Title: Harmonizing descriptor protocol documentation
Type: enhancement Stage: needs patch
Components: Documentation Versions: Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 2.7
Status: open Resolution:
Dependencies: Superseder:
Assigned To: rhettinger Nosy List: Jay.Parlar, daniel.urban, davide.rizzo, docs@python, eric.araujo, ezio.melotti, franck, martin.panter, nedbat, rhettinger
Priority: low Keywords: patch

Created on 2011-05-14 13:56 by davide.rizzo, last changed 2017-05-06 15:16 by rhettinger.

File name Uploaded Description Edit
12077_descriptor_howto_python3.patch franck, 2013-02-11 17:37 Patch for python 3 doc review
12077_descriptor_howto_python2.patch franck, 2013-02-11 17:38 Patch for python 2 doc review
Messages (6)
msg135974 - (view) 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.
msg136172 - (view) 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
msg136295 - (view) 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.
msg181915 - (view) 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!
msg293161 - (view) Author: Martin Panter (martin.panter) * (Python committer) 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.
msg293164 - (view) Author: Raymond Hettinger (rhettinger) * (Python committer) Date: 2017-05-06 15:16
I will have a chance to work on this before long.
Date User Action Args
2017-05-20 09:35:51martin.panterlinkissue20751 dependencies
2017-05-06 15:16:26rhettingersetmessages: + msg293164
2017-05-06 14:16:47martin.pantersetnosy: + martin.panter
messages: + msg293161
2013-05-03 18:41:12nedbatsetnosy: + nedbat
2013-05-03 09:48:34rhettingersetpriority: normal -> low
assignee: rhettinger
2013-02-12 08:25:53rhettingersetassignee: rhettinger -> (no value)
2013-02-11 17:38:18francksetfiles: + 12077_descriptor_howto_python2.patch
2013-02-11 17:37:59francksetfiles: + 12077_descriptor_howto_python3.patch
versions: + Python 3.5
nosy: + franck

messages: + msg181915

keywords: + patch
2012-11-08 08:37:48ezio.melottisettype: enhancement
versions: + Python 3.4
2011-05-23 14:41:31eric.araujosetnosy: + eric.araujo
2011-05-19 14:16:48Jay.Parlarsetmessages: + msg136295
2011-05-17 18:17:19Jay.Parlarsetnosy: + Jay.Parlar
messages: + msg136172
2011-05-17 05:16:58daniel.urbansetnosy: + daniel.urban
2011-05-17 02:08:01rhettingersetassignee: docs@python -> rhettinger

nosy: + rhettinger
2011-05-16 11:45:58ezio.melottisetnosy: + ezio.melotti

stage: needs patch
2011-05-14 13:56:25davide.rizzocreate