Issue 12077: Harmonizing descriptor protocol documentation (original) (raw)
Created on 2011-05-14 13:56 by davide.rizzo, last changed 2022-04-11 14:57 by admin. This issue is now closed.
Messages (8)
Author: Davide Rizzo (davide.rizzo) *
Date: 2011-05-14 13:56
There are three sources of information for the descriptor protocol:
- Data model reference (Doc/reference/datamodel.rst)
- Descriptor HowTo guide (Doc/howto/descriptor.rst)
- PEP 252
A developer who already knows descriptor tipically reads the first one: object.get(self, instance, owner) "... owner is always the owner class ..." Reading a bit further there are the ways a descriptor can be called, and the "direct call" is x.get(a). That is, without the third argument (owner) specified.
The how-to definition is slightly different: descr.get(self, obj, type=None) --> value Here the arguments have different names ("type" shadowing the type bultin) and it seems to be implied that the third argument is optional. The ClassMethod example at the end of the document seems to confirm this: def get(self, obj, klass=None): (though another example doesn't). And the method contains an explicit check on klass being None. Also it could be confusing that through the examples in the same document many different names are used for the same argument (type, objtype, klass), and all different than the name used in the reference.
Lastly the PEP is more explicit: get(): a function callable with one or two arguments. [...] When X is None, the optional second argument, T, should be meta-object. [...] When both X and T are specified ...
One more quirk: the reference explains the distinction between data and non-data descriptors, though says nothing about set raising AttributeError for read-only data descriptors.
My proposal:
- use the same names for get arguments throughout the documentation (both the reference and the tutorial), e.g. get(self, instance, owner)
- decide whether the third argument should be optional, or state the common usage in the reference, and fix accordingly the examples in the howto
- explain data, non-data and read-only descriptors in the set reference, or more simply, how the defintion of set affects these things.
Author: Jay Parlar (Jay.Parlar)
Date: 2011-05-17 18:17
While working on this, I believe it would also make sense to remove all instances of the terms "new-style" and "old-style" from the Descriptor HowTo (and wherever else they might be present)
It still makes sense for them to be present in the 2.7 documentation, but they're concepts that don't exist in 3.x
Author: Jay Parlar (Jay.Parlar)
Date: 2011-05-19 14:16
Another problem is that the examples and text in the section "Functions and Methods" is no longer correct in 3.x. Namely the the references to unbound methods, and the example showing an unbound method being returned when accessing a method of a class.
Author: Franck Michea (franck) *
Date: 2013-02-11 17:37
Here is at least a correction of Descriptors' HowTo. There are two versions since some stuff differs (object inheritance, ...).
Here are some of my interrogations though:
- RevealAccess is not using instance parameter, so value is shared. Is this intended?
- Don't really know what to do with "Function and Methods" part. First I don't really understand the relevance of this in a descriptor how-to. Also it is know outdated in python3 version (unbound thing, ...), so what do?
- In getattribute function, I don't really understand the paramters given to get, why None and the instance? But this is probably my fault.
This also doesn't answer the question about the real source that should be kept. What to do?
I also need a proof-read, since english is not my first language... Anyway it's clearly not enough to be published like that
Have a nice day!
Author: Martin Panter (martin.panter) * 
Date: 2017-05-06 14:16
See Issue 23702 specifically about unbound methods in Python 3, and Issue 25435 about general problems with the how-to in Python 3.
Author: Raymond Hettinger (rhettinger) *
Date: 2017-05-06 15:16
I will have a chance to work on this before long.
Author: Raymond Hettinger (rhettinger) *
Date: 2019-08-29 06:07
[Davide]
- use the same names for get arguments throughout the documentation (both the reference and the tutorial), e.g. get(self, instance, owner)
Early on the choice of variable names diverged (including in various PEPs and in the C source). I will harmonize where I can but the cat is out of the bag.
- decide whether the third argument should be optional, or state the common usage in the reference, and fix accordingly the examples in the howto
PEP 252 specifies that it is optional. Various builtin descriptors also make it optional (function_get, staticmethod_get, classmethod_get, and property_get).
I'm fixing the main docs and non-compliant code in PR 12992
explain data, non-data and read-only descriptors in the set reference, or more simply, how the > defintion of set affects these things.
That is reasonable. Will add to the datamodel docs.
[Jay Parlar]
Another problem is that the examples and text in the section "Functions and Methods" is no longer correct in 3.x. Namely the the references to unbound methods
That was fixed a good while ago.
Author: Raymond Hettinger (rhettinger) *
Date: 2019-08-29 06:22
See PR 12992 for the cross-reference from the set docs to the section covering data and non-data descriptors.
History
Date
User
Action
Args
2022-04-11 14:57:17
admin
set
github: 56286
2019-08-29 08:29:30
rhettinger
set
status: open -> closed
resolution: fixed
stage: needs patch -> resolved
2019-08-29 06:22:12
rhettinger
set
messages: +
2019-08-29 06:07:50
rhettinger
set
messages: +
2017-05-20 09:35:51
martin.panter
link
2017-05-06 15:16:26
rhettinger
set
messages: +
2017-05-06 14:16:47
martin.panter
set
nosy: + martin.panter
messages: +
2013-05-03 18:41:12
nedbat
set
nosy: + nedbat
2013-05-03 09:48:34
rhettinger
set
priority: normal -> low
assignee: rhettinger
2013-02-12 08:25:53
rhettinger
set
assignee: rhettinger -> (no value)
2013-02-11 17:38:18
franck
set
files: + 12077_descriptor_howto_python2.patch
2013-02-11 17:37:59
franck
set
files: + 12077_descriptor_howto_python3.patch
versions: + Python 3.5
nosy: + franck
messages: +
keywords: + patch
2012-11-08 08:37:48
ezio.melotti
set
type: enhancement
versions: + Python 3.4
2011-05-23 14:41:31
eric.araujo
set
nosy: + eric.araujo
2011-05-19 14:16:48
Jay.Parlar
set
messages: +
2011-05-17 18:17:19
Jay.Parlar
set
nosy: + Jay.Parlar
messages: +
2011-05-17 05:16:58
daniel.urban
set
nosy: + daniel.urban
2011-05-17 02:08:01
rhettinger
set
assignee: docs@python -> rhettinger
nosy: + rhettinger
2011-05-16 11:45:58
ezio.melotti
set
nosy: + ezio.melotti
stage: needs patch
2011-05-14 13:56:25
davide.rizzo
create