[BUG] "__doc__" attribute of static method is not set. #2403

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1. bstaletic commented on Aug 17, 2020

   

   bstaletic

   on Aug 17, 2020

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   I'm not at my computer right now, but I think this is a duplicate of a very old bug report. I might be wrong.
2. YannickJadoul commented on Aug 17, 2020

   

   YannickJadoul

   on Aug 17, 2020

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   This seems quite complex too me. Wouldn't this work just as well?

       template <typename Func, typename... Extra> class_ &
       def_static(const char *name_, Func &&f, const Extra&... extra) {
           static_assert(!std::is_member_function_pointer<Func>::value,
                   "def_static(...) called with a non-static member function pointer");
           cpp_function cf(std::forward<Func>(f), name(name_), scope(*this),
                           sibling(getattr(*this, name_, none())), extra...);
           auto static_cf = staticmethod(cf);
           if (py::hasattr(cf, "__doc__"))
               cf.attr("__doc__") = static_cf.attr("__doc__");
           attr(cf.name()) = static_cf;
           return *this;
       }

   But I'd still like to figure out why PyStaticMethod_New doesn't do the same as @staticmethod in Python, before getting that PR in. Maybe this mistake was made in other places as well.
3. YannickJadoul commented on Aug 17, 2020

   

   YannickJadoul

   on Aug 17, 2020

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   @pkerichang, is this on Python 2 or Python 3 or both, btw?
4. pkerichang commented on Aug 18, 2020

   

   pkerichang

   on Aug 18, 2020

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   Hello,

   I only tested on Python 3 (3.7 FYI).

   as for using attr() versus ptr->m_ml->ml_doc to get the docstring, that's where my uncertainty comes in. Other places that deal with cpp_function obtain the doc string with the second approach, where-as for static method the only way to set the doc string is through the first approach. I'm not knowledgeable enough about the code base to know what is preferred versus not.
5. YannickJadoul commented on Aug 18, 2020

   

   YannickJadoul

   on Aug 18, 2020

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   Hi @pkerichang. I can't reproduce this, though. What version of pybind11 are you on?

   I just tried this:

   #include <pybind11/pybind11.h>
   
   namespace py = pybind11;
   
   struct X {
       static bool f(int x, float b) { return true; }
   };
   
   PYBIND11_MODULE(example, m)
   {
       py::class_<X>(m, "X")
           .def_static("f", &X::f, "This is an amazing docstring");
   }

   $ python3.7
   Python 3.7.6 (default, Dec 19 2019, 23:50:13) 
   [GCC 7.4.0] on linux
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import example
   >>> example.X
   <class 'example.X'>
   >>> example.X.f
   <built-in method f of PyCapsule object at 0x7f97e8638f30>
   >>> example.X.f.__doc__
   'f(arg0: int, arg1: float) -> bool\n\nThis is an amazing docstring\n'

   And help(example.X.f) shows me:

   Help on built-in function f in module example:
   
   f(...) method of builtins.PyCapsule instance
       f(arg0: int, arg1: float) -> bool
       
       This is an amazing docstring
   (END)
   

   Am I doing something different than you are?
6. pkerichang commented on Aug 18, 2020

   

   pkerichang

   on Aug 18, 2020

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   Ah I figure out the problem, my bad, this is not a bug after all.

   In python, if f_inst is an instance method of class Foo, then Foo.f_inst is equivalent to Foo.__dict__['f_inst']. However, for f_static, a static method of class Foo, Foo.__dict__['f_static'] points to the static method decorator around the function object instead, not the function object itself. You need to use Foo.__dict__['f_static'].__func__ to access the underlying function object, Foo.f_static. I do not know this before, and my docstring parsing script is finding the docstring of the staticmethod decorator, not the docstring of the underlying object.

   I'm closing this issue now, thanks for all the quick responses!

   👍React with 👍1Reacted by Yannick Jadoul
7. pkerichang

   closed this as completedon Aug 18, 2020
8. YannickJadoul commented on Aug 18, 2020

   

   YannickJadoul

   on Aug 18, 2020

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   You need to use Foo.__dict__['f_static'].__func__ to access the underlying function object, Foo.f_static. I do not know this before, and my docstring parsing script is finding the docstring of the staticmethod decorator, not the docstring of the underlying object.

   Yes, the staticmethod class implements the descriptor protocol (see here for the details), so you won't notice if you call Foo.f_static (or getattr(Foo, 'f_static')), but if you go through __dict__, it doesn't resolve the descriptor protocol for you.

   But glad to hear I'm not going crazy and it's not a bug in pybind11; thanks! :-)