[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Géry]

Change by Géry :


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versions: +Python 3.9 -Python 3.7

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Géry]

Change by Géry :


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resolution: not a bug -> fixed

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Géry]

Change by Géry :


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pull_requests: +21866
pull_request: https://github.com/python/cpython/pull/22934

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Raymond Hettinger]

Raymond Hettinger  added the comment:

How about taking another look at this after I've finished my more extensive 
rewrite based on my course materials.

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Géry]

Géry  added the comment:

@Raymond Hettinger

> The goal in the descriptor how-to is to give an understanding of how 
> descriptors work.

Okay. So I don't know if that was clear in my last message but that also means 
replacing the current "Function" implementation:

class Function(object):
. . .
def __get__(self, obj, objtype=None):
"Simulate func_descr_get() in Objects/funcobject.c"
if obj is None:
return self
return types.MethodType(self, obj)

with something like this:

class Function(object):
. . .
def __get__(self, obj, objtype=None):
"Simulate func_descr_get() in Objects/funcobject.c"
if obj is None:
return self
def newfunc(*args, **kwargs):
return self(obj, *args, **kwargs)
return newfunc
# "newfunc" emulates "types.MethodType(self, obj)"

And as you said, adding a similar comment to the "ClassMethod" implementation 
(and **kwargs):

class ClassMethod(object):
"Emulate PyClassMethod_Type() in Objects/funcobject.c"

def __init__(self, f):
self.f = f

def __get__(self, obj, klass=None):
if klass is None:
klass = type(obj)
def newfunc(*args, **kwargs):
return self.f(klass, *args, **kwargs)
return newfunc
# "newfunc" emulates "types.MethodType(self.f, klass)"

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I'm going to close this because 1) I'm working on a somewhat major set of 
updates this guide already and 2) I think this tracker issue misses the point 
of what those guide is trying to do (communicating that the class is prepended 
to the argument stream but not trying to be a fully accurate drop in 
substitute).  The responsibility for describing in detail what @classmethod 
does belongs in libstdtypes.rst.  The goal in the descriptor how-to is to give 
an understanding of how descriptors work.

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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Géry]

Géry  added the comment:

@Raymond Hettinger

> Though less accurate, the current version communicates better

I agree that types.MethodType is more accurate but may be less understandable. 
But in this case I think that the Function class for emulating instance methods 
should not use types.MethodType either, but a custom newfunc function as well. 
And **kwargs parameters should be added to both newfunc functions.

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Raymond Hettinger]

Raymond Hettinger  added the comment:

This wasn't intended to be an exact equivalent. Instead, it focuses on the key 
action of classmethod which is prepending the class to the call.  Though less 
accurate, the current version communicates better.  I suggest adding a short 
comment to the effect that "newfunc" emulates "types.MethodType(self.f, klass)".

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Raymond Hettinger]

Change by Raymond Hettinger :


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assignee: docs@python -> rhettinger

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[issue37203] Correct classmethod emulation in Descriptor HowTo Guide

[Géry]

New submission from Géry :

With the current Python equivalent `ClassMethod` implementation of 
`classmethod` given in Raymond Hettinger's _Descriptor HowTo Guide_, the 
following code snippet:

```
class A:
@ClassMethod
def f(cls, *, x): pass

print(A.f)
A.f(x=3)
```

prints:

> .newfunc at 0x106b76268>

and raises:

> TypeError: newfunc() got an unexpected keyword argument 'x'

instead of only printing:

> >

like the `@classmethod` decorator would do.

So the `ClassMethod` implementation fails in two regards:
* it does not return a bound method to a class;
* it does not handle keyword-only arguments.

With this PR `ClassMethod` will correctly emulate `classmethod`. This approach 
(`types.MethodType`) is already used in the Python equivalent `Function` 
implementation of functions given earlier in the same guide.

--
assignee: docs@python
components: Documentation
messages: 345031
nosy: docs@python, eric.araujo, ezio.melotti, maggyero, mdk, rhettinger, 
willingc
priority: normal
pull_requests: 13785
severity: normal
status: open
title: Correct classmethod emulation in Descriptor HowTo Guide
type: behavior
versions: Python 3.7

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