
print("Done with MyA Constructor")
def SayHello(self):
return '\n\nHello from Class MyA\n\n'
class MyB(B):
def __init__(self):
super().__init__()
print("Done With MyB Constructor")
def AllocateAnAClass(self):
print("In MyB::AllocateAnAClass!!!")
return MyA()
#x = B()
#x.Run()
y = MyB()
y.Run()
print("done with test script\n")
The correct (I think) way to use std::shared_ptr<A> as the A holder is to add it to class_<A...> arguments.
You also want to replace every instance of A* with std::shared_ptr<A>, and new with std::make_shared. I think non-default return policies are not needed in this case, so I have removed them; YMMV.
Working module below (with minor errors corrected).
#include <pybind11/pybind11.h>
#include <memory>
#include <iostream>
namespace py = pybind11;
class A
{
public:
A(){};
A(const A&) { std::cout << "Copying A\n"; }
virtual ~A()
{
std::cout << "In A::~A()\n";
};
virtual const char* SayHello()
{
const char* x = "\n\nHello from Class A\n\n";
return x;
}
};
class ATrampoline : public A
{
public:
using A::A;
const char* SayHello() override
{
PYBIND11_OVERLOAD( const char*,A,SayHello,);
}
};
class B
{
public:
B()
{
std::cout << "In Class B Constructor\n";
}
B(const B&) { std::cout << "Copying B\n"; }
void Run()
{
aBase = AllocateAnAClass();
std::cout << aBase->SayHello();
}
virtual ~B()
{
}
std::shared_ptr<A> aBase;
virtual std::shared_ptr<A> AllocateAnAClass()
{
return std::make_shared<A>();
}
};
class BTrampoline : public B
{
public:
using B::B;
std::shared_ptr<A> AllocateAnAClass() override
{
PYBIND11_OVERLOAD(std::shared_ptr<A>,B,AllocateAnAClass,);
}
};
PYBIND11_MODULE(TestModule,m)
{
py::class_<A,std::shared_ptr<A>, ATrampoline>(m,"A")
.def(py::init<>())
.def("SayHello",&A::SayHello);
py::class_<B, BTrampoline>(m,"B")
.def(py::init<>())
.def("Run",&B::Run)
.def("AllocateAnAClass",&B::AllocateAnAClass);
}
py::nodelete was the solution. While n.m's answer DOES work, it would require going back and chaning all of the pointer in an existing libary to smart pointers, which isn't a viable option for me. Using py::nodelete allows me to do everything on the pybind11 side.
py::class_<A,ATramploline,std::unique_ptr<A,py::nodelete> >(m,"A")
.def(py::init<>())
.def("SayHello",&A::SayHello);
I'm re-writing a string class (called XXXString) that implements many Pythonic operations like ==.
When overloading the [] operator I want it to return a XXXString& to its ith character, rather than a char& like in std::string.
In this way, I can do both
a_string[0] = "a"
and
a_string[0] == "a"
(assuming I have defined the == operator to compare a XXXString with a char*).
I know many C++ enthusiastic people will say no your "a" is not a 'a'. There is a '\0' in there balabala. I just want to ignore this and think Pythonic.
If you really want to do this, the solution should be to use a proxy and not return the actual XXXString itself.
#include <iostream>
#include <string>
#include <cassert>
class XXXString {
public:
class StringRefProxy {
public:
StringRefProxy(char& ch_in) : ch{ch_in} {}
bool operator==(const std::string& other) {
assert(other.length() == 1);
return other[0] == this->ch;
}
StringRefProxy& operator=(const std::string& other) {
assert(other.length() == 1);
this->ch = other[0];
return *this;
}
private:
char& ch;
};
XXXString(const std::string& str_in) : str{str_in} {}
StringRefProxy operator[](std::size_t index) {
return StringRefProxy{this->str[index]};
}
operator std::string() {
return this->str;
}
private:
std::string str;
};
int main() {
auto str = XXXString{"something"};
assert(str[0] == "s");
str[0] = "a";
assert(static_cast<std::string>(str) == "aomething");
}
Note that when C++17 is freely available, you can substitute all the const std::string& with std::string_view in the above code for better performance
Also note that as most approaches go that extract references that point to elements inside container. This too is subject to invalidation issues.
I have a list of base classes in C++, I want to access them in Python as a list of their derived most classes.
Is there a build in means to cater for this in Boost.Python?
I've made an example the problem I ma facing:
// ------------------------------- Code ----------------------------------//
#include<memory>
#include<iostream>
#include<vector>
namespace boost { template<class T> T* get_pointer(std::shared_ptr<T>& p){ return p.get(); }}
struct Vehicle{ virtual ~Vehicle(){} friend bool operator==(const Vehicle& lhs, const Vehicle& rhs) { return true; }};
struct Boat: public Vehicle{
virtual ~Boat(){}
friend bool operator==(const Boat& lhs, const Boat& rhs) { return true; }
char const* testBoatSpecificMethod() { return "Floating."; }
};
struct Truck: public Vehicle{
virtual ~Truck(){}
friend bool operator==(const Truck& lhs, const Truck& rhs) { return true; }
char const* testTruckSpecificMethod() { return "Trucking."; }
};
class Garage
{
public:
Garage() {};
~Garage() {};
char const* test() { std::string temp = "Vehicle List Size: " + std::to_string(m_VehicleList.size()); return temp.c_str(); }
friend bool operator==(const Garage& lhs, const Garage& rhs) { return true; }
std::vector<std::shared_ptr<Vehicle>>& vehicleList() { return m_VehicleList; }
private:
std::vector<std::shared_ptr<Vehicle>> m_VehicleList;
};
#include <boost/python.hpp>
#include <boost/python/suite/indexing/vector_indexing_suite.hpp>
BOOST_PYTHON_MODULE(garage_ext)
{
using namespace boost::python;
class_<Garage>("Garage")
.def("test", &Garage::test)
.def("vehicleList", &Garage::vehicleList, return_internal_reference<1>());
class_<Vehicle,std::shared_ptr<Vehicle>>("Vehicle");
class_<Boat,std::shared_ptr<Boat>,bases<Vehicle>>("Boat")
.def("testBoatSpecificMethod", &Boat::testBoatSpecificMethod);
class_<Truck,std::shared_ptr<Truck>,bases<Vehicle>>("Truck")
.def("testTruckSpecificMethod", &Truck::testTruckSpecificMethod);
implicitly_convertible<std::shared_ptr<Boat>,std::shared_ptr<Vehicle>>();
implicitly_convertible<std::shared_ptr<Truck>,std::shared_ptr<Vehicle>>();
class_<std::vector<std::shared_ptr<Vehicle>> >("stl_vector_Vehicle")
.def(vector_indexing_suite<std::vector<std::shared_ptr<Vehicle>> >());
}
// --------------------------- Test Script -------------------------------//
import garage_ext
g = garage_ext.Garage()
l = g.vehicleList()
l.append(garage_ext.Boat())
print "Expecting a Boat object:"
print str(l[0])
print g.vehicleList()[0].testBoatSpecificMethod()
garage_ext.f2("Done.")
// ------------------------------ Output ---------------------------------//
Expecting a Boat object
Traceback (most recent call last):
File "test_garage.py", line 7, in
print g.vehicleList()[0].testBoatSpecificMethod()
AttributeError: 'Vehicle' object has no attribute 'testBoatSpecificMethod'
'Vehicle' object has no attribute 'testBoatSpecificMethod'
Here I want Vehicle to be a Boat object.
If there is not a build-in or recommended/known Boost.Python means to handle this problem,
I'll try wrapping the list (Lots of wrapping to be done then in my library.) with a get accessor returning a boost::python::list, storing the derived most types in the python list object. Getting the derived most type possibly by calling overriden 'getAsDerivedClass' method.
I would like to avoid this. I dislike having to add python usage specific methods to the library, for our design and vision values / reasons. Another concern is that this way will introduce a lot of extramaintenance work.
EDIT:
What I want works when I am using raw pointers instead of smart pointers.
For what I feel are obvious reasons,I do not want to use raw pointers in place of smart pointers.
This does give me a relieve in that knowing what I want this concept isn't so far-fetched as I started to fear. (I am struggling still to make it work with smart pointers. The python object asks for a converter, too much work to write one by hand.)