[Python-ideas] Are we supposed to be able to have our own class dictionary in python 3?

[Daniel Moisset]

Sorry, should have replied to the list too

On Sat, 3 Nov 2018, 23:55 Daniel Moisset <dfmoisset at gmail.com wrote:

> If I understood correctly what you want, it's possible with a metaclass.
> Check the __prepare__ method at
> https://docs.python.org/3/reference/datamodel.html#preparing-the-class-namespace
> and Pep 3115
>
> On Sat, 3 Nov 2018, 20:55 Joy Diamond <python.gem at gmail.com wrote:
>
>> Team,
>>
>> Are we supposed to be able to have our own class dictionary in python 3?
>>
>> If we currently cannot -- do we want to be able to?
>>
>> That we can have out own class dictionary in python 3 is strongly implied
>> in the following at https://www.python.org/dev/peps/pep-3115/ where it
>> says:
>>
>> """
>>
>>     # The metaclass invocation
>>     def __new__(cls, name, bases, classdict):
>>         # Note that we replace the classdict with a regular
>>         # dict before passing it to the superclass, so that we
>>         # don't continue to record member names after the class
>>         # has been created.
>>         result = type.__new__(cls, name, bases, dict(classdict))
>>         result.member_names = classdict.member_names
>>         return result
>>
>> """
>>
>> I don't understand this.  As far as I can tell, no matter what class
>> dictionary you pass into `type.__new__` it creates a copy of it.
>>
>> Am I missing something?  Is this supposed to work?  Is the documentation
>> wrong?
>>
>> Thanks,
>>
>> Joy Diamond.
>>
>> Program that shows that the class dictionary created is not what we pass
>> in --- Shows the actual symbol table is `dict` not `SymbolTable`
>>
>> class SymbolTable(dict):
>>     pass
>>
>> members = SymbolTable(a = 1)
>>
>> X = type('X', ((object,)), members)
>>
>> members['b'] = 2
>>
>> print('X.a: {}'.format(X.a))
>>
>> try:
>>     print('X.b: {}'.format(X.b))
>> except AttributeError as e:
>>     print('X.b: does not exist')
>>
>> #
>> #   Get the actual symbol table of `X`, bypassing the mapping proxy.
>> #
>> X__symbol_table = __import__('gc').get_referents(X.__dict__)[0]
>>
>> print('The type of the actual symbol table of X is: {} with keys:
>> {}'.format(
>>       type(X__symbol_table),
>>       X__symbol_table.keys()))
>>
>>
>> # Prints out
>> # X.a: 1
>> # X.b: does not exist
>> # The type of the actual symbol table of X is: <class 'dict'> with keys:
>> dict_keys(['a', '__module__', '__dict__', '__weakref__', '__doc__'])
>>
>>
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