[Python-Dev] Preserving the definition order of class namespaces. (original) (raw)

Eric Snow ericsnowcurrently at gmail.com
Sun May 24 03:15:35 CEST 2015

tl;dr Are there any objections to making making the default cls.prepare return OrderedDict instead of dict (and preserve that order in a list on the class)?

A couple years ago [1][2] I proposed making class definition namespaces use OrderedDict by default. Said Guido [3]:

I'm fine with doing this by default for a class namespace; the type of
cls.__dict__ is already a non-dict (it's a proxy) and it's unlikely to
have 100,000 entries.

It turns out making cls.dict an OrderedDict isn't reasonably tractable (due to the concrete API v. subclasses), but really that isn't what I was looking for anyway.

Regardless, since it's been a while I just want to run the proposal by the group again. I'm hopeful about landing my C implementation of OrderedDict [4] in the next few days. Also, I have a patch up [5] that implements using OrderedDict for class definitions. So mostly I just want to double check that I'm still good to go.

Just to be clear on what I'm proposing specifically, I've summed it up below.

-eric

Currently if you want to preserve the order of a class definition you have to use a metaclass with a prepare method (see PEP 3115). However, as that PEP points out [6], the common case for prepare is to use OrderedDict. I'm proposing that we return OrderedDict() by default from prepare. Considering the common case, we should also expose that definition order on the class afterward since otherwise the extra information from the class definition namespace is discarded (type.new copies it into a dict which is then used for cls.dict).

So the key changes are:

(Note that I will not be changing the actual type of cls.dict (i.e. tp_dict) which will remain a dict.)

The effect of the change would be that the following are basically equivalent (relative to the the definition namespace):

class Meta(type):
    @classmethod.
    def __prepare__(meta, *args, **kwargs):
        return OrderedDict()

class SpamOld(metaclass=Meta):
    a = 1
    b = 2
    c = 3
    __definition_order__ = list(locals())

class SpamNew:
    a = 1
    b = 2
    c = 3

assert SpamOld.__definition__order == SpamNew.__definition_order__

The key differences are:

The patch for the change is pretty minimal. [5]

Also, Nick Coghlan recently expressed that he favored using OrderedDict by default over the alternative presented by PEP 422/487. [10]

[1] https://mail.python.org/pipermail/python-ideas/2013-February/019690.html [2] https://mail.python.org/pipermail/python-dev/2013-June/127103.html [3] Guido: https://mail.python.org/pipermail/python-ideas/2013-February/019704.html [4] http://bugs.python.org/issue16991 [5] http://bugs.python.org/issue24254 [6] see the "Alternate Proposals" section of https://www.python.org/dev/peps/pep-3115/ [7] PEPs 422 and 487 relatedly focus on the benefits of reducing the need to use metaclasses [8] https://mail.python.org/pipermail/python-ideas/2013-February/019706.html [9] see "Key points" on https://mail.python.org/pipermail/python-dev/2013-February/124439.html [10] Nick: https://mail.python.org/pipermail/python-ideas/2015-March/032254.html

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