Not understood error with __set_name__ in a descriptor class

[ast]

Le 19/04/2020 à 17:06, ast a écrit :

I understood where the problem is.

Once __get__ is defined, it is used to read self._name in method
__set_name__ and it is not défined yet. That's why I have
an error "'NoneType' object is not callable"
Thats a nice bug.
I need to think for a workaround

> Hello
> 
> This code is working well: (I am using python 3.6.3)
> 
> --------------------------------------------------------------------
> class my_property:
>      def __init__(self, fget=None, fset=None, fdel=None):
>          self.fget = fget
>          self.fset = fset
>          self.fdel = fdel
>      def __set_name__(self, owner, name):
>          self._name = name
>          print("Inside __set_name__ method:  ", self._name)
> 
> ##    def __get__(self, instance, owner):
> ##        return self.fget(instance)
> 
> 
> class Test:
>      celsius = my_property()
> 
> print("Outside __set_name__ method: ", Test.celsius._name)
> 
> 
> Here is the output:
> 
> ========= RESTART: C:/Users/jm/Desktop/python/my_property.py =========
> Inside __set_name__ method:   celsius
> Outside __set_name__ method:  celsius
> --------------------------------------------------------------------
> 
> And this one doesn't. (I just removed ## to uncomment the 2 lines)
> Do you understand why ?
> 
> class my_property:
>      def __init__(self, fget=None, fset=None, fdel=None):
>          self.fget = fget
>          self.fset = fset
>          self.fdel = fdel
>      def __set_name__(self, owner, name):
>          self._name = name
>          print("Inside __set_name__ method:  ", self._name)
> 
>      def __get__(self, instance, owner):
>          return self.fget(instance)
> 
> 
> class Test:
>      celsius = my_property()
> 
> print("Outside __set_name__ method: ", Test.celsius._name)
> 
> Here is the output:
> 
> ========= RESTART: C:/Users/jm/Desktop/python/my_property.py =========
> Inside __set_name__ method:   celsius
> Traceback (most recent call last):
>    File "C:/Users/jm/Desktop/python/my_property.py", line 17, in <module>
>      print("Outside __set_name__ method: ", Test.celsius._name)
>    File "C:/Users/jm/Desktop/python/my_property.py", line 11, in __get__
>      return self.fget(instance)
> TypeError: 'NoneType' object is not callable