>On Sun, Jul 31, 2016 at 6:33 AM, Gerald Britton ><gerald.britton at gmail.com> wrote: >> Today, I was reading RH's Descriptor HowTo Guide at >> >> https://docs.python.org/3/howto/descriptor.html?highlight=descriptors >> >> I just really want to fully "get" this. >> >> So I put together a little test from scratch. Looks like this: >> >> class The: >> class Answer: >> def __get__(self, obj, type=None): >> return 42 >> >>>>> The.Answer >> <class '__main__.The.Answer'> >>>>> >> >> but, I expected to see 42. >> >> So, digging deeper I read: >> >> For classes, the machinery is in type.__getattribute__() which transforms >> B.x into B.__dict__['x'].__get__(None, B). In pure Python, it looks like: >> >> def __getattribute__(self, key): >> "Emulate type_getattro() in Objects/typeobject.c" >> v = object.__getattribute__(self, key) >> if hasattr(v, '__get__'): >> return v.__get__(None, self) >> return v >> >> OK, so I copied this function, then ran it and got: >> >>>>> __getattribute__(The, 'Answer') >> 42 >> >> So, what I don't get is why the "B.x into B.__dict__['x'].__get__(None, B)" >> part doesn't work in my case. >> >> I'm sure I'm missing something here (that`s usually the case for me <:‑|) , >> but what? > >Obviously that __getattribute__ is not exactly like the real one. The >real one looks up __get__ as a *method* of B.__dict__['x'], which >requires that B.__dict__['x'] be an instance of some class that >defines __get__, not the class itself. Try this:
Thanks Ian, that helps, though I suppose I should point out that the article covers both objects and classes. It was the section about classes I was referring to. Reading it again, it's obvious that 'B' is a class and 'x' is some attribute of B. What's not clear is that 'x' must be an *instance* of something for this to work as I expected. Even so, If I translate my The.Answer to The.__dict__['Answer'].__get__(None, The), (as per the article) it works as I expect: >>> The.__dict__['Answer'].__get__(None, The) 42 Note that 'Answer' is not an instance; it's another class. So I suppose the document is a little ambiguous on that point. That's OK. I just need a mental note to remember what it really means. > >py> class Answer: >... def __get__(self, obj, type): >... return 42 >... >py> class The: >... Answer = Answer() >... >py> The.Answer >42 On Sun, Jul 31, 2016 at 8:33 AM, Gerald Britton <gerald.brit...@gmail.com> wrote: > Today, I was reading RH's Descriptor HowTo Guide at > > https://docs.python.org/3/howto/descriptor.html?highlight=descriptors > > I just really want to fully "get" this. > > So I put together a little test from scratch. Looks like this: > > class The: > class Answer: > def __get__(self, obj, type=None): > return 42 > > >>> The.Answer > <class '__main__.The.Answer'> > >>> > > but, I expected to see 42. > > So, digging deeper I read: > > For classes, the machinery is in type.__getattribute__() which transforms > B.x into B.__dict__['x'].__get__(None, B). In pure Python, it looks like: > > def __getattribute__(self, key): > "Emulate type_getattro() in Objects/typeobject.c" > v = object.__getattribute__(self, key) > if hasattr(v, '__get__'): > return v.__get__(None, self) > return v > > OK, so I copied this function, then ran it and got: > > >>> __getattribute__(The, 'Answer') > 42 > > So, what I don't get is why the "B.x into B.__dict__['x'].__get__(None, > B)" part doesn't work in my case. > > I'm sure I'm missing something here (that`s usually the case for me <:‑|) , > but what? > > -- Gerald Britton, MCSE-DP, MVP LinkedIn Profile: http://ca.linkedin.com/in/geraldbritton -- https://mail.python.org/mailman/listinfo/python-list
